Here’s an easier version of yesterday’s post.

The Facts on Relative Humidity vs. Dew Point

Relative humidity doesn’t tell you anything. That percentage of humidity you hear on the news every day is worthless.

Dew point is a much better gauge of humidity.

Why Those Facts Are True

The rest of yesterday’s post explained why that was true, but here’s a shorter explanation …

• Relative humidity measures the amount of water in the air in comparison to what the air can hold. Thus, it changes with the temperature.
• Dew point tells you at what temperature the air will be fully saturated so that water will begin to condense out of the air into dew.

You don’t have to understand that last sentence. You only have to know that dew point tells you exactly how much water is in the air.

What Dew Point Tells You

Dew Point over 80^o

Rarely happens in US. Often happens near Persian Gulf and Red Sea and in southeastern Asia. Oppressive. When temperatures are in the 90’s, dew points this high produce heat indices over 130

Dew Point over 75^o

Very humid and uncomfortable

Dew Point 70 to 75^o

Noticeably humid and uncomfortable

Dew Point 60 to 69^o

Slightly humid

Dew Point below 60^o

You won’t think it’s humid

Selmer, Tennesse

Dew point in Selmer was 78^o today. That’s very, very high humidity for the US. It was worse than Houston or Dallas today.

Because the dew point was so high, the heat index was 102^o even when it was only 88^o in temperature.

Compare that with Sacramento that had a temperature of 94^o, but the dew point was only 45^o. The result was a heat index of 90^o—less than the actual temperature.