Relative humidity can be a difficult concept to understand. It is expressed as a percentage...but a percentage of what? In the simplest terms I can manage relative humidity is a measure of how full the atmosphere is of water vapor at a given temperature. The temperature is the key because warm air will hold more water vapor than cold air. Say we have a parcel of air that is 80 degrees with a relative humidity of 60%. If we raise the temperature of that parcel by a couple of degrees we can actually lower the relative humidity by about 5%. We still have the exact same amount of water vapor in the parcel but we have raised the water vapor capacity by simply raising the temperature. So now the parcel is not as full. Get it? (remember that warmer air has a higher capacity for water vapor). Another way of looking at it is that we had a glass of water that was 60% full. Now we've increased the size of the glass (by incresing the temperature) so it's not quite as full.

But what does all this have to do with the dewpoint? The dew point is the temperature at which a given parcel of air would reach 100% humidity. Huh? Say we have a parcel of air that is 70 degrees with a dewpoint of 55 degrees. The relative humidity would be 59%. If we lowered the temperature of our parcel to 55 degrees (the dewpoint temperature) the relative humidity would reach 100%. After that something's got to give. If we continue to try to lower the temperature water vapor will condense (condensation) out of the air as visible droplets of water. That's how clouds, fog and dew (as in dewpoint) are produced.

During the warm season in our area dewpoints in the 50s with tempreatures in the 80s and 90s are tolerable. When the dewpoints rise above the mid 60s with very warm temperatures that's when it gets oppressive. When the temperature/dewpoint spread is close, our bodies perspiration does not evaporate as easily so we have trouble maintaining a comfortable body temperature. That's why we calculate the Heat Index which is actually a measure of our bodies ability to cool itself via perspiration. Meteorologists use the evening dewpoint to make a first guess as to how cool it might get overnight because the temperature can not fall below the dewpoint.

So now you know...

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