Weatherz School: Dew point and humidity

On a sweltering summer day, humidity becomes public enemy number one. Everyone complains about it.

I say we should cut humidity a break, because it turns out, what we’re really complaining about is the dew point. Let’s establish some definitions.

Dew point is a measure of how much moisture is in the air. It’s the temperature that the air must be cooled to in order to reach saturation.

Humidity is a measure of how saturated the air is. It gives us an idea of how close the temperature is to the dew point, but it doesn’t directly represent the amount of moisture present.

We can visualize this with a plot of dew point based on the temperature and humidity. The lower the temperature, the more humid it can be while still being rather comfortable.

At 50 °F, 90% humidity still isn’t “humid” enough for griping because the dew point is only 47 °F. Even though we’re close to saturated, the moisture content of the air is pretty low.

The hotter it gets, the lower the humidity needs to be to start a riot. At 100 °F, you can have relative humidity as low as 50%, but it’s still going to be downright oppressive. With a dew point of 78 °F, you practically need gills to breath in that air.

The next time you want to complain about the humidity, redirect that frustration to the true measure of moisture.