Humidity When It Rains

A couple of morings I awoke here in L.A. to the sound of rain on the roof. As I was getting ready for work the TV news confirmed what I already knew–it was raining. The weatherman also reported that the humidity at that moment (and it was coming down pretty good) was 89%. Later that evening the rain had stopped and I could see stars between the billowy moonlit clouds. The 10:00 pm news weather reported that the relative humidity at that time was 100% even though it had stopped raining hours earlier. Whay is it that the relative humidity was only 89% when it was coming down, and 100% after it had stopped? Seems to me like it should have been the other way around.

Steven Kinzler, Los Angeles

Was that the same weatherman who said that the La Brea Tar Pits were hurling fireballs 1500 yards into the sky? If I were you, I’d get cable T.V. and watch the Weather Channel. Now, THOSE people know their cumulo-nimbii. :smiley:


RH is the ratio of absolute humidity (AH = grams of H2O per kg of air) to water vapor capacity (VC), expressed as a percentage. VC is a function of temperature and increases with temperature. AH increases by evaporation, which can only occur if there is water present to evaporate (a lake or puddle, or raindrops).

Thus RH can increase with evaporation from some liquid source, or it can increase through a decrease in temperature. In SoCal you don’t see 100% humidity often 'cuz it’s generally warm and dry. The exception is foggy mornings; dew falls when it’s cold enough right at the ground level for condensation to occur.

Raindrops obviously form in conditions of at least 100% humidity, but this is up above the cloud deck, a km or so up. The air near the ground doesn’t have to be at 100% for it to rain, but drops (high surface area) will evaporate some as they fall, so you should expect RH to increase as long as it rains, and possibly for a bit afterward.

This explains why you will have RH values below 100% while it’s raining. When RH at the ground reaches 100% condensation occurs and it will get foggy. The report of 100% on the news may have referred to conditions at LAX, since fog is more common near the ocean.

The humidity could be 100% without rain. The condensation would be in the form of heavy dew and/or fog. I guess it’s possible to rain with less than 100% humidity along the ground. Rains are precipitated (no pun intended) from the clouds by way of some irritating factor, such as the way you look at them.

“Air” is actually a mixture of dry air and water vapor. The amount of water vapor which can conceivably be contained in (mixed) air varies based upon temperature. Warm air is capable of holding more moisture than cold air is. (That’s partly why your skin and lips get so dried out in winter. The colder air is capable of holding less moisture than you’ve been used to during the rest of the year. To add insult to injury, the heating system in your house tends to dry it out more ------- Assuming you live in a climate where it gets cold in winter, which I guess you don’t.)

Relative humidity is a measure of how much moisture is contained in a pound of air divided by how much moisture a pound of air at that temperature would contain if it were completely saturated. So, let’s assume that you have roughly the same amount of moisture in the air at the 10:00 PM newscast as you did at the 6:00 one. The temperature has probably gone down a few degrees. Since cooler air is capable of containing less moisture than warmer air, then, the relative humidity will be higher, even though the amount of moisture hasn’t changed. (Actually, you don’t even necessarily have to have the same amount of moisture at both times. The specific humidity could actually have gone down a bit, but the relative humidity could still be higher than it was at an earlier time.)

Our local meterologists also like to toss out the term “dew point temperature” a lot. The dew point temperature is the temperature that, given the present amount of water vapor in the air, would result in a saturated air (100% relative humidity.)

I’m too lazy to look up an online reference for you now, but you might try looking up the term “psychrometric chart” (don’t forget the “r” in there) in your search engine. It’s a weird looking little graph that plots several different properties of mixed air — wet bulb temperature, dry bulb temperature, dew point temperature, specific humidity, relative humidity, air volume, enthalpy, etc. When you specify any two of the above, you can find out all of the other properties. If you think better visually, this could give you a much better picture of what’s going on. (Dry bulb temp. is the one you’re used to.)

Spouse of kpm
(Who grumpily is using his moniker because she can’t figure out how to get the board to allow a second registered name…)