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Written by: Brian Neudorff
Ever wonder why television meteorologist and weather broadcasters rarely show the current humidity numbers? During the summer, especially when it is as hot and sticky like it has been we will get a few emails maybe even a phone call or two asking why aren't you showing the humidity? Even though we use the term "Humid" to describe the air outside, humidity is actually a bad indication of just how comfortable or uncomfortable the air really is.
Relative humidity is just that... relative. It is relative to the temperature and moisture content of the air. Here is an example of what I mean.
On a hot, sticky, rather uncomfortable summer day you could have an outside temperature of 92 with a dew point temperature of 68 (which is listed in the uncomfortable zone in the chart above) this would give you a relative humidity of about 46%
On a pleasant, warm yet comfortable spring day you could have an outside temperature of 72 with a dew point temperature of 55 (which is listed as pleasant in the above chart) you would have a relative humidity of about 55% Although the summer day feels more humid than the spring day its relative humidity is actually lower.
The dew point temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. At 100% relative humidity, the dew point temperature and the air temperature are the same, and clouds or fog can begin to form. While relative humidity is a relative measure of how humid it is, the dew point temperature is an absolute measure of how much water vapor is in the air (how humid it is). In very warm, humid conditions, the dew point temperature can reach 75 to 77 degrees F, but rarely exceeds 80 degrees.
That means if you hear someone say it is 90 degrees and 90 or 100% humidity they are absolutely wrong. For that to happen you would need a dew point temperature of 86 to 90 degrees. Around here and most of the United States that is nearly impossible.
Dhahran, Saudi Arabia, on the Persian Gulf, recorded a dew point of 95°F on July 8, 2003. In the USA, the highest dew points (above 80°F) occur near the Gulf of Mexico and in parts of the upper Mississippi Valley.
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Turner, Patricia. I Heard It Through the Grapevine .
Berkeley, CA: Univ. of California, 1993. ISBN 0-520-08185-4 (pp. 103-104).