Clouds, radiation and sun cream

By Mark Fielding

The summer holidays are still with us, and no doubt many of us have spent time lying on a beach, drink in hand, topping up our tans. However, if your skin is as pasty-white as mine, no doubt you burn easily. While it’s a mild inconvenience to lather yourself in sun cream, it’s irrefutably worth the reward to not look (and possibly walk) like a lobster. But what other forms of protection exist to guard us from potentially harmful solar rays?

Nuclear fusion within the sun provides the energy to churn out a wide spectrum of radiation that is conventionally split into three bands based on their frequency: ultraviolet (UV), visible and near infra-red. The shortest wavelength, UV, while only accounting for a few percent of the light emitted from the Sun, contains the most energy and is the most damaging to our skin.

After an 8 minute journey from the Sun, the first line of defense incoming photons (small packets of light) will encounter is in the stratosphere, around 20–50 km above the Earth’s surface. Here a layer of ozone readily absorbs UV light through a photochemical reaction that breaks the ozone down into smaller oxygen molecules. These smaller oxygen molecules react with UV light again to form more ozone molecules, thus forming a delicately balanced cycle.

Next, the photons that escaped absorption in the ozone layer face another obstacle. As the density of air in the atmosphere increases towards the surface, further protection is offered by a process known as Rayleigh scattering. Light partially polarizes molecules of air (mainly Nitrogen) causing them to oscillate and produce their own electromagnetic energy. This light is scattered in all directions, allowing some photons to be reflected away from the Earth’s surface back into space.

There is one final major hurdle for our intrepid photons and their sun-burning destiny. Cloud droplets are also very effective scatterers of solar radiation. For example, a typical stratocumulus cloud, while only a few hundred metres thick, can easily reflect over half of the incident solar radiation. This makes clouds of vital importance to climate: just a seemingly small change in cloud properties can have a large impact in the radiation budget of the Earth. It also means that on a dull, overcast day, the amount of UV radiation reaching the surface is likely to be minimal.

In conclusion, if there are clouds in the sky you don’t need to put on suncream? Well, in some cases it turns out you should put on more …

Cumulus ‘fair weather’ clouds are a common sight anywhere across the world. They tend to pop up in the afternoon as surface heating creates the thermals that drive them. As with any other cloud type, they quickly extinguish the direct beam solar radiation, as is evidenced by the shadows they cast (Figure 1). However, as cumuli cover only a small fraction of the sky, it is possible for photons to escape out the side of the cloud, rather than only emerging from the cloud bottom or top. This horizontal escape of photons can actually enhance the solar radiation at the surface. For example, if you are sitting on a beach in direct sun, clouds that are not casting a shadow on you are in fact increasing the total amount of radiation that you are experiencing. This effect is also known to solar farms, where on a partly cloudy day, it is possible for more electricity to produced by the panels than on a completely clear day.

2015 08 17 Mark Fielding - shadow in the sky

Figure 1. Cloud shadows cast by cumulus clouds.

Inside climate models, clouds are currently treated as homogeneous layers and so these 3D effects cannot be explicitly represented. Clearly, this approximation is not particularly physical for cumulus clouds that cover vast swathes of the tropics. Unfortunately, modelling 3D radiative transfer is computationally expensive, so research is underway to investigate how to parameterize (approximate) 3D effects into climate models.

Creating a parameterization for 3D radiative transfer requires knowledge of the true shapes and sizes of clouds in nature. My research uses radar and other remote sensing instruments to create a ‘cloud library’, which can then be used to study the 3D structure of clouds and their influence on all wavelengths of light, including UV. Understanding how clouds might evolve in the future, including their structure, is of vital importance to reduce uncertainty in climate change projections.

So next time you’re lying on a beach, watching the clouds drift by, spare a thought for the perilous journey of the photons meeting a sticky end in your sun cream. Then get back to that drink. You deserve it.

Further reading

Hogan R. J. and J. K. P. Shonk, 2013. Incorporating the Effects of 3D Radiative Transfer in the Presence of Clouds into Two-Stream Multilayer Radiation Schemes. J. Atmos. Sci., 70, 708–724, doi: http://dx.doi.org/10.1175/JAS-D-12-041.1

Fielding M. D., J. Chiu, R. J. Hogan, and G Feingold, 2014. A novel ensemble method for retrieving properties of warm cloud in 3-D using ground-based scanning radar and zenith radiances. J. Geophys. Res. , 119(18), doi:10.1002/2014JD021742.

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