It's not the heat, it's the dew point!

We’ve heard it a million times.

“It’s not the heat, it’s the humidity.”

That’s mostly true. But dew point is actually a better indicator of how comfortable we feel in any given air mass.

This week’s air mass over Minnesota feels delightful. That’s not just because temperatures are in the 60s and 70s.

Dew points this week in Minnesota are in the very comfortable 30s and 40s.

That means there’s less than half the amount of water vapor in the atmosphere than in tropical air masses when dew points hover around 70 degrees.

Dew point and humidity

A note about using humidity as a comfort guide. What we call humidity is actually relative humidity. It’s relative compared to the air temperatures at any given time. Dew point is an absolute measure of moisture in the atmosphere, thus a better indicator of comfort. Relative humidity can vary greatly throughout the day, even though the dew point stays the same.

Here’s an example. The relative humidity at Minneapolis-St. Paul International Airport at 2:53 a.m. Wednesday morning was 81 percent. The temperature was 58 degrees and the dew point was 50 degrees.

At 9:53 a.m., the relative humidity dropped to 53 percent as the temperature rose to 68 degrees. But the dew point remained at 50 degrees. That means there was still the same amount of water vapor in the air as at 2:53 a.m. when the humidity was 81 percent!

Here’s another example from the National Weather Service.

The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly affects how "comfortable" it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

General comfort levels USING DEW POINT that can be expected during the summer months:

less than or equal to 55: dry and comfortable

between 55 and 65: becoming "sticky" with muggy evenings

greater than or equal to 65: lots of moisture in the air, becoming oppressive

So when you want to gauge the comfort of a summer air mass in Minnesota it’s usually better to use the dew point. It’s a better measure of just how much moisture is in the air at any given time.