Not yet Christmas, but has spring sprung?
By Roger Brugge
This may seem a strange question, but it was prompted by the sight of flowering daffodils alongside the A4 through Maidenhead on 12 December 2015 (Figure 1).
Figure 1. Flowering daffodils along the A4 in Maidenhead, on 12 December 2015
In order to bloom, daffodils and other spring-flowering bulbs must be exposed to a drop in temperature after the previous flowering period – often a temperature below 8 °C is quoted for this threshold. However, once spring arrives it has been shown that a cool/cold winter will lead to a delayed onset in flowering (Seasonal weather, by Lionel P. Smith, published in 1968 by George Allen and Unwin Ltd). Warmer weather in spring has, according to plant scientists at Kew Gardens and Wakehurst (Sussex), led to an earlier onset of the flowering dates of daffodils in recent years: in the 1980s daffodils commonly flowered around 12 February, but by 2008 this date had shifted to 27 January, 16 days earlier. In the 1960s the flowers appeared some 3-4 weeks later than they do nowadays, so an appearance of the flowers in mid-December is remarkable.
Has the warmth of recent weeks been to blame for the flowering observed in Maidenhead? Weather conditions in autumn and winter so far (17 December) have been unusually mild. In Reading, at the University’s climatological station, just two air frosts have been recorded since the beginning of September (see Figure 2) while the mean air temperature has been about 5 degC above average for December so far.
Figure 2. Red: mean air temperature for 1-17 December during each year from 1918 to 2015. Black: The number of air frosts recorded during autumn and winter up to 17 December for the same years. Dashed lines show the 30 year averages for 1981-2010. Note that the air frost axis on the right has been reversed so that a low incidence of air frost and higher temperatures tend to coincide. Observations made at the University of Reading.
Following a mild November (the third mildest in the period 1908-2015) the first 17 days of December have been the mildest on record – see Figure 2 – by a truly remarkable 1.1 degC. The average temperature during this period of 10.6 °C in 2015 is similar to the mean temperature normally to be expected around the beginning of May! During the afternoon of the 17th, the air temperature rose to 15.5 °C at the University, the second highest ever recorded here in December (after 15.8 °C in 1985).
As mentioned earlier, winter bulbs usually need a period of cold weather in order for the shoots of the bulb to sprout and produce flowers; such a spell has not occurred yet – since late September average temperatures have remained fairly constant, give or take a degree or so (Figure 3). The only exception to this was a cold spell around 20-25 November, when the two air frosts and the only snow shower of the season (so far) occurred in Reading.
Figure 3. Five day running mean temperatures °C for 2015 (blue) and the 1981-2010 average (red) at the University of Reading.
Of course, soil temperatures may also be playing a role in encouraging the bulbs to produce growth and flowers. Table 1 shows the anomalous warmth of the soil during December so far.
|Measurement (depth)||10 cm||30 cm||100 cm|
|1981-2010 climatological averages|
|1st-10th||4.9 °C||7.1 °C||9.1 °C|
|1st-10th||9.2 °C||10.3 °C||11.1 °C|
|1st-10th||+4.3 degC||+3.2 degC||+2.0 degC|
Table 1. Soil temperatures during 1-17 December 2015 in Reading.
The soil temperature averages for the month of December 2015 so far are currently averaging close to 4.5 degC above average at daffodil bulb depth; in fact, the soil temperatures so far this month are currently those normally seen in mid-October/mid-April (at 10 cm depth), late-October/late April (30 cm) and mid-November/mid-May (100 cm). Peak temperatures at depths of 30 and 50 cm this month have also exceeded anything previously measured at this time of year since observations at these depths began in 1910 and 1971, respectively.
This unusually persistent warmth has been the result of a higher-than-normal frequency of winds from the south-west quadrant (a mild direction in autumn/winter) across southern England – see Figure 4. With Christmas fast approaching, there is no sign of any change to this pattern of winds and temperatures. Indeed, the chances of a White Christmas in Reading seem very slim. Possibly a rerun of the Christmases of 1911 (when temperatures rose to 15.0 °C on Boxing Day) or 1935 (when the temperatures reached at least 10 °C every day from Christmas Day until New Year’s Eve) might be more likely this year? Even another air frost looks unlikely in Reading before the start of 2016.
Figure 4. Wind rose of winds at the University of Reading based on observations every 5 minutes during 1 November – 16 December 2015. Mean wind speeds: 0-2 m/s (red), 2-4 m/s (magenta), 4-6 m/s (cyan) and 6 m/s and above (blue). Radial axis shows the percentage occurrence during the period in 30 degree-wide bands.
It will be interesting to see how the Maidenhead daffodil clump copes with any cold spells this winter. Most flowering daffodils can handle a small amount of frost – but if the winter does produce a long, cold snap …
Further reading about Reading’s weather:
R Brugge and S Burt, 2015. One hundred years of Reading weather