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Deep and crisp and even

Given the fact we had about a week of lying snow last month, and that I’ve already been asked ‘is that it for this winter?’, I thought some statistics on lying snow here in Reading might not go amiss.

We have complete day-to-day records of the dates with lying snow (defined as a coverage of at least 50 per cent over the ground at 0900 GMT) at the University going back to 1948 with the actual measured snow depth on all but 38 days back to 1951. Note that the observing site moved to the Whiteknights campus from the London Road area in 1968.

So how often does snow lie in Reading at 9 a.m.? Over the period 1981-2010 (our most recent 30-year climatological period) the figures (shown in blue) are as follows:

The climatology of lying snow days in Reading.

This recent period shows a decline in lying snow with respect to 1951-80 (red), while anyone who has only been in Reading for a few years will only be familiar with the green plotted bars – and may not realise how fortunate they are (assuming they are snow lovers!). My own daughters – now both at university – had many a winter in which their sledges remained in the garage. Unfortunately, across southern England this is about as good as it gets, unless you venture into the hills – see this chart from the Met Office’s current 1981-2010 averages webpages (

Met Office: UK-wide snow lying days each year.

So how much variation is there in the annual incidence of lying snow? The next figure gives the answer:

The variation of lying snow days in Reading, 1948-9 onwards.

Winters without lying snow occur roughly every nine years. Winters with ten days of lying snow occur quite frequently – once every 3.8 years on average although only twice from 1986 to 2008. Winters with 20 days of lying snow are rare (5 years in 64) while the winter of 1962-63 stands in a class of its own – having 54 mornings with 50% snow cover (with most of those on consecutive mornings in January and February). Recent winters do seem to indicate that lying snow has become more prevalent.

Of course ‘days with lying snow’ gives no indication of the snow depth and its persistence. One way to judge that is to follow the suggestion of Philip Eden and to note the cumulative snow depth during the season (the cumulative snowfall is very difficult to measure in the UK as often rain and snow fall within a short space of time and separating the one from the other requires keen and frequent observations).  The cumulative snow depth is simply the sum of all the 0900 GMT snow depths added together during the (autumn-) winter (-spring).

Cumulative winter snow depths in Reading, 1951-52.

Again, 1962-63 stands out – but 2009-10 also makes an appearance. This is due to the great depths of snow recorded that winter, for example

  • 11 cm on 22 December 2009 and 10 cm on 23 December 2009;
  • 27 cm on 6 January 2010, 26 cm on 7 January 2010 – decreasing slowly to 9 cm on 15 January before clearing the next day.

This 27 cm was the greatest snow depth at 9 a.m. in Reading since 31 cm on 3 January 1963 (this latter depth following depths of 28 cm and 27 cm on the previous two days) – and this depth in 1963 was the greatest any University of Reading observer had ever measured.

The other winter that stands out in this image is that of 1981-82 – that winter lying snow occurred from 8 December, on and off, to 17 January. (Even more memorable that winter was the cold – minimum temperatures of -14.5 °C on 14 January following on from -13.4 °C on 13 December, temperatures which helped the snow cover to persist.) The former temperature was the lowest air temperature ever recorded at the university’s climatological station.

So what about the remainder of the winter? – forecasts currently suggest a ‘slight’ cooling in the air temperature over the next couple of weeks and statistically February (March) sees lying snow in about one year in two (three). In February 1986 snow lay for 15 days in Reading having been virtually absent up that point. Thus we may yet see some more of the white stuff lying – in 2008 snow lay 6 cm deep as late as 6 April (the deepest April depth in Reading since before 1951)…

Snow cover of 6 April 2008.

…and some were not amused!

Written 6 February 2013.

Mountain Pine Beetles, Climate and the Carbon Cycle

Mountain Pine Beetle infested tree at Niwot Ridge, Colorado.

From the early 1990s, high levels of tree mortality have been observed across large areas of forest in North America infested by species of bark beetles. The majority of the damage originates from the native Mountain Pine Beetle Dendroctonus ponderosae. The beetles burrow underneath the bark of the trees to lay their eggs, disrupting water and nutrient flows within the sapwood during the processes. In addition they often carry blue stain fungus which further infects the phloem and sapwood. These effects lead to a slow death for the tree and infected areas are easily identified by the symptomatic red colouration in the forest canopy. Although the exact cause of the outbreak is unknown it appears to be in response to changing climatic conditions with warmer temperatures acting to increase the beetle’s natural range and to reduce levels of winter mortality which normally helps keep the population in check.

Impacts of the beetle infestation on human activity are numerous. A chief concern is damage to timber based economies, such as in British Columbia where forest product exports totalled $9.95 billion Canadian Dollars. There is also strong evidence relating beetle induced mortality to increased likelihood of wild fires: the dead trees remain standing for several years and dry out during the summer months leading to increased risk of combustion. In addition it has been hypothesised that the infestation could lead to a large source of carbon to the atmosphere, potentially acting a positive feedback to climate change. A paper published in Nature by Kurz et al. (2008) projected that between 2000–2020 there will be 270 megatonnes (Mt) carbon released to the atmosphere from infected forests in British Columbia alone. Whilst this is relatively small compared to annual emissions of carbon from global fossil fuel combustion this figure becomes more daunting when scaled up over the whole of North America. The Kurz et al. paper was sufficiently high profile to be reported in the New York Times.

But are these claims concerning large releases of carbon to the atmosphere really correct? A group of scientists, including myself, have been working to address this question for several years now. A key assumption in the calculations of post infestation carbon release is that respiration from soil micro-organisms (known as heterotrophic respiration) will continue unabated once the forest dies. This is the main route by which dead organic matter is returned to the atmosphere in a normal, undisturbed ecosystem. In other words heterotrophic respiration is a major term in the carbon balance of a mature forest. However, careful experiments carried out by Nicole Trahan as part of her PhD at the University of Colorado suggested that this may not be true for forests that have been attacked by the mountain pine beetle. By simulating beetle attacks on a small patch of healthy forest at Niwot Ridge, Colorado, and measuring the corresponding heterotrophic CO2 flux over a number of years, Nicole observed that respiration from soil microbes slowed down as the trees died. She then went on to show the same things happening in a nearby beetle infested forest in Fraser Valley. The explanation for this appears to be that as photosynthesis in a tree decreases, the amount of carbohydrates (such as glucose and sucrose) that are made available to the fungal mycorrhiza surrounding its root system also decreases. Without these priming agents the activity of the microbial community slows down.

More information on this component of the research can be seen in The Pine Beetle Project video on YouTube.

Will these field scale observations hold out over wider areas? This is an important question if we are to understand the impact of the beetle infestation on the carbon balance of North America and its potential to act as a positive feedback to climate change. Unfortunately both photosynthesis and heterotrophic respiration are notoriously difficult to measure on large scales. To address this we used two approaches: 1) satellite derived estimates of photosynthesis and 2) measurements of the night-time accumulation of CO2 at the bottom of Fraser valley as a proxy for heterotrophic respiration.

Satellite estimates of photosynthesis (or more specifically, Gross Primary Productivity, GPP) are now derived operationally for the whole global roughly every two weeks using sensors such as NASA’s Moderate Resolution Imaging Spectrometer (MODIS). However preliminary analysis of this data suggested that, for a number of reasons, it was not capturing the pattern of disturbance that we were observing on the ground. Instead we opted to calibrate a recently published model of satellite derived GPP against estimates of photosynthesis derived using eddy covariance trace gas flux measurement techniques at the Niwot Ridge site (which is a relatively healthy forest). This model was then applied over the infested Fraser Valley forest. The resulting estimates of annual GPP are shown in the figure below. The forest at Niwot Ridge shows no decline in photosynthesis for the period 2002-2011 whereas Fraser Valley exhibits the same temporal pattern as the Niwot Ridge forest from 2002 to 2006, but rapidly declines after 2006 (the year in which the beetle infestation started).

Starting in 2006 CO2 concentrations have been measured on the valley floor in Fraser. We used the averaged night-time accumulation as a proxy for heterotrophic respiration. During the night autotrophs (i.e. vegetation) stop respiring CO2 and hence the night-time accumulation is only produced by heterotrophs (primarily soil microbes). This assumes that the boundary layer is relatively stable so that CO2 is not being lost from the valley, and also that the valley is flushed out during the day. Meteorological observations were used to filter the data for nights where this was potentially a problem.

Estimated photosynthesis from satellite data at Niwot Ridge (blue line) and Fraser Valley (green line). Night time accumulation of CO2 in Fraser Valley is used as a proxy for heterotrophic respiration and assumes a relatively stable atmosphere. The absolute magnitudes of each of these data series are different and so values relative to 2006 (the first year of major beetle infestation in Fraser Valley) are shown to facilitate comparison.

The plot above shows our results from the scaling up exercise and corroborates the field studies: as photosynthesis declines so does heterotrophic respiration. Consequently we can be more confident in our assertion that the Kurz et al. result is overstating the impact of the D. ponderosae in terms of the potential release of carbon to the atmosphere. Longer term the carbon in the dead wood will be released to the atmosphere, but the regrowth of new trees, which is a strong sink of carbon, is likely to offset much of this on similar time scales. This isn’t to say the beetles are not a problem, they most certainly are, but that the ecological puzzle is inevitably more complex than it first appears.

A manuscript is currently under consideration for Ecology Letters (Moore et al., submitted) that describes all of this in much more detail. I will update this post when that paper is accepted. Our next steps are two-fold: first, to try and scale our estimates up over even larger areas and second to incorporate our new knowledge into a prognostic model so that we can make better constrained projections of the interaction with climate.

To find out more:

Kurz W.A., Dymond C.C., Stinson G., Rampley G.J., Neilson E.T., Carroll A.L., Ebata T. and Safranyik L. (2008) Mountain pine beetle and forest carbon feedback to climate change. Nature 452, 987-990, doi:10.1038/nature06777

Moore D.J.P., Trahan N.A., Wilkes P., Quaife T., Desai A.R., Negron J.F., Stephens B.B., Elder K. and Monson R.K. (2013, submitted) Changes in carbon balance after insect disturbance in Western U.S. forests. Submitted to Ecology Letters.

Growing bio-fuels in Africa: Sugarcane in Ghana

Figure 1: Location of the proposed sugarcane cultivation in Ghana

The cultivation of bio-energy crops, such as sugarcane is a potentially lucrative activity.  But sugarcane is a thirsty crop, and the infrastructure required to refine it is expensive.  These issues raise serious questions about sustainability and profitability.  Nowhere is this more true than in Africa.

Assessing the environmental and economic feasibility of sugarcane cultivation is especially urgent in Ghana.  In 2008 Brazil’s agriculture research agency, EMBRAPA, opened an office in Accra with the intention of helping Ghana to build up its nascent ethanol industry. In 2010 Brazil made a $300m investment in exporting Brazilian sugar-cane cultivation and refinement technology to Ghana.

There is no doubt that growing sugarcane in Ghana will require irrigation – and a dam has been proposed to provide this (see Figure 1 for the location of the River Daka cultivation – the subject of our case study).  But how much irrigation is required?  Will these requirements increase, in the future, as the climate changes?

We used a new computer model to investigate sugarcane cultivation both now, and for a simplified future scenario, in which temperature and CO2 increase, but rainfall and other climate variables are held constant.  Importantly, our model is based on the land-surface component of the Met Office climate model.  This means that it is capable of representing both the growth of plants in the present day, and their response to climate change.

The results were surprising (Figure 2).   We found that in the present day, sugarcane can be grown in Ghana – with an irrigation requirement of 3-4mm/day (in agreement with pilot field studies, carried out by our Ghanaian collaborators).   When temperatures were warmed by 4 degrees Celsius, as expected, the higher evaporative demand increased irrigation requirements.   However, when CO2 concentration was doubled, the irrigation requirements returned to present day levels.

Figure 2: Irrigation versus mean yield for current climate, 40 warming with present day CO2 and 40 warming with doubled CO2. The error bars represent the interannual standard deviation in yield.

To understand why this is, we need to think about the way that plants respond to climate change.  Plants’ response to elevated CO2 largely depends on their mechanism of photosynthesis.  Most plants fall into one of two categories:  those that follow the C3 photosynthetic pathway and those (such as sugarcane) that follow the C4 pathway.  In C3 plants, carbon is fixed by the action of the enzyme Rubisco. The rate at which this reaction happens is enhanced by higher ambient CO2/O2 ratios. There is therefore a theoretical mechanism for greater biomass production under scenarios of higher atmospheric concentration of CO2.

Sugarcane assimilates carbon through the C4 photosynthetic pathway. Unlike C3 plants, C4 plants concentrate CO2 from the atmosphere in sheath (outer) cells via a biochemical pathway that does not have a strong dependence on atmospheric CO2 levels.  There is therefore no obvious mechanism for a direct link between elevated CO2 and enhanced photosynthesis. Despite this, there is abundant observational evidence that production of biomass by C4 plants is increased by rising concentrations of CO2 – especially under water-stressed conditions. This is likely to be primarily because of the impact of CO2 concentrations on transpiration and hence on water use efficiency (WUE).

Specifically, plants take in CO2 through microscopic openings on their leaves called stomata, but whenever stomata are open, plants lose water. However, when CO2 concentrations are raised, fewer stomata need to be open in order to maintain optimal CO2 levels within the plant, which leads to a reduction in the rate of transpiration (see Figure 3).  For given meteorological conditions, the plant will therefore lose less water to the atmosphere – and hence have greater WUE.  Hence, over the course of a growing season, the plant will be less stressed and accumulate more biomass.

Figure 3 (Left): A photograph of a stomata; (right) a schematic illustrating that stomata take in CO2 and release water and oxygen

It follows from the previous discussion, that under scenarios of climate change, in which both temperature and CO2 are higher than the present day, there are competing effects on C4 plants.  Specifically, higher temperatures increase soil and canopy evaporation, potentially reducing the soil moisture available to plants, while higher CO2 levels reduce transpiration rates.

There remain uncertainties about how these competing factors interact to determine the response of C4 vegetation to climate change. Our results tell us about the specific case of sugarcane cultivation in Ghana, and raise interesting questions about the more general interplay between CO2, temperature and water stress.

Returning to our Ghana case study… Of course looking at irrigation requirements cannot, in itself, tell us whether or not growing sugarcane is sustainable.  Ongoing work within our project is looking at the wider economic context and the hydrological impacts of the damming and irrigation.  Preliminary results suggest that, properly managed, the dam could provide sufficient irrigation water for sugarcane.  Furthermore, the dam would reduce the risk of flash floods, and provide a more perennial water supply to the local population.

Bio-energy crop cultivation is a controversial issue – especially in Africa.  However, in this case, our modeling results, coupled with local field studies, suggest that sugarcane cultivation is environmentally feasible, and could bring much needed investment into this desperately poor region.

To find out more:

Paper on this topic:  Emily Black, Pier Luigi Vidale, Anne Verhoef, Santiago Vianna Cuadra, Tom Osborne and Catherine Van den Hoof (2012) Cultivating C4 crops in a changing climate: sugarcane in Ghana Environ. Res. Lett. 7 044027

News article on Environmental Research Web: Can Ghana match Brazilian sugarcane yields?

Sudden Stratospheric Stirrings

For a stratospheric scientist, winter is a worrying time.  For most of the year we can enjoy whatever weather the atmosphere decides to throw at us, secure in the knowledge that none of it is our fault.  But occasionally winter throws in a surprise, and one glance at last weekend’s weather forecast tells you that this is one of those years:  Squeezed between the icy graphics and blue-tone temperatures was an unequivocal message:  this time a Stratospheric Sudden Warming is to blame.

Sudden Stratospheric Warmings (SSWs) are conjured from the darkness of the winter polar stratosphere.  Without the Sun’s energy, the ozone-rich air can’t maintain its characteristically high temperatures, and a cold pool develops over the pole, contained by strong cyclonic winds from the temperature gradient.  But the vortex isn’t left to its own devices.  Planetary waves can propagate from the troposphere, and slow the vortex as they break.  As with the winds of a surface cyclone, this creates convergence; in the stratosphere air is forced to descend, and adiabatically warm.

The majority of SSWs are minor, but occasionally the westerly winds can be slowed so much that they reverse – the definition of a major warming.  Major warmings are a Northern Hemisphere phenomenon; on average there are 6 a decade, but only one has ever been recorded in the Southern Hemisphere.  With very little land mass to the Antarctic stratosphere is starved of that key ingredient for SSWs – planetary waves.  (This too is why the Antarctic ozone hole is so much larger than its Arctic neighbour; with such a strong vortex there is little dilution of cold-pool air, so destroyed ozone can’t be replaced.)

It’s important to remember that the pace of the stratosphere is slower than the transient weather systems below.  Though SSWs only hit the headlines in the past few days, they’ve been above our heads for several weeks.  Turn back the calendar to 2012, and the first was in full swing just as we were throwing the turkey in the oven on Christmas day.  This was a vortex displacement, where the whole vortex moves equator-ward, taking on a distinctive comma shape (Figure 1.a), and it persisted for over a week, before the vortex started to recover.  The second type of warming, a vortex split (e.g. Figure 1.b), was just around the corner, with an onset date somewhere around 6th January.  In fact, this wasn’t so much a split as a complete disintegration.  Either way, the result was the same; significant down-welling of air, and a warming of the upper and middle stratosphere.

FIGURE_1 - The typical evolution of a vortex displacement (top), and a vortex split (bottom), shown as stereographic plots of geopotential height (contours) on the 10-hPa pressure surface.

With those sorts of time scales, it’s clear that the effects at the surface aren’t instantaneous.  It takes about 15 to 20 days for an SSW to express itself in our terrestrial weather, but when it does, the changes can be dramatic. Weaker polar vortices can generate surface anomalies very similar to those of the negative phase of the Arctic Oscillation (a cooling at most mid-latitudes in the Northern Hemisphere, with a warming in Eastern Canada) (Figure 2.b). Atlantic and Pacific storm tracks also tend to move equator-ward.  A weaker polar vortex/SSW won’t always generate such changes, but this time it has, and they show all the signs of lasting:  Figure 2.a shows predicted Northern Hemisphere temperature anomalies for February, from the Canadian Middle Atmosphere Model.

FIGURE 2 - (a) Surface temperature anomalies for February 2013, from the Environment Canada on Dec. 31, 2012. (b) Mean surface temperature anomalies 16-60 days after SSWs, from the Canadian Middle Atmosphere Model (Fig. 1a of Sigmond et al.; 2013). The boxes indicate the regions of maximum surface temperature response.)

But how confident can we be in such long-term predictions, when most of us will have noticed the forecasts for the next couple of days fluctuating wildly?  Though we may have lost the short-term detail, medium-range forecast can actually improve if you pick the right day to start your model!  10 to 60 day forecast skill improves significantly if you initialise a forecast during an SSW.  There are implications for the type of model you choose too.  Those with poorly resolved stratospheres struggle to replicate SSWs (Figure 3); better then to opt for a ‘high-top’ version, if you want to capture the intricacies of the stratosphere.

FIGURE 3 - Climatological mean decadal frequency of Stratospheric Sudden Warming events, 1960-2000 in 19 historical simulations of CMIP5 models. Coloured bars show the number of SSW events per month calculated by the Charlton and Polvani algorithm, along with 95 % confidence intervals for each estimate. Models shown in red are classified as high-top models, those shown in blue as low-top models. The climatological mean decadal frequency in the ERA-40 re-analysis dataset is shown in the horizontal dashed black line and the 95 % confidence interval for this estimate in grey. On the right of the plot, median estimates for the low-top and high-top ensembles are shown

But what of the current sudden stratospheric warming?  Whilst we can forecast the onset of SSWs with reasonable accuracy, the recovery is something of a dark art.  This time last week, I wrote a summary of the ECMWF forecast for colleagues who are making stratospheric measurements in Southern France.  It concluded that a vortex recovery seemed to be well underway; the next day’s forecast showed the vortex disintegrating again, and there’s been little improvement since.  Either way, this bout of stratospheric weather is likely to continue for a couple of weeks at least, as it continues to work its way to the surface.  Until then, you’ll find stratospheric scientists locked in their offices, nervously waiting for spring.

A White Christmas in Reading?

What is a “White Christmas”?

For many people a White Christmas is a Christmas Day (25 December) with a ‘traditional’ look to the day – that is one with snow covering the ground. For those whose fancy a flutter at the bookmakers (although it is probably too late to get good odds now) a White Christmas is one when snow or sleet is observed during the 0000-2359 h day. This latter definition includes a wet and damp with a few flakes during a shower of sleet on a day when rain is the dominant form of precipitation. In fact William Hill are slightly more restrictive with their White Christmas betting rules which state they pay out only when ‘… one flake of snow will fall on Met office monitoring stations over the 24 hr period of the 25th of December’.

Weather on past Christmas days

Here at Reading weather observations are made at 0900 GMT (9 a.m. in the morning in winter) every day of the year and in this note we will examine the incidence of

  1. Snow/sleet falling at some time during the 24 hours beginning 0000 GMT [SF annotation below],
  2. Lying snow at 0900 GMT [SL annotation below], and
  3. A measurable depth of snow at 0900 GMT.

We currently have records for every Christmas back to 1917 (in 1916 there was almost certainly a fall of sleet/snow on Christmas Day) and these indicate the following years when one or more of the above events was noted on 25 December:

1925 [SF] – The 9 a.m. air temperature in a NE’ly wind was 0.8C (following a slight air frost) and this subsequently rose to 8.2C in the next 24 hours. Some light snow fell during the day but it then turned dull in Reading with light rain. Further north, snow lay for in both Manchester and Glasgow. The state of the ground is not known for this day in Reading although it is not thought that a 50% snow cover prevailed.

1927 [SF] – According to Robin Stirling (in “The Weather of Britain”) “A depression from the Atlantic moved from Ireland to the English Channel and then across France to the Mediterranean. It caused a great snowstorm in southern England. About 18 h on Christmas Day, rain in the south turned so heavy that roads were hopelessly blocked by midnight, and a train was snowbound between Alton and Winchester.” As the storm continued some 15 cm of snow fell in central London. In Reading the 25th began with an air temperature of 6.3C at 9 a.m. – then rising only to 6.8C before dropping to -0.4C the following night. There was moderate rain falling in an E’ly wind at 9 a.m.; the rain became heavy at times and turned to snow fell later in the day (9.9 mm of water equivalent precipitation fell in 24 hours) – although, again, snow depths were not reported at Reading. However, newspaper accounts report that locally, a considerable stretch of the Reading to Henley road was under five feet of snow by Boxing Day (26th). “The Berkshire weather book” by Currie et al. carries an interesting account of the Boxing Day blizzard in Berkshire and adjacent areas.

For intensity, extent,damage and disruption many would argue that this snowstorm was the most damaging since March 1891 and probably the most severe snowstorm of the twentieth century in the UK. South-east of a line from Norwich to Dorchester over 30 cm of snow fell (Eden, 2008, “Great British weather disasters”) with 60-75 cm over parts of the North Downs in Surrey and the Weald in Kent. A-roads were blocked for up to a week – minor roads for much longer. The snow was of a high water content and hundreds of kilometres of lines came down as a result.

1938 [SL] – The 25th dawned with a minimum temperature of -0.3C and lying snow (depth unknown, although it had been lying from the 21st) after several falls of snow that fell over several days back to the 18th and in which time the temperature had only reached 2.3C. The temperature rose to just 0.6C on the 25th; no snow fell on Christmas Day (which was dull and cold) – but did fall on the 26th. Across the local area it was more the cold that caused problems in the run up to Christmas – with many frozen pipes and a very large demand for paraffin for heating.

1956 [SF] – After a dull, cold start to the day, a SE’ly flow brought some snowfall during the 25th with the temperature rising from -1.4C to 1.8C. Some 5.3 mm of water-equivalent precipitation fell leading to about 5 cm of lying snow on the 26th at 9 a.m. (so almost all of this precipitation would have been snow). The snow was initially light and turned heavier later. Locally snow was ‘several inches [~10 cm] deep’ (Currie et al.) later on the 25th in parts of Berkshire.

1968 [SF, SL] – This was one of the few Christmas Days when the Reading weather observer had to contend with falling sleet and snow at the observation time. At 9 a.m. conditions were air temperature 0.6C (after a minimum of 0.1C), sky obscured by falling light rain and snow (there had been a spell of continuous snowfall early in the morning), with a NE wind of 2 kn, visibility of about 400 m and 8 cm of lying (but thawing) snow. The temperature rose to 2.8C later in the day as the clouds thinned out and just 0.3 mm of precipitation fell in the next 24 hours. As a depression moved ESE’wards across Cornwall some places were cut off by blizzards in N England.

1970 [SF, SL] – Christmas Day began with a minimum temperature of -3.9C and 1 cm of lying snow. The temperature rose later to 0.9C and snow fell during the day to give a depth of 5 cm the next morning. However, on the 25th itself the snow was in the form of a shower before noon – more persistent snow was to fall next morning. Despite the morning snow, some 3.5 h of bright sunshine was recorded – with only 2 oktas of cloud at 9 a.m.

1981 [SL] – There was a cold start to this Christmas morning – the temperature had risen from a minimum of -4.5C to -3.4C by 9 a.m., although the grass minimum temperature had been as low as -10.2C. The ground was frozen to a depth of just over 5 cm under 5 cm of lying snow. No snow fell during the day, which was dry and sunny with 4.5 h of bright sunshine. This was one of the snowiest December’s of the twentieth century – on the 12th the snow depth in Reading had been 18 cm.

1996 [SF] – By now many people had begun betting on the likelihood of a ‘White Christmas’ and this year brought a pay-out. In Reading, after a slight air frost, some snow fell – but not enough to give a 50% ground cover at 9 a.m. With 3.5 h of sunshine during the day any lying snow had melted by Boxing Day morning.

1999 [SF] – Another day when the bookmakers had to pay out – certainly snow fell in Reading but not enough to lie on a ground surface that was ‘moist’ after some 43 mm of rain in the preceding 72 hours. In Reading rain turned to snow around 11 a.m., only to turn back to rain after about ten minutes. It was a cloudy day with another 7.2 mm of precipitation being credited to the 25th. It was a windy day with a gust of 43 kn measured at the university.

2009 [SL] – After a minimum temperature of 0.9C it warmed up to reach 8.4C later, the warmth helping to reduce the 5 cm of complete ground cover of snow (this had persisted since the 18th, having caused some severe pre-Christmas disruption to Reading) to a broken cover of 2 cm depth by the morning of the 26th. No snow fell during Christmas Day, which brought some rain and some sunshine.

2010 [SL] – Again, there was no snowfall but lying snow existed at 9 a.m. to a depth of 2 cm. This lying snow remained rather broken in cover and helped the temperature to drop sharply into Boxing Day down to -6.5C in the screen and to -11.3C on the grass.

A White Christmas in 2012?

In summary then, in the past 95 years there have been seven Christmas Days with snow falling and six with snow lying in the morning (1968 and 1970 fall into both camps). So the likelihood of a White Christmas as the bookies judge it is about one in 13.6 – the last one was in 1999 so we are (maybe) due for one soon…

Those who would like to read about the incidence of a White Christmas elsewhere in the UK should start also read Martin Rowley’s account at .

2012 Hurricane season wrap-up

By Ray Bell

The 2012 Atlantic hurricane season officially ended on Saturday as a ‘very unusual‘ year with an abnormally large 19 tropical storms forming and curiously only 1 making it to Category 3 strength. A typical hurricane season has 12 tropical storms, 6 of which are hurricanes and 3 of which are major hurricanes. This year also saw a large proportion of tropical storms forming at high latitudes.  However, 2012 will be long remembered for producing hurricane Sandy: a ‘Frankenstorm’ (a cross between a nor-easter and a hurricane) that was the largest, most powerful, and second most destructive Atlantic hurricane on record, say Jeff Masters.

North Atlantic tropical storms tracks 2012 (National Hurricane Centre)

Based on the combined number, intensity, and duration of all tropical storms and hurricanes (ACE) the season was above-average, but the limited major Hurricane activity helped keep ACE down to 30% above average.

Since 1995 the US has on average been hit by 4 storms per year, with 1.5 of these being hurricanes and 0.6 being major hurricanes. This year, Beryl, Debby and Isaac hit the US. No major hurricanes hit resulting in 2012 being the 7th consecutive year with a major hurricane strike, the longest since 1861-1868 (see here).

Back in June I wrote a blog on the hurricane seasonal forecast that the Met Office issued, as well as the forecast from other centres. The Met Office was at the low end of the seasonal forecasts going for 10 tropical storms (7-13 most likely). Colorado State University were at the high end only predicting 13 tropical storms. So why did all of the seasonal forecasts significantly under predicted tropical storm activity?

Atmospheric-Oceanic conditions

Phil Klotzbach from Colorado State University has great documentation on the Atmospheric-Oceanic conditions over the North Atlantic this season ( One reason was than the predicted El Niño did not develop, which usually calms tropical storm activity in the North Atlantic. A heavy influx of dry Saharan air over the tropical North Atlantic is also likely to have damped the intensity of storms forming in the tropics (see here). The figure below shows how stable the air has been in the tropical North Atlantic this season compared to the past 8 years.

Vertical instability over the tropical Atlantic in 2004 - 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from a climatological temperature profile (Jeff Masters)

Dr. Landsea claims the tropical storm count this year is ‘slightly misleading’ due to satellites and an array of technological advances that allows systems to be identified that likely would have gone undetected decades ago (see here). 7 out of the 9 tropical storms this year reached tropical storms strength for only 2 days or less. The method in which the Met Office use currently excludes any systems with a duration of less than two days to prevent picking up transient disturbances and depressions. Planned increases to the horizontal resolution of the Met Office seasonal forecasting system in December 2012 may allow for a better distinction between weak tropical storms and tropical depressions (Jo Camp, Pers. Comm.).

Atlantic tropical cyclones between 1878 - 2012 that spent two days or less at tropical storm strength (top) and more than two days at tropical storm strength or hurricane strength (bottom.) Figure updated from Villarini, G., G. A. Vecchi, T. R. Knutson, and J. A. Smith (2011), "Is the recorded increase in short-duration North Atlantic tropical storms spurious?", J. Geophys. Res., 116, D10114, doi:10.1029/2010JD015493. (Jeff Masters)

Hurricane seasonal forecasts for 2013 will be released in May.

I would like to thank ex-Reading University student and colleague Joanne Camp for proof reading.


Fifty years ago – The winter of 1962-1963 in Reading

The winter of 1962/63 was one of the coldest on the UK instrumental record, with the mean January temperature ranking fifth coldest (out of 354 months) and the mean February temperature ranking the seventh coldest, according to the Central England Temperature record [1]. Across the country as a whole gas and electricity supplies failed frequently, there were some noteworthy snowfalls and freezing rain (a relatively rare event in the UK) occurred on occasions – notably across southern England on 3 January. Here is the winter temperature in the context of recent ones in East Berkshire [2].

Winter mean temperature in East Berkshire, 1863-4 to 2011-12.

Winter mean temperature in East Berkshire, 1863-4 to 2011-12.

The cold weather started just before the Christmas of 1962, as temperatures from the University’s climatological station (then in London Road) show.

Daily maximum and minimum air temperatures, Reading, 1 December 1962 to 7 March 1963.

Daily maximum and minimum air temperatures, Reading, 1 December 1962 to 7 March 1963.

During early December it had been had been changeable and stormy, although the daily weather diary written at the time by the Reading observer Arthur Moon noted that 6 December (-2.3 C maximum; the third consecutive ice day) had the lowest daily December maximum temperature since 20 December 1938 (-2.8 C). There was also dense fog around Reading on the 4th to 8th – at the 0900 GMT observation on these days a visibility of 50 metres was reported. Indeed, the Reading weather diary notes that later on the 3rd visibility was a low as 15 yards (15 m) with ice needles being observed on the 4th.

(In London “the final ‘major’ old style London smog occurred in this month (4th to 6th): i.e. a combination of domestic coal smoke plus sulphur dioxide products producing an acidic fog droplet, which in turn caused major respiratory problems. About a thousand people died [in London] as a result. During the fog, the smoke & sulphur dioxide content in the atmosphere increased to a maximum of 10 to 14 times the normal concentration. This was noted at the time as the ‘worst’ since December, 1952”.[10])

Then on 22 December a high pressure system moved to the north-east of the British Isles, dragging bitterly cold winds across the country – as the NCEP reanalysis chart shows [3]. As the Reading weather observer noted “Later in the day colder conditions began to set in…”.

NCEP reanalysis surface analysis, 0000 GMT 22 Dec 1962 (white contours).

NCEP reanalysis surface analysis, 0000 GMT 22 Dec 1962 (white contours).

This NE’ly to E’ly flow was to persist for much of the winter, as the hodograph for the 0900 GMT observations at Reading from 22 December to 7 March shows.

Wind hodograph for Reading, 22 December 1962 to 7 March 1963.

Wind hodograph for Reading, 22 December 1962 to 7 March 1963. Hodograph drawn using the eight principal compass points.

The largely E/NE’ly surface flow is also confirmed by the mean MSL pressure maps for January-February 1963 – with MSL pressure being especially high across the British Isles in January. At Lerwick the mean MSL pressure during January was the highest for any January month during 1961-2012 – although at Heathrow the years of 2000, 1989 and 1964 have seen highest January values.

Winter MSL pressure, 1962-1963.

Mean MSL pressure (hPa), (left) January-February 1963 and (right) January 1963.

There was not to be a White Christmas in Reading, After some snow on the 12th, wintry weather began with a vengeance in Reading around noon on 26 December (Boxing Day) – which was the first of ten consecutive days with snowfall. This snow soon began to lie, the ground being frozen.

There was a blizzard on 29-30 December across Wales and south-west England; by the morning of the 30th the snow depth had doubled to 21 cm by 0900 h while the Reading observer noted on the 30th “drifts of 2.5 to 3 feet [75 to 90 cm] were present, the strong wind causing considerable drifting and blowing of snow.” The snow depth reached 31 cm at the 0900 GMT observation on 3 January. Lying snow (50% or more of ground cover) lasted continuously at Reading from 27 December to 14 February. Altogether, snow fell on 38 days during the three winter months (the average for the period 1981-2010 was just 7.6 days [6]). In spite of these snowy conditions it is reported that there were no delays to Reading’s trolley buses by the 28th [8].

The weather diary for 3 January  contains the following: “Dull conditions continued and overnight snow turned to freezing rain mixed with ice pellets (clear ice) at times. This coated walls and trees with a smooth ice glaze. … The ice covering twigs measured around 3 mm thick and where dripping occurred blunt icicles about one inch [2.5 cm] long formed. Trees so covered emitted a cracking sound when blown by the wind.”

However, the Reading and Berkshire Chronicle reported on 4 January that “staggering sums of money and huge quantities of material have had to be used in the fight to clear the snow in Reading and Berkshire: villages on the Downs have been isolated; public transport services and motorists have had difficulty in keeping going; milk, paraffin and vegetables have been in short supply. It is just a small part of the picture presented by this week of Arctic conditions” [8].

The 12th gave the lowest January maximum temperature (-2.2 C) since 20 January 1946 – but colder was to come. The weather diary for 23 January notes the lowest night minimum temperature (-12.5 C) since 15 February 1929 with the 24th noting the lowest day maximum temperature (-5.6 C) yet recorded. Fog on the 23rd led to riming of trees.

Images from the winter can be seen at [12] – an example is shown here of people walking on the Thames at Windsor. The website at [12] also relates many stories and news items about the winter.

Skating on the ice on the Thames in Windsor, 1963.

Skating on the ice on the Thames in Windsor, 1963.

Level lying snow depth.

Level lying snow depth, Reading, 1 December 1962 to 7 March 1963.A similar pattern of lying snow occurred at nearby Hurley (just outside Maidenhead) ref 9.

On 18 January -22.2 C was recorded at Braemar in Scotland. The long bitterly cold spell caused lakes and rivers to freeze and sea water froze in some of England’s harbours. This is also reflected in the soil temperatures measured here in Reading. At a depth of 4 feet (122 cm) the temperature fell throughout the winter, reaching the lowest level in early March. Close to the surface temperatures were sometimes close to -5 C, and frequently below -2 C. Even at a depth of 30 cm the soil froze for a while in mid-January, the coldest part of the winter.

0900 GMT soil temperatures, Reading, 1 December 1962 to 7 March 1963.

0900 GMT soil temperatures, Reading, 1 December 1962 to 7 March 1963.

Minimum temperatures recorded at Reading during the months of December to February 1962-3 were as follows:

Value (C) Date
Lowest maximum temperature -5.6 24 Jan
Minimum air temperature -12.5 23 Jan
Grass minimum temperature -15.1 23 Jan
Lowest bare soil surface temperature -13.9 23 Jan
Lowest 5 cm soil temperature -5.0 23 Jan
Lowest 122 cm (4 feet) soil temperature -2.6 9 Mar

Lower temperatures have been recorded more recently at Whiteknights – for example the air minimum temperature of -14.5 C on 14 January 1982; the grass minimum temperature was -20.1 C that morning. On 12 January -6.8 C was the highest temperature of the day in Reading. But in 1962-1963 it was the persistence of the cold and lying snow that was remarkable.

During January the Thames was almost frozen over at Caversham Bridge while in Wargrave snow ploughs struggled to clear tons of snow in the High Street. On 16 January a USAF officer was killed in a weather-caused accident on the Twyford-Henley Road and, on the same day, two Tilehurst men died in an accident involving a coach on the Bath Road at Sonning.  British Railways Western Region complained of the wrong type of weather for testing their new anti-freeze for points. A N’ly wind was blamed for blowing back coke fumes into Holy Trinity Church in Theale on 13 January; several people collapsed and were taken to hospital. By 18 January the Thames had frozen over completely at Reading and Windsor.

The cold continued into February – on the 2nd the maximum temperature of -1.7 C was the lowest in February since 2 February 1956. Ice days (when the temperature remained below 0 C all day) are very rare in Reading in February – there were two in February 1963 along with another three days that failed to rise above 0.5 C. As days lengthened and temperatures started to rise (slowly) in February the snow slowly thawed; the Reading weather diary notes that by the evening of 14 February the snow cover on the ground was below 50 % for the first time since early on Boxing Day (a period of 50 days with 50 % cover at 0900 h.)

High pressure for much of the wintry part of the winter led to some sunny days, even if they remained cold. 208 h of bright sunshine were recorded during the three winter months (despite problems keeping the snow and frost off the glass sphere of the sunshine recorder) – this amounts to about 20 % more than average.

Sunshine duration, Reading, 1 December 1962 to 7 March 1963.

Sunshine duration, Reading, 1 December 1962 to 7 March 1963.Note the sunshine around the end of February which helped to melt much of the lying snow.

Once the snow at the end of December/start of January had finished it was a mainly dry winter with just 33 mm of precipitation falling in the first two months of 1963 – or about 33 % of average – and most of that fell by 4 January.

Daily precipitation totals (water equivalents), Reading, 1 December 1962 to 7 March 1963.These are for the 24 hours beginning on the date shown.

Daily precipitation totals (water equivalents), Reading, 1 December 1962 to 7 March 1963.These are for the 24 hours beginning on the date shown.

NCEP reanalysis surface analysis, 0000 GMT 4 March 1963 (white contours).

NCEP reanalysis surface analysis, 0000 GMT 4 March 1963 (white contours).

On 4 March a mild SW’ly flow finally reached the British Isles and temperatures gradually rose, allowing snow to melt and winter to end. There had been quite sunny conditions in Reading towards the end of February and in early March so much of the snow had already disappeared – preventing the severe flooding along the Thames that sometimes occur after a cold, snowy spell.

So how cold was it here? This image, extracted from the annual report for 1964 from the Reading & District Natural History Society [11] gives the raw numbers for January.

Notes on the 1962-3 winter by A Moon, the observer at the time.

Notes on the 1962-3 winter by A Moon, the observer at the time.

For comparison, 1931-60 soil and air temperature anomalies, and 1921-50 rainfall anomalies ([4], [5], [7]) for the three winter months of 1962-63 are shown in the next table.

Month Mean max temp (C) Mean min temp (C) Mean temp (C) Mean temp anomaly (degC) Total Precip (mm) Precip anomaly (mm) Mean 30 cm soil temp (C) Mean 30 cm temp anomaly (degC) Mean 122 cm soil temp (C) Mean 122 cm temp anomaly (degC)
Dec 4.7 -0.9 1.9 -3.1 64.1 +5.7 3.7 -1.9 7.0 -0.6
Jan 0.4 -4.4 -2.0 -5.9 22.7 -38.5 0.5 -3.8 4.1 -1.6
Feb 2.6 -2.4 0.1 -4.3 10.6 -34.6 0.3 -3.8 2.9 -2.3

Finally, it is worth noting that it was not only the British Isles that was affected by cold weather this winter. January 1963 was memorable for the extreme severity of the cold weather which simultaneously gripped North America, Europe, and the Far East. [14] In Europe  it  was  one  of  the  coldest  months  ever  recorded,  resulting  in  shortages  of  coal  and  food  as  snowdrifts blocked roads and ports  and waterways were blocked by ice  or  were completely frozen  over. Shortages of  water and  gas  also  occurred  as  a result  of  damage by frost  to exposed  pipelines  in  normally  milder  climates.  Many died or were hospitalized from exposure to the cold. Average temperatures for the month were in excess of 5 degC below normal from southern England across Europe to the Urals. Warsaw reported an average temperature of -12.4C for January, or 10.3 degC below normal, while Paris averaged -2.9C, or -5.5 degC below normal.  Even Mediterranean regions averaged about 3 degC below normal.

700 hPa mean height in January.

700 hPa mean height in January, from ref. 14. Note the ridges to the west of North America and Europe.

In the  Far  East,  abnormal  cold was  accompanied  by blizzards,  notably in western Japan, where snowdrifts of 4 m depth  in  some  districts  paralysed  transportation  and collapsed roofs.  Soldiers and students were pressed into service to dig out trains and remove snow from roofs to save schools and other structures.

What about the winter of 2012-13? Who knows? While you’re awaiting developments why not relive the winter of 1962-3 in realtime via the observation archive at [13]?


The author is indebted to the intrepid observers at the time, who made daily observations without fail. The chief observer at the time was Arthur Moon who also assiduously compiled a daily weather diary. These observations, and both earlier and later observations, are currently being compiled by the author into a database showing the daily weather in Reading since 1908. The graphs were plotted using this database.

Note that the observation site in 1962-3 was on the London Road; as such mean maximum and mean minimum temperatures were about 0.4 degC and 0.6 degC, respectively, warmer than at Whiteknights.





[4] Averages of earth temperature at depths of 30 cm and 122 cm for the United Kingdom 1931-60. 1968. HMSO, London.

[5] Averages of temperature for Great Britain and Northern Ireland 1931-60. 1963. HMSO, London.

[6] Burt, S.D. and Brugge, R., 2011. Climatological Averages for 1981-2010 and 2001-2010 for stations appearing in the monthly bulletin of the Climatological Observers Link. ISBN 9780956948502.

[7] Averages of rainfall for Great Britain and Northern Ireland 1916-50. 1958, reprinted 1961. HMSO, London.

[8] Currie, I., Davison, M. and Ogley, R., 1994. The Berkshire weather book. Froglets publications. ISBN 1872337481.






[14] : J F Connor (1963). The weather and circulation of January 1963. Monthly Weather Review, vol 91, p 209.

The 2012 Antarctic Ozone Hole

A remarkable event occurs each springtime in the Antarctic (Sept-Oct-Nov) lower stratosphere – a dramatic depletion of ozone in a period of just a few weeks.

It has not always been this way. Ozone amounts can be measured in many ways. One useful measure is the total number of ozone molecules above a square centimetre of the surface (“column ozone”) – this is reported in Dobson Units (DU), named after a pioneering stratospheric ozone researcher. The global average value is about 300 DU. The British Antarctic Survey (BAS) have routinely measured column ozone since the late 1950s at their base at Halley. Until the early 1980s, Antarctic  ozone had a weak annual cycle, rising from about 300 DU in late spring to around 370 DU in early summer, before returning back to about 300 DU, with some year-to-year variation in these values.

In the late 1970s and early 1980s,  springtime column ozone seemed to be falling year on year. By 1983, monthly mean values had fallen to less than 200 DU, one-third of  “normal”. Was this just natural variability or  was it to do with human activity? In 1985, three BAS scientists (Farman, Gardiner and Shanklin) published a landmark paper reporting this decline and proposing chemical reactions which implicated human activity as a cause. The paper ignited a huge scientific effort to understand the nature and causes of the ozone hole (as it was, by then, known). Analysis of NASA’s satellite data showed that the ozone hole was an Antarctic-wide phenomena; balloon-borne ozonesondes showed the ozone depletion to be concentrated in the lower stratosphere.

Theories for its cause abounded – from changes in the stratospheric winds to changes in chemical processes (due to both natural effects and human activity). Human activity was expected to cause ozone depletion.  Stratospheric ozone is in a “dynamic equilibrium” being constantly created and destroyed by a set of chemical reactions (as well as being blown from the equator to the poles by stratospheric winds). From the 1970s onwards, it was realised that a number of additional chemicals could destroy ozone – importantly, human activity was increasing their concentrations. These chemicals included the chlorofluorcarbons (CFCs) which were widely used for both domestic and industrial purposes (in aerosol sprays, refrigerators, air conditioning …). Three pioneers of this early work (Crutzen, Molina and Rowland) received the 1995 Nobel Prize for Chemistry.

But there was a problem. This earlier ozone depletion theory did not predict the rapidity of ozone depletion (with about half of the column ozone being lost in a few weeks in the Antarctic spring), why it happened in the lower, rather than the middle and upper, stratosphere and why it occurred preferentially in the Antarctic. Airborne and ground-based measurements in the Antarctic quickly found a “smoking gun” which pointed to the role of chlorine released from the CFCs; lab measurements and theoretical work showed that hitherto unsuspected chemical reactions could rapidly deplete ozone. The key ingredients were as follows: during the winter darkness, the Antarctic lower stratosphere cools to very low temperatures (typically 190 K). Although the stratosphere is very dry, tenuous polar stratospheric clouds (PSCs) could form at such temperatures. Chemical reactions on the cloud particles released chlorine, which was normally locked up in a more inactive form, into a form that could “attack” ozone. These reactions needed sunlight, so it was only as the sun returned in the Antarctic spring that the final ingredient was in place.

The unique conditions in the Antarctic lower stratosphere were ideal for ozone depletion. Subsequently, a more modest global-ozone depletion was detected. The Arctic lower stratosphere is usually much warmer than the Antarctic in winter, so PSCs are rarer – nonetheless during occasional winter/springs, Arctic conditions are conducive to a less severe form of the ozone hole, as happened in 2011.

The political response to the overwhelming evidence of the human impact on the stratospheric ozone was swift, and has been hailed as one of the great successes of the United Nations. Under the Montreal Protocol on Substances that Deplete the Ozone Layer, and its subsequent adjustments and amendments, the emissions of CFCs have been reduced to almost zero.  Because the CFCs are relatively stable molecules (having atmospheric lifetimes of typically 50-100 years), their atmospheric concentrations respond rather slowly to changes in emissions. Nevertheless, the concentrations of the major CFCs are now  falling, in line with expectations.

Back in the Antarctic, the years following the ozone hole’s discovery saw ozone levels fall further as shown in the figure. This shows the October and December monthly-mean column ozone at Halley. The data is taken from Jonathan Shanklin’s BAS webpage using the monthly-means for 1957-2011 and the average of the daily-data for October 2012. Note that this 2012 daily data is still provisional, and subject to up to 10% adjustments following further analysis.

The figure shows that the ozone hole hit rock bottom in the early 1990s. This was partly because the amount of chlorine in the stratosphere was close to its peak. But it is also believed to be due the eruption of Mount Pinatubo, in the Philipines, which resulted in an increased number of stratospheric aerosol particles (tiny droplets of sulphuric acid) for a few years after the eruption. These enhanced the chemical reactions that released chlorine into its active form and deepened the ozone hole. The figure also shows that December values have remained depressed since the early 1980s, showing that a less severe ozone depletion persists beyond spring time

Since 2000, ozone amounts have been more variable – on average higher than the 1990s, but still only around half the values seen before the 1980s. 2012, on the provisional data shown in the figure, was one of the less deep ozone holes of recent times.

There are many ways that the ozone hole behaviour can be visualized. The Halley daily minimum each year (shown on Jonathan Shanklin’s webpage) seems to hint at a gradual recovery. Satellite data gives a more Antarctic-wide view. Animations of the 2012 hole show its dynamic nature (note the dark blue areas that develop during August), as it is influenced by the winds –  on occasions, low ozone values sweep over  populated areas of South America. Mixtures of satellite data and sophisticated atmospheric models also reveal large-scale features such as the Antarctic-wide ozone minima  and  the hole’s area (defined as the area with column ozone less than 220 Du). These plots consistently point to the 2012 ozone hole being less severe than in many recent years. During October 2012, temperatures were higher, and hence the amount of PSCs were lower, leading to less severe ozone depletion. Nevertheless, the strong ozone depletion in the lower stratosphere from late August onwards is clear in animations of ozonesonde observations made at the South Pole.

All things being equal, the expectation is that the ozone layer will continue to recover in years to come, as CFC concentrations decrease,  but their long atmospheric lifetimes means that the ozone will not likely return to pre-1980s levels before about 2050. Continued monitoring of the ozone layer is necessary, as all things are not likely to be equal. First, ozone depletion could be affected if there were further large volcanic eruptions. Second, concentrations of some ozone depleting substances are  rising. Third, climate change will lead to changes in stratospheric temperatures and winds, which could impact on ozone amounts.

Some Web Resources

Jonathan Shanklin’s BAS pages –  a wonderful collection of up-to-date and archive material on Antarctic ozone depletion, at BAS stations and beyond.

NASA’s Ozone Hole Watch pages

NOAA’s Climate Prediction Center stratosphere pages

WMO’s Antarctic Ozone Bulletins

NOAA’s Earth System Research Laboratory ozone pages

The United Nations Environment Programme (UNEP) organises scientific assessments of the state of the ozone layer, the most recent in 2010. Its “20 questions and answers about the ozone layer” has much background information on ozone depletion. The UNEP website includes much useful information on the political and technological aspects of the response to ozone depletion.

Nature – a collection of some of the key papers in the “ozone story” brought together for the 25th anniversary of the Farman, Gardiner and Shanklin paper

summer weather and food production

It can’t have escaped anyone’s notice that summer 2012 has seen some pretty unusual conditions. In the UK its officially been the second wettest summer (June, July and August) since records began with only 1912 being wetter. The only exception was the NW of Scotland with somewhat drier than average conditions. This of course followed a winter and early spring in which we were being warned of possible water shortages over the coming summer thanks to a couple of drier than normal winters.

The wet conditions this summer have had a major impact on cereal, fruit and vegetable crops. In England the National Farmer’s Union estimates that wheat production was down 15% from the 5 year average ( Wet weather around harvest time made the headlines with floods, particularly in the north-east, meaning that some crops couldn’t be harvested at all. The grain that was brought in often had to be dried artificially, substantially adding to production costs. The Scottish strawberry crop was also reported to be decimated, and cold wet weather around apple blossom time in April and May meant that there were less pollinating insects flying and so apples have experienced reduced yields this year.

However, personal experience suggests it hasn’t all been bad news. The raspberry crop in my garden was the biggest I’ve ever seen as long as I could pick them fast enough before they started to mildew. Raspberies are a British native plant growing on woodland margins and have evolved to do well in cool damp conditions. Hence they were quite happy with this summer’s weather. My sweetcorn yield was also up on previous years and potatoes seemed to do pretty well. Tomatoes growing ouside came to very little though. I’ll be starting to dig up the parsnip crop this weekend so we’ll see how well they’ve done fairly soon.

My own apple tree produced only two edible apples, but talking to the farmer at Cross Lanes Orchard ( just outside Reading suggests that it hasn’t all been bad with apples either. Whilst yields were significantly down for some varieties others have done very well. Cross Lanes is an orchard that grows over 40 different varieties of apples, as well as plums and pears, and so this highlights the importance of diversity in the crop with earlier and later blossoming varieties meaning that there’s a good chance that some varieties will do well.

Over in the USA its been a drought summer, with some big wheat producing states such as Kansas and Nebraska experiencing “exceptional” drought conditions – the most extreme drought category according to the US Drought Monitor at the University of Nebraska-Lincoln ( As some of our undergraduate students reported in Weather and Climate Discussion last week, this has had an impact on local food production but also potentially on global food prices. Whilst US wheat yields are actually up, corn yields have been down and the quality of pasture for cattle is also very poor (US Department of Agriculture). Reduced grain yields lead to increased prices and because a  lot of grain is used in winter feed for livestock production the impact goes beyond the grain price itself.

These stories all highlight our dependence on the weather for maintaining food production, an issue that has concerned mankind since the earliest civilizations. In ancient Egypt for instance the fertility of the fields along the Nile was dependent on the annual floods, which were themselves dependent on rainfall many hundreds of miles south around the river’s headwaters. These days we have more sophisticated agricultural methods and are able to develop varieties of crops which are more tolerant of particular weather conditions. However, we still have some way to go in our ability to make weather predictions on the seasonal timescale that will allow farmers to make decisions on what crop varieties to plant ahead of time.

Blood Rain for Halloween?

Last week you might have been forgiven for thinking there would be something spooky in the weather for Halloween, with several news websites were pushing headlines along the lines of:

So what is ‘blood rain’ and was there any truth in these headlines?

Every year, millions of tonnes of Saharan dust are lifted from the desert surface of the Sahara, and transported by the atmosphere. Mostly, this dust makes its way westwards, across the Atlantic. However, some of it can be transported northwards towards Europe. This is quite a common affair for our southern European neighbours such as Spain, Italy and Greece, but dust outbreaks rarely make it as far north as the UK – on average only around 2-3 times a year, when you might notice a light coating of reddish dust on your car. On its journey through the atmosphere, dust eventually either settles naturally due to gravity (dry deposition), or it encounters cloud and rain, which cause it to be ‘rained out’ (wet deposition). If dust mixes with rain, it can frequently change the colour of raindrops giving them a reddish hue, due to the high amounts of iron in Saharan soils, thus creating the phrase, ‘blood rain.’ Sometimes it can be noticed on the ground or cars after rain has fallen, leaving a reddish residue after the water has evaporated.

Last week, the articles getting excited about this phenomenon associated it with warm air over the UK originating from the Sahara. Indeed, going back to Sunday 21st October, there was a low pressure centred over eastern Spain, with associated southerly winds over the Mediterranean bringing warm air and dust from the Sahara to Europe, as shown by the surface pressure chart below.

An overpass of the CALIPSO satellite, with its onboard lidar, provides us with a vertical cross-section of the atmosphere over the French-German border on that day. The image below shows a layer of dust from the surface up to around 4km at latitudes of around 45-50 degrees north, shown by the yellow colouring here, confirming that there was dust in the air over Europe.

Accordingly, the ECMWF were forecasting a plume of dust to be transported from the Sahara over Europe for Monday 22nd October, part of which reached as far north as Kent and Suffolk in the UK:

For the part of this dust plume which made it as far as the UK, forecasts were for it to be deposited by rainfall during Monday and Tuesday (22nd and 23rd October) last week. There was a warm front over the UK on Tuesday, and Reading did receive a few millimetres of rainfall, but I didn’t see any sign of ‘blood rain’, and nothing seemed to be mentioned further by the press. It’s quite possible that the dust didn’t make it quite as far as the UK, or perhaps more likely that I wasn’t inclined to stand outside in the wet looking for redness in the rain. Did anybody else see any blood rain? The dust event over the UK certainly seemed to be a weak one, if it did indeed happen – for example, the Met Office describe a much larger event that affected the UK in June this year here.

Either way, I think the statement ‘blood rain for Halloween’ is a bit optimistic more than a week in advance of Halloween. As it is, quite a different weather phenomenon has stolen the Halloween show on the other side of the Atlantic…