Relative Humidity vs Dewpoint â" Relative Points

Dewpoints indicate water vapor content. Relative humidity is dependent on temperature. / Mike Morrison

Many of my viewers are accustomed to the term relative humidity rather than dewpoint when describing the water content of the air but using dewpoint to describe how much water the air holds is actually more accurate.

Dew point is an indicator of how much water is in the air and relative humidity is a measurement of how close the air is to saturation. The higher the dewpoint temperature the more moisture in the air at that time, however relative humidity is dependent on the temperature of the air and not directly with the moisture in it. This is why dewpoint favored over relative humidity in some cases.

For example let's say at noon in Albany Georgia the temperature is 93, the dewpoint is 68 and the relative humidity is 44%. Then at 8pm that same day the temperature is 72, the dewpoint is 64 and the relative humidity is 76%. Over that 8 hour period the relative humidity has greatly increased but the air is actually less humid at 8pm because the dewpoint has dropped a couple of degrees.

Dew point temperature is defined as: "the temperature at which dew begins to form." Dew is the water you find on your grass or car early in the morning. If the temperature reaches the dew point temperature then dew can form.
If the air temperature equals the dewpoint temperature then the relative humidity will be at 100%. If the dewpoint temperature is much lower than the temperature (it can never be higher) then the relative humidity will be much less than 100%.

RH just tells how near the air is to becoming completely saturated. It doesn't tell you how much moisture is in the air, nor does it tell you if there are large amounts of moisture.

Both dewpoint and relative humidity are used to describe one's expected comfort level especially in the heat of the summer.

Here in southwest Georgia it's not just the heat, it's the humidity. This is very true during the summer months as a statement of how moisture content of the air can impact how hot it actually feels.

During times of hot weather increased humidity in the air will make it feel much warmer than if the humidity is lower at the same temperature. But why?

It all has to do with how the human body cools itself. The human body cools itself through evaporation of perspiration. Evaporation of perspiration (and water) into the air is dependent on how close the air is to saturation. If the relative humidity is high, perspiration cannot evaporate quickly enough and the environment feels warmer than the actual temperature. That's where the heat index becomes important.

Heat Index:
The combination of high heat and high humidity can take its toll on the human body. The human body cools itself by sweating but more specifically the sweat evaporation off the surface of the skin. When the air has a large amount of moisture in it the evaporation of sweat will be reduced greatly and the cooling from evaporating sweat will be minimal. The heat index is a calculation that includes the humidity and temperature of the air to give a value related to how efficiently the human body can cool itself. Many times during summer the temperatures and humidity are high and the apparent temperature (how it feels to a person) is greatly increased. For example if the air temperature 94 and the relative humidity is 70% the air would feel like 119 because of the limited cooling provided by perspiration.

Here in southwest Georgia the temperatures rarely climb above 105 but the heat index values can climb above 120 making it feel as hot as Phoenix Arizona with an actual temperature of 120.

Now a high relative humidity number on a hot day indicates plenty of moisture in the air and is uncomfortable. The air with a high relative humidity number on a cool or cold day say in winter would contain much less moisture and would not feel that bad.
Relative humidity of 90% on a 95 day would be oppressive.
Relative humidity of 90% on a 55 day would be fairly comfortable.
Between 40% and 80% Relative humidity is comfortable if the temperature is also comfortable.

When the dewpoints are above 65 it will feel humid outside. Now if temperatures climb higher than 90 or 100 the relative humidity would be low but the dewpoint would still be at 65 and the air would contain a high quantity of moisture.

The worst combination for human comfort is a high dewpoint (65 F or above) combined with a high RH. If the dewpoint is above 65, it will generally always feel uncomfortably humid outside. Obviously, the temperature could climb to over 100 and result in a low RH, but the quantity of moisture in the air is still high and will be noticed.

One last point I would like to make is keep you and your relatives safe this summer.