Dew point vs. relative humidity – which is more useful to determine comfort level?

Undoubtedly, the answer is the dew point temperature, but it’s not perfect in every instance. Let’s look at relative humidity first.

Both relative humidity and dew point measure how much water vapor is in the atmosphere. Relative humidity, however, is temperature-dependent while Dew point is not. Dew point is simply a measure of how much moisture is in the air at a given time.

Take the example where the dew point is 65 degrees and the air temp is 65 degrees. Relative humidity in this situation is 100%.

Hold that dew point at 65 with the air temp rising to 95. Relative humidity drops to about 37%. In this scenario, it would feel quite humid or muggy and even uncomfortable despite the relative humidity being fairly low. In fact, the heat index temperature – what it actually feels like outside – would be 98 degrees!

We can see from these examples that the closer the temp is to the dew point, the higher the relative humidity. The farther apart they are the lower the relative humidity.

Let’s now look at dew point temperature:

The general rule of thumb is that dew points in the 50s or lower is comfortable during the warm months. 60 to 65 and it feels sticky or humid. Dews above 65 are downright muggy and even tropical when they reach the 70s.

Now, a dew point in the 50s at night with temps in the 50s will feel damp with high relative humidity in this scenario. You tend to get at least patchy fog and dewy grass galore.

We’ve also had days where the dew point is around 70 along with a similar air temp and it’s been raining. In this scenario relative humidity is high and it would tend to feel damp and clammy, but not so much muggy or tropical.

Both measurements are useful, but as a general guide on and warm or hot day it’s the dew point temp that will be a more reliable guide for determining comfort level.