Why Don't You Show the Relative Humidity?

Written by: Brian Neudorff

That is a question I have been asked a lot in my career. Back in the day it was common practice for meteorologist and weathercasters on television to give the current relative humidity showing how close the atmosphere was to saturation. Unfortunately it doesn't give you any indication of just how comfortable or uncomfortable the air really is.

You see relative humidity is just that... relative. It is relative to the temperature and moisture content of the air. Here is an example of what I mean.

On a hot, sticky, rather uncomfortable summer day you could have an outside temperature of 92 with a dew point temperature of 68 (which is listed in the uncomfortable zone in the chart above) this would give you a relative humidity of about 46%

On a pleasant, warm yet comfortable spring day you could have an outside temperature of 72 with a dew point temperature of 55 (which is listed as pleasant in the chart above) you would have a relative humidity of about 55%

Although the summer day feels more humid than the spring day its relative humidity is actually lower.

The dew point temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. At 100% relative humidity, the dewpoint temperature and the air temperature are the same, and clouds or fog can begin to form. While relative humidity is a relative measure of how humid it is, the dewpoint temperature is an absolute measure of how much water vapor is in the air (how humid it is). In very warm, humid conditions, the dewpoint temperature can reach 75 to 77 degrees F, but rarely exceeds 80 degrees.