There are certain days, more noticeable in summer than at other times,
when the air is heavily laden with water vapor; and there is little or
no cooling of the body due to evaporation. We perspire freely but as the
sweat does not evaporate, there is a constantly increasing amount of
water on the skin.
[Illustration: FIG. 15. BLUE HILL OBSERVATORY DURING ICE STORM, NOVEMBER
29-30, 1922]
It is not altogether a question of temperature, for another day may have
as high or even higher temperature. It is essentially a matter of
ventilation. On muggy days we are somewhat in the condition of the
unfortunate prisoners in the Black Hole at Calcutta. They did not die by
poisoning, as has generally been accepted, that is, lack of sufficient
oxygen and an excess of carbon dioxide; but because they were unable to
keep the skin sufficiently cool. There was no ventilation; no movement
of the air and the body became over-heated and exhaustion followed. No
matter how much water there may be on the skin if the surrounding space
is saturated, one feels oppressed. A vigorous fanning of the air helps
evaporation and cools us. That is why a brisk northwest wind routs a
muggy condition.