Re: What factors make St.Louis so Humid, but somewhere like Los Angeles isn't?

Hi Anthony

What a great question! That humidity thing is really awful when you hit 
one of those 90 degree days and it feels like you'll never be cool again!

Why don't those guys over in LA have such high humidity when they are 
right on the beach? The real reason is that they have low humidity BECAUSE 
they are near the beach, and because the Pacific ocean is cold. 

This is a little complicated, you have to pay attention to the difference 
between how much water is in the air, and how much water COULD be in the 
air. 

Humidity is a measure of how much water is in the air compared to the 
maximum amount that it could possibly have. So 90% humidity means that 
the air is holding almost as much water as it possible can hold. But the 
amount it can hold depends on the temperature as well. If the air is warmer,
then it can hold a lot more evaporated water. If the air is cold, then it can't 
hold as much. So, when you are talking about 90% humidity in warm air, that's a
lot more water than 90% humidity in cold air. 

You already knew this, right? That's the reason we dry our clothes in hot 
air in a clothes dryer, or out on the line on a bright sunny day. Because 
the hot air can soak up more water than cold air and so the clothes will 
get dried faster in a hot dryer.

What if there's water in the air and the temperature of the air changes?

I was out in L.A. a few days ago. It was hot! but the temperature of the 
ocean was cold. The air was 90 and the ocean water was 60. 

So, let's say the air over the cold ocean was 99% humidity in the early 
morning. That means the ocean air was holding 99% of the water it could 
possibly hold at 60 degrees. But wait, when the breeze started and the air 
came over the land, the air warmed up to 90 degrees. See, the 90 degree 
air could hold much more moisture, but it only had the same amount of 
water that it had when it was 60 degrees. So, once it warmed up, compared 
to how much water it COULD hold, it was actually holding much less. 

Let's say that again. The air over the ocean at 60 degrees can hold about 
1% - 2% of its weight in water. But air that is 90 degrees can hold about 
3% of its weight in water. So if the ocean air comes on shore and warms 
up, it is actually holding much less water than it COULD hold at the 
warmer temperature... 

Here's a chart of the humidity in LA
 http://www.cityrating.com/cityhumidity.asp?City=Los+Angeles

See, in the cool of the morning, the humidity is fairly high - the air is 
holding almost as much water as it can, but during the day as the sun 
warms up the land and the air over the city gets warmer, the amount of 
water it COULD hold gets greater, so that the amount of water it actually 
is holding is a smaller percentage of the maximum it COULD hold.

This idea is a little complicated. I hope you can think about it for a 
while and understand it. It is always fun trying to understand the 
weather!  

In St Louis, the air that is coming over the city is coming from a place 
that is about the same temperature. So, if it is wet, it will still be wet 
in St Louis and everyone will feel uncomfortable. 

Here's a link to a good article about humidity that was in the USA Today
 http://www.usatoday.com/weather/whumdef.htm
 
Keep cool!

Dr. Matt McConeghy