Sometimes I hear people say "99 degrees, 99 per cent relative humidity"
or words to that effect. Wrong! Let me explain.
Wikipedia defines relative humidity (RH) as "the ratio of the partial pressure
of water vapor to the equilibrium vapor pressure of water at a given
temperature." The definition is correct but hardly understandable.
When RH is 100%, there is balance between evaporation
and condensation. The two occur at the same rate, and therefore
water on your skin stays on your skin.
When RH is a little less than 100%, evaporation will slowly
outrun condensation. On a hot day in the South, when you're doing yard work,
the production of sweat exceeds evaporation minus condensation;
therefore, your sweat tends to drip or to dampen your clothing.
When RH is 35% or less, evaporation quickly
outruns condensation. When you step
out of a swimming pool into air with low RH, your skin dries almost
instantly and you will feel
chill regardless of the prevailing temperature. I've experienced that
in Arizona in summer.
Vapor pressures are difficult to measure, so instead we use a chart that
indirectly calculates RH using dewpoint and temperature.
Assuming that RH is not 100% already, dewpoint is the lower temperature at
which evaporation and condensation balance. This is easily measured.
Dewpoints in the southern U.S. during summer are generally in the 70s. I asked a local
meteorologist what was the highest dewpoint he had ever seen here, and
he said that outside of temporary events like the aftermath of a
thunderstorm, the maximum dewpoint is about 82. There may
be places in the world where dewpoints routinely are higher; an example
is the Arabian peninsula adjacent to a very warm Red Sea or Persian
Gulf. But reports in the U.S. of dewpoints in the high 80s or low 90s
are very rare and always transient. Why? Because inevitably a
thunderstorm will form in such moist air, causing both a drastic drop in
temperature and a reduction in water in the air through rainfall.
In other words, the atmosphere self-corrects
excesses in dewpoint.
In the absence of precipitation, the dewpoint is usually close to the
temperature at dawn. Why? Because as the temperature drops
slowly overnight, condensation increases. The process of condensation adds
heat to the air. In effect, water vapor cushions the fall of overnight
temperatures. For example, in Raleigh this morning the temperature
bottomed at 78 with a dewpoint of 77… meaning that RH at sunrise was
almost 100%.
As the day goes on, however, the temperature rises but the dewpoint doesn't change
much, absent a storm or significant inflow of different air such as a cold front or
a sea breeze (which occasionally does get this far inland). This means
that RH falls during the day. Suppose the dewpoint at 3 pm is still 77
and the temperature is 97. The RH will be 53%. Uncomfortable and potentially
dangerous, definitely. But nowhere close to 99/99.
For the temperature and RH both to be 99, the dewpoint would have to
be 98. That won't happen! Even a temperature of 90 and an RH of 90
requires a dewpoint of 87, and the likelihood of an 87 dewpoint in
Raleigh would be one in a million — and short-lived.
So, while we complain about two weeks straight of moist heat and
explain to our European friends (who rarely experience that kind of
prolonged moist heat) why we like air conditioning so much and are
so willing to accept the CO2 production that most air conditioning
requires, let's not be unscientific by quoting exaggerations like
99/99.