Sweating outside on a humid day.

Everyday learning. Trying to spread the wealth of knowledge one blog at a time.

Let’s take a couple minutes to explore the difference between relative humidity and dew point. Relative humidity, despite its name, is not a good measure of how humid it is outside. Rather, dew point is a much more accurate representation of how humid most people would describe it feels outside. The big difference between the two is how much moisture the air can hold.

When the temperature is 55 degrees, the relative amount of moisture the air can hold (relative humidity), is much smaller than the amount of moisture or water vapor the air mass at 95 degrees can hold. The relative humidity of 55 degrees of 50 percent, is a much smaller amount of water vapor or moisture in the air, than a 50 percent relative humidity at 95 degrees. Even though the RH is the same it’s considerably more moisture when the temperature is 95 degrees.

When you look at the measure of how much moisture we’re talking about, dew point is the value we often refer to. So a 37 degree dew point is what you get with a temperature of 55 and a relative humidity of 50%. While a dew point gets up to 74 by the time you have a temperature of 95 with a 50% relative humidity.

Water vapor reaching its fail point on the window

This is roughly a subjective scale on how most people would describe a given dew point to feel outside. Generally speaking, a dew point below 60 feels pretty comfortable and dry to most people. When the dew point starts climbing through the 60’s, the humidity becomes increasingly noticeable. As the dew point climbs through the 70’s, the humidity feels downright unbearable outside.

So the next time somebody asks you how humid it is outside remember, relative humidity is a relatively poor measure of how humid it is. Dew point is a much more accurate representation of what the humidity feels like when you step outdoors.