We all know that some hot days feel fairly comfortable while others at the same temperature feel clammy and muggy. We tend to identify the difference as caused by the relative humidity and this sentiment is often expressed by the statement “It’s not the heat, it’s the humidity”. But actually, the humidity is not the best measure to use to measure the level of discomfort. A better measure is the lesser-known ‘dew point’. The National Weather Service explains what ‘dew point’ is and why this is the case.
The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.
The higher the dew point rises, the greater the amount of moisture in the air. This directly effects how “comfortable” it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more “humid” on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.
So if you want a real judge of just how “dry” or “humid” it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.
General comfort levels that can be expected during the summer months:
- less than or equal to 55: dry and comfortable
- between 55 and 65: becoming “sticky” with muggy evenings
- greater than or equal to 65: lots of moisture in the air, becoming oppressive
For those who like to get their information in emoji form, here’s one.
I noticed this myself about ten days ago when it was hot. I was outside and noticed that it felt very muggy. When I checked, the temperature was 91F but the relative humidity was only 42%, which would normally be felt as comfortable. But the dew point was 65F.
Of course the three things (actual temperature T, the dew point temperature TD, and the relative humidity RH) are related. Given any two, you can calculate the third. This website enables you to obtain any one of them given the other two and provides the actual formulas used, where temperatures used in the formulas are measured in Celsius.
The reason why the dew point may be a better indicator than the relative humidity, even though they are related to each other, is that the relationship is not linear. For a given temperature, the relative humidity rises more rapidly as the dew point rises.