IT鈥橲 not the heat, it鈥檚 the humidity. Across great stretches of the US, the
old clich茅 is putting in its annual appearance as sweltering weather sets
in.
Having languished through 15 uncomfortable summers around Washington DC and
in the upper Midwest, I can testify that like all the best clich茅s, this
one鈥檚 true. And for a sound reason. When the air鈥檚 moist, evaporative
cooling鈥攕weating to you and me鈥攄oesn鈥檛 work as well. You feel
hotter.
But American weather forecasters don鈥檛 like qualitative statements. If they
can find a way of putting a number on something, they will. And so, in recent
years, we began to hear the word 鈥渉umiture鈥�. Warning of a nasty day to come,
forecasters will declare that a temperature of 95 掳F (this is the US,
remember) will be accompanied by stultifying humidity, giving a humiture of 103
掳F. Meaning, they claim, that it 鈥渇eels as if鈥� the air had an actual
temperature of 103 掳F but was bone dry.
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This sounds very precise and sophisticated, and there鈥檚 even some physics in
it. But does it really mean anything? Humidity, and again I speak from personal
experience, brings unpleasantness all its own. The air is heavy, breathing is
laboured, the shoulders sag. It鈥檚 as much as a man can do to twist the cap off a
lite beer.
Humiture is, of course, the summer counterpart to the wind-chill factor. On a
winter鈥檚 day when it鈥檚 20 掳F and a sharp wind鈥檚 blowing, it will, we are
informed, 鈥渇eel as if鈥� it鈥檚 5 掳F. Same idea: wind sucks away heat from
exposed skin, making you feel colder.
Earlier this year, however, it was revealed that a major correction to the
wind-chill calculation was needed. The 1940s experiments on which the scale is
based, involving plastic containers of water hung out in howling Antarctic
gales, were found not to relate all that closely to human skin exposed to more
moderate conditions. Wind chills, the staple of American weather forecasts for
decades, might be off by 10 掳F or even 15 掳F.
My first thought on hearing this alarming news was: they鈥檝e been endangering
our health for years! My second thought was, if the wind chill was off by 15
掳F and nobody noticed, what can it matter?
The problem, as I said, is that American weather forecasters love to quantify
the unquantifiable. They don鈥檛 say it鈥檚 likely to rain tomorrow, they say
there鈥檚 a 70 per cent chance of rain. I was never able to fathom whether this
probability was an expression of the confidence the forecasters have in their
computer programs, the accuracy of the data they鈥檙e feeding in, or the vagaries
of the weather. Or maybe all of the above.
And does anyone ever decide they鈥檒l hold their barbecue if the chance of rain
is 50 per cent, but call it off if it rises to 70 per cent? Of course not. You
cross your fingers and pray to the weather gods, as sensible people have done
for millennia.
Having now returned to Britain, I am delighted to see television weather
people standing in front of quaint maps, with nursery-school pictures of sun and
clouds and raindrops. They say tomorrow will have scattered showers and sunny
intervals. Now that鈥檚 a prediction I can make sense of.