Much Ado About Dew Point

Norm Dvoskin

In June this year, we had an extreme heat wave on Long Island with four straight days of temperatures close to 100 degrees and very high humidity. It got so bad, News 12 Long Island reporters were running dangerously low on people to ask, "How do you beat the heat?"

When you feel miserable during the Summer months, people will often remind you," It’s not the heat but the humidity." I hate these people. However, in reality, it’s the heat AND the humidity that make you miserable.

To solve the problem of describing the effects of heat and humidity on human comfort, meteorologists have resorted to various indices to quantify human comfort into a single number.

The most traditional way of reporting moisture in the air is to state the relative humidity. The problem with relative humidity is that it doesn’t tell us how we feel. It’s a measure of how much water vapor is in the air as a percentage of the maximum amount the air can hold at a given temperature.

There’s the temperature-humidity index (THI), which was presented to the public in 1976. It was abandoned because people found it confusing.

The heat index, developed in 1979 and released in 1984, is often used today. It gives you the "apparent temperature." For example, "It’s 92 degrees but if you factor in the humidity, it feels like 99." This is less than satisfactory as it doesn’t describe the effects of heat and humidity on how you feel. For example, when it’s very hot and dry (high apparent temperature) many people feel comfortable. The major benefit of using the apparent temperature is, with an exceptionally high value, it gets people’s attention.

Most meteorologists will tell you, "In Summer, the best measure for describing the moisture in the air and it’s effects on comfort is the dew point. The dew point is the temperature to which the air must be cooled at constant pressure, for water vapor to condense into liquid water. It can be converted directly into how much moisture is in the air. Of course, the only people who may want to know this are engineers and meteorologists. I talk about dew point on TV only when it is extreme, that is, when it’s under 10 in winter and over 70 in Summer. The scale below, describes in general, how the air feels as a function of dew point.

DEW POINT	PERCEPTION
52 & lower	COMFORTABLE
53-56	SLIGHTLY NOTICEABLE
57-59	QUITE NOTICEABLE
60-63	STICKY
64-69	UNCOMFORTABLE
70 +	INTOLERABLE

The highest summer dew points in the USA,(80 and above), occur mostly near the Gulf of Mexico and in portions of the upper Mississippi Valley.  Whenever dew points rise above 75 degrees, I usually issue a "severe hug warning."  This means that if you hug someone, there's a 60% chance you'll stick together permanently.