Weather Blog

Just what is "relative humidity"?

Just what is "relative humidity"?
Handyman Gary Cochran, 55, wipes sweat from his face while working on the roof of Trinity Lutheran Church in Victorville, Calif. (AP Photo/The Los Angeles Times, Irfan Khan)

In Wednesday's blog I touched on how Tucson and South Texas were both having sweltering springs, but South Teaxs' was worse due to their oppressive humidity.

But just what is relative humidity?  We all know it's an indicator of how humid it is outside, but many have a misconception about it.

A lot of people think it's a percentage of water vapor in the air, but it's actually the ratio between water evaporating from liquid water to water vapor and condensating from water vapor back to liquid form at any given time. (Yes, meteorology always seems to take the more complex route when given the chance.)

A relative humidity of 50% means that twice as much air is evaporating than going from air vapor to liquid water. So that means over time, a puddle of water will evaporate.

As the humidity drops, the evaporating rate increases, and vice versa.

If the humidity is 100%, you have saturated air -- equal evaporating and condensation, at which point a body of water won't evaporate.

If you have more condensation than evaporation, you have a humidity of higher than 100% (called super-saturation), which is how clouds form.

Why should we care?

Probably the most popular use of relative humidity is related to human comfort. As humidity increases, the rate of evaporation slows. This is important on warm to hot days because evaporation is a crucial process in how humans cool off.

When we sweat, the water on our skin is intended to evaporate. As the sweat evaporates, the energy required to do so takes heat away from the skin.

As humidity increases, the sweat sticks around on your skin instead of evaporating, and you don't get the cooling effect. Whereas in the dry desert with low humidity, evaporation works really well and that's why a dry heat is more comfortable, because you can cool off easier.

Aside from humidity, another measurement seen in weather conditions is the dew point.  This is the temperature at which the air would have to cool to reach 100% humidity.

The greater the difference between the dew point and temperature, the lower the humidity. (A real rough guesstimate is 3% points for each degree spread when the spread isn't that great).  And the lower the dew point, the more comfortable the air feels.

For example, if you have a temperature of 80 degrees and a dew point of 60, that feels quite humid. But if it's 80 degrees and a dew point of 45 degrees, that's much drier and more comfortable.

Once dew points climb over 58-60, it starts to be noticeably humid, especially around the Northwest. Going over 70 is very rare in the Northwest, but in the southeast and Midwest summers where they get influence from the very warm Gulf of Mexico, dew points commonly range from 65-75 degrees -- even approaching 80 along the Gulf Coast -- which is very humid and feels quite miserable.

But around the Northwest, our dew points tend to be between 45 and 55 degrees in summer, which keeps our summers very comfortable -- even on warm days.

On the other hand, in the Desert Southwest, sometimes dew points are in single digits -- or even negative degrees (We've seen it where it'll be 68 degrees with a -4 dew point in Phoenix.)

Why doesn't it feel humid here next to the ocean?

The previous paragraphs seem counterintuitive to the idea that it's usually pretty humid around here due to our rainy climate and the big Pacific Ocean nearby.  But there's a couple of caveats.

One, the Pacific Ocean is much colder (50-55 degrees) along our coast than the Atlantic Ocean and Gulf of Mexico along the U.S. east and south coasts (65-85 in the summer).  So we get a much cooler, moist flow. 

With dew points generally staying in the 40s to low 50s, it might be humid/foggy in the early morning when temperatures have dropped to the 40s, but that's cool enough that the body isn't overheating and we aren't sweating, so the high humidity is not a comfort factor. 

Once daytime temperatures warm into the 60s, 70s, or even 80s, we have enough of a spread between the temperature and dew point that the humidity is now lower and not as noticeable -- really the perfect weather situation.

On top of that, when it gets really hot here, it's due to an east wind coming from the dry interior, which gets dried further when it sinks down the Cascade Mountains.  So it really IS the perfect situation where if it's going to be humid, it'll be cooler due to ocean breeze, but if it's going to be hot, it'll be dry. 

It is very rare to get a warm and humid day here -- about the only way is if we get an area of low pressure centered off the California/Oregon coast that taps into and then pulls up tropical air from the south -- especially during the Desert Southwest's monsoon season.