Enhancement of UV by cloud

and UV reductions too ...

A thought for the summer holidays when UV is highest and our exposure to it is worst …

The news article I mentioned here included the dramatic photograph below. It’s a great example of a cloud situation that leads to the UVI being greater than for clear skies.

The image below, from an all-sky camera at Lauder, New Zealand, is another good example. The colour balance is quite different at this pristine site, and the field of view is much larger; but the message is the same.

In both cases, the clouds appear brighter than the surrounding areas, showing that cloud-covered areas divert more light to the camera than similar areas of unobstructed sky. The same applies in the UV region, though at those shorter wavelengths, there’s a lot more scattered skylight, so the contrast between sky and cloud is smaller. While around 90 percent of visible radiation is in the direct beam of sunlight, in the UV region responsible for skin damage, that fraction is usually less than 50 percent, with the majority of the radiation coming from scattered skylight.

Despite that, situations like those pictured above can result in overall UVI enhancements of 20 percent or more compared with clear-sky values. Don’t assume you’re safe from the sun just because there are clouds about. At times it can be more dangerous than for clear skies. In fact, a good rule of thumb is whenever there are clouds present that do not block direct sunlight, the UVI will be GREATER than for a cloudless sky. At least in New Zealand’s pristine air. The only exception might be those threatenly-dark thunderclouds that herald imminent rain.

Overcast conditions are different. If clouds obscure the sun, the UV will typically be around half the corresponding clear-sky value. But the range of cloud-effects is broad.

These points are illustrated below, using results from more than 23 thousand high-sun measurements of UVI taken over a 25 year period at Lauder New Zealand (latitude 45 S). The plot shows the relative frequency of occurrence of all UVI values obtained at sun elevation angles greater than 60 degrees above the horizon (i.e., high sun conditions, with the sun less than 30 degrees from the zenith).

The solid red line is the distribution of those measured UVI values, while the dashed blue line is the distribution of corresponding calculated clear-sky values.

For clear skies (blue), the values are clustered around a peak of UVI = 10, while for the measurements that include cloud-effects (red) there’s a secondary much-broader peak around UVI = 5. As you can see - by the displacement of the red curve to the right of the blue for high UVI - the presence of clouds can also lead to an increase over clear-sky values. In the extreme, while the maximum for clear-skies is around UVI = 13.5, the measured maximum was UVI = 14.5. At the other end of the scale, the minima are occasionally less than UVI = 0.5, even for these measurements near noon in summer.

The distribution is of course site-dependent. At sunnier sites, the measured distribution (red) is closer to the calculated one (blue) - and vice versa. More on that next week.

Nice, eh? And you’re the only ones I’ve shown it to so far … .

It’s difficult to calculate these cloud effects, so the predicted UVI is usually just for clear skies. So, if there are puffy white clouds about that don’t obstruct the sun, the UVI can exceed the predicted value by 20 percent or more. If your smartphone app gives a peak clear-sky forecast of UVI = 10, the real-world peak value could occasionally exceed UVI = 12.

Fortunately, over the course of the day, the highs tend to be balanced by lows when those cloud do obscure the sun. But you can’t predict exactly when or by how much … Care is (clearly?) needed. Take it 😊.