Hot weather is bad enough, but when tropical moisture is added to the mix,it becomes unbearable.Relative humidity is a poor measure of discomfort. On a hot, humid day, howoften have you heard the weathercaster say that the humidity is 50 percent.
That's actually a very uncomfortable reading on a hot day, but you can't tellthat from the number. The reason that relative humidity is misleading follows.
Understand that the relative humidity is the product of a simple fraction. Thenumerator is how much water vapor is in the air; the denominator is how muchmoisture the air can hold. The second key point is that that hot air has agreater capacity for water vapor than cool air. It follows that as thetemperature goes up, the relative humidity must go down (if no additionalmoisture is added).
It follows that the best tool for determining discomfort in hot weather is thedew point. The dew point is the temperature at which condensation occurs andtherefore is an accurate measure of how much water vapor is in the air.
A dew point above 70 degrees makes it feel pretty miserable on a day when thetemperature is in the 90s. On the other hand, when the dew point is below 60,it feels much better.
Story by AccuWeather.com Senior Meteorologist John Kocet.
