Friday, August 3, 2012

Explanation of Humidity and Dew Point

I found this heat index chart online. For people who live elsewhere than in the more hot and humid areas of the U.S., it can be hard to convey just how awful the day is going to be when the humidity is 88% and the temperature is 82F at 7:30 in the morning.

For the past month (July) our average dew point was 72 degrees. The high was 79 and the low was 60. The low dew point seems to have occurred on two different days at the beginning of the month, but other than those two days, the dew point did not dip below 70.

I think this is just one of those worthwhile things to know and understand so I'm going to brave the crickets chirping in the deadly silence after this post and go ahead and do a public service announcement. Stick with me if you can.

From Wikipedia:
The dew point is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface.

The dew point is a water-to-air saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity decreases.
 Everyone knows what dew is: when you run out to get the paper in the morning and the grass is all wet, that's dew. When there's enough water in the air and the temperature gets just low enough, it forms. Obviously, if the temperature at which it will form (the dew point) is high, that means two things: it's pretty darn hot and it's pretty darn humid. Numbers are just abstract things though, so I provided the nifty chart above.

"So what?" you may be thinking. "It's the heat that will get to you, right?" Well, yes, heat certainly plays a part, but what cools you off in the heat? Evaporation. Water vapor has more energy than liquid water. As sweat evaporates, in other words, turns from a liquid into a gas, it absorbs heat and you feel cooler. You feel the opposite when you stick your hand over some steam. As the water vapor condenses back into liquid water on your hand it releases heat and you get a steam burn. The drier the air is (the lower the humidity) the more "room" there is for water to evaporate. If you stick a bone-dry sponge near some water it will suck it up immediately. So when the air is dry sweat evaporates quickly and you lose the heat to evaporation.

But what if it's humid? Humidity is like measuring the amount of water already in the sponge. The more water the sponge contains, the less water it can absorb. Saturation occurs when the sponge can't absorb one more drop of water. If you stick a saturated sponge in a puddle of water, the two just sit there. There's no more room in the sponge for any of the water. The weather men on TV don't just talk about humidity though - they talk about relative humidity. Remember when I said that humidity is like measuring the water in the sponge? Well, one thing you have to know is that the warmer the sponge, the more water it can absorb. (This metaphor starts to break down a bit here because that doesn't actually happen with the sponge...I think.) So the warmer the air, the more water vapor it can hold.

The practical application to this is watching what happens when you are outside on a hot day with high dew points. You exercise, your body sweats to try to cool you off...and the sweat just sits there. You find yourself wiping it out of your eyes, your clothes are wet, etc. The air simply can't hold any more water. You can't cool off because you can't use evaporation to remove heat from your body. One thing that helps evaporation is wind so if it's a breezy day you will lose a little more heat than you would have without it. Otherwise, you're going to feel pretty icky.

All that is just to say that I can't wait for fall because these morning and evening walks with dew points of 79 and 80 are not fun.

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