From May to September people have to be careful about
how long they work or play in the sun. The sun's ultraviolet
(UV) rays during these months can cause painful sunburn on
unprotected skin, and this exposure may lead to premature aging
of the skin and even skin cancer later in life.
How can we have fun outdoors in the summer sun without
suffering bad consequences? Every summer doctors advise us to
check our clocks before going out into the sun. This summer try
checking your shadow instead. Shadows are more accurate gauges
of the strength of the sun's dangerous UV than clock time.
As the sun descends slowly in the west on a sunny summer
afternoon, its rays must pass through more and more atmosphere,
which in turn filters more harmful UV out of sunlight. During
the afternoon, shadows also gradually increase in length.
Therefore, weakening solar UV comes with lengthening shadows.
Conversely, short shadows indicate a strong sun.
In summer we are often told to avoid sun exposure in
midday within about two hours of noon. But in summer, the sun
is not highest in the sky and thus not strongest at noon
by clocks since during this season we observe daylight saving
time. On daylight time the highest sun occurs much closer to
1 p.m. than noon.
Furthermore, the sun remains high in the sky longer in
June than in September. In addition to the seasons, your latitude
and longitude also have much to do with when you should be careful
in the sun. You practically have to be a geographer or an
astronomer to sort all of this out.
Fortunately, there is a simple rule of thumb for sunburn
protection: "Shun the sun when your shadow is short." How short?
Shorter than your height. Accordingly, when your shadow is
shorter than you are tall, seek protection with proper clothing,
sunscreens, or other means.
This simple shadow rule works by estimating the height
of the sun directly by a crude but accurate enough method. Standing
in the sun and looking at your shadow, you find out all you
need to know about the sun's strength without even looking at a
watch.
Measuring your shadow does not require a ruler, but make
sure the shadows you use are on a level surface. Then imagine
yourself lying beside your shadow on the ground and make a mental
comparison. When your shadow appears as long as your height, its
length is changing so rapidly that you are unlikely to misjudge
(by more minutes than matter) the time the two are equal.
Of course, other factors besides the sun's height
influence the strength of sunburning UV in solar rays: changes
in the amount of ozone in the upper atmosphere, the abundance
and type of clouds and air pollution. These things are generally
not as important as the height of the sun in the sky.
Another thing to consider is the type of skin you have.
People with red or blond hair, blue eyes and/or freckles are more
sensitive to UV and should be more careful in the sun than
persons with darker hair, eyes and skin who have better natural
protection. More vulnerable people will have to take protective
action when shadows are somewhat longer than the rule says. All
persons must be more careful when the daily UV Index is high.
The National Cancer Institute and the American Academy
of Dermatology have endorsed the shadow rule in their publications.
In a scientific journal article I demonstrated this rule's
validity by an analysis of actual UV observations made in the
Washington, D.C. area by the Smithsonian Institution's SERC Solar
Radiation Laboratory.
My study shows that on a sunny, midsummer day in Washington
about 75 percent of that day's total sunburning UV reaches
the ground during the midday period when a person's shadow is
shorter than his height. My results are based on local data, but
I think the shadow rule is dependable in most of the United
States.
It is wise to use the shadow rule because too much exposure
to ultraviolet damages your skin. Following the shadow
rule, you still have four hours of daylight in the early morning
and four more in the late afternoon to enjoy a sunny day outside.
Your shadow follows you around all day as a reminder of
when to look for protection from the sun.
So in summary: "SHORT SHADOW? SEEK SHADE!"