Sunshine on a Cloudy Day
At one time or another, most of us have proved empirically, and
painfully, the old mother's tale that it's possible to get sunburned
on a cloudy day. On average, clouds do reduce the amount of
ultraviolet A and B radiation that reaches the Earth's surface and
our skin, but it far from stops the damaging rays. Indeed, clouds
are generally better at blocking visible light than UV.
Unfortunately, the average can, in some cases, be a pretty bad deal.
Investigators have known since 1964 that clouds can have paradoxical
effects on incident UV radiation. In more than a dozen studies since
then, every data set includes at least some examples of what is
known as cloud enhancement of UV. For people hoping to
avoid skin photoaging and cancer, this can be a confounding
characteristic. How much increase in radiation? It depends how you
look at it.
Although the mechanisms aren't yet entirely clear, the degree of
enhancement can be significant. Forrest Mims III with the Sun
Photometer Atmospheric Network and John Frederick with the
University of Chicago reported in a 1994 Nature article
measurements of UVB at the Mauna Loa Observatory as much as 29.8
percent above modeled clear-sky levels. In various other studies,
the range has been reported as a few percent up to 50 percent. Some
of the most prolific authors on the subject are Jeff Sabburg, Alfio
Parisi and Michael Kimlin at the University of Southern Queensland.
According to Sabburg, "In our latest research [soon to appear
in the Journal of Atmospheric Research], we use new
equipment and refine our methodology, and the highest UVI [an index
of skin reddening] enhancement we found was 25 percent."
But those values are with respect to expected clear-sky UV.
Compared with the level of attenuation usually seen when clouds are
present, such measurements can actually be 50 to 75 percent higher
than predicted, says Sabburg. And therein lies a conundrum for those
who work or recreate outdoors and depend on UV forecasts. No
national forecast based on the World Health Organization's numerical
scale for UV takes enhancement into account. Indeed, although
several mention the possibility on their Web sites, the calculations
instead assume that clouds reduce UV exposure. The U.S. National
Weather Service and Environmental Protection Agency, for example,
figure 89 percent transmission for scattered clouds, 73 percent
transmission for broken clouds and 32 percent transmission for
So how do we get more rather than less? Several studies suggest that
reflection off the sides of cumulus clouds is one mechanism by which
UV radiation can become focused. Sabburg and Joe Wong (also with the
University of Southern Queensland) have also postulated that
refraction and scattering of direct and diffuse radiation could
result in markedly increased enhancement. Thus cloud conditions that
include cirrus clouds thin enough not to completely obscure
the solar disk, along with lower-altitude cumulus clouds, may lead
to the perfect UV storm.
Just how common is cloud enhancement? The various studies have found
that between 1.4 and 8 percent of all measurements show cloud
enhancement compared with clear-sky values, depending on geographic
location, but as many as 25 percent of those made on partly cloudy
days may show it. Most often the enhancement lasts for 10 minutes or
less—not a concern for the sun worshiper—but it has been
known to persist for an hour.
The problem, of course, is predicting something that is influenced
by systems as dynamic as clouds. A model was developed in 1974 that
accounts reasonably well for observed results. The trouble is, just
for starters one must know the fraction of the sky covered and the
cloud optical depth—not characteristics that are predictable
well in advance. As Sabburg says, "Our research is more aimed
at measured, rather than forecast, UVI, but I think it would be
almost impossible to predict these cases."
He does think, however, that authorities should do more to educate
the public on the subject. Asked whether he wears sunblock on cloudy
days, he replied "Yes! Unless it is overcast and the sun is not
producing shadows—that is, it is not visible in some
form—and there is no chance of the sun shining through broken
patches of the cloud cover, I am still very conscious of the effects
Bottom line: On a perfectly clear day, that UV forecast is likely to
be quite accurate. Add condensed water vapor to the atmospheric
mixture, though, and the outlook can become, well, cloudy.