Why was the sky Orange?

By William Davies

I was sitting in my house one morning in October 2017, engrossed in what I was doing. Gradually I noticed that an eerie darkness was smothering the natural light in the room. I stopped and looked outside. The sky was a dark orange! What was going on and where could I go for answers?

Earth observation satellites! These are good for this sort of event. From my experience using remote sensing instruments to study the atmosphere I know the value of this resource. This link: https://earthdata.nasa.gov/earth-observation-data/imagery provides access to a range of data useful for seeing what is going on with our planet. To see what ‘Worldview’ could tell us about 16th October 2017 look at Figure 1.

Figure 1. Worldview satellite image from 16th October 2017

The first thing Figure 1 tells us is that ex-hurricane Ophelia was playing a part – that’s the swirl of cloud to the west of the UK. There is a saying – ‘Red sky in the morning, shepherd’s warning’. That’s because our weather systems usually come from the west and the low morning sun to the east colours them red, as explained further below. But there is more – compare the dirty air over the UK with the whiteness of the storm’s centre. This air was coming from the south west – from the dusty Sahara and from Portugal and Spain where there were reports of wildfires.

These dusty, smoky particles are referred to by scientists as ‘aerosols’ along with other particles such as salt from the sea and nitrates and sulphates from pollution. Aerosols have a direct and indirect effect on the Earth’s climate. Their indirect effect comes about by playing the part of condensation nuclei that cause clouds to form (Davies et al., 2010). An increase in condensation nuclei means an increase in cloud formation with a reduction in water droplet size. This leads to an increase in cloud reflectance of sunlight – a cooling effect. The increase in reflectance happens because the total surface area of the water is greater when spread over more droplets (Twomey, 1974). The direct effect is that sunlight is absorbed and scattered by these aerosols (Davies and North, 2015). When aerosols absorb sunlight, this increases the atmospheric temperature – a warming effect. Some of the scattered sunlight is reflected back into space and this will also have a cooling effect. Understanding ‘aerosol – cloud’ interactions and the way that aerosols absorb and scatter sunlight is crucial in our understanding of the climate.

The red sky is caused by the way that light is scattered. Blue light has a shorter wavelength than red light and is scattered more easily by the molecules in the air (this is why a clear sky appears blue). Red light has a longer wavelength than blue light and is not scattered as easily. The orange sky was caused by the contribution the aerosols were making to the way the light was being scattered.

Here at University of Reading I am working on two projects that study the effect of aerosols. The CLoud-Aerosol-Radiation Interactions and Forcing Year 2017 (CLARIFY) field campaign flew from Ascension Island in the south eastern Atlantic. This has delivered airborne, surface-based and satellite measurements which will improve representation of aerosols and clouds and reduce uncertainty in their radiative effects in climate models. The Copernicus Atmosphere Monitoring Service (CAMS) provides analyses and forecasts that address environmental concerns relating to the composition of the atmosphere. At University of Reading we are producing climate forcing estimates for CAMS but there are many other CAMS teams across Europe covering a range of service themes.

Figure 2.  A recent CAMS dust aerosol forecast

Figure 2 displays a recent forecast for dust aerosol where it can be seen that dust off the north west coast of Africa was being blown over Spain and France towards the UK. By clicking on the icon at the top right of the global map one can choose a different type of aerosol to view.

Figure 3. A recent CAMS biomass burning aerosol forecast

In Figure 3 we can see the smoky aerosols that are the focus of the CLARIFY project. These biomass burning aerosols from Africa are emitted in August and September by agricultural waste burning and forest clearing. We can also see smoke from the wild fires in Northern California.

When I looked at these aerosol forecasts on 16th October 2017 the presence of dust and biomass burning aerosol over the UK was confirmed.

This orange sky was due to a combination of Ophelia, Saharan dust and wildfires over Portugal and Spain and was an unusual event which generated a lot of interest in the media (see https://www.bbc.co.uk/news/uk-england-41635906 ). It serves to remind us of the importance of aerosol research and the effect that varying aerosol optical properties can have on sunlight and on our climate.

References

Davies, W. H., North, P. R. J., Grey, W. M. F., and Barnsley, M. J.,2010. Improvements in aerosol optical depth estimation using multiangle CHRIS/PROBA images. IEEE T. Geosci. Remote, 48, 18–24. https://doi.org/10.1109/TGRS.2009.2027024

Davies, W. H., and North, P. R. J., 2015. Synergistic angular and spectral estimation of aerosol properties using CHRIS/PROBA-1 and simulated Sentinel-3 data. Atmos. Meas. Tech., 8, 1719–1731. https://doi.org/10.5194/amt-8-1719-2015

TWOMEY, S. A. ,1974. Pollution and the Planetary Albedo. Atmospheric Environment, 8, 1251–56. https://doi.org/10.1016/0004-6981(74)90004-3

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