Thursday, 20 December 2007

Hot under the collar. Apparently.

This is a wet and dry bulb thermometer.

The dry bulb is the normal thermometer while the wet bulb has a fabric sleeve over it that is kept wet. When air passes over the wet sleeve, it cools due to evaporation and you can determine the humidity in the air by the difference in the two temperatures.

A wind chill factor, if you like.

In a totally humid area, like the tropics, little water would evaporate and the temperatures would be the same. In a dry climate there would be high evaporation and there would be a large difference between the two temperatures.

Which brings me to the local weather. There is a widget I have on my desktop that displays real and apparent temperature. Like so:

What I can't figure out is why should the apparent temperature be warmer than the actual temperature?

Any meteorologists out there?


  1. I have no science to offer.
    But I will absolutely testify to the truth of that.
    My body seems to have a broken thermostat, but on a humid day a temp of 22 can certainly feel about 4 degrees higher than it does on a dry one!
    Could it be that the moisture heats up?

  2. I'm confused by meggie's notion of humidity. I moved from Michigan (cold to -10) to SF (cold to 39) and froze my ass off for a year. A native SF'er tells me its because the humidity here makes it seem colder than it is.

    I have no idea what "really" does it, but the cold here isn't bad on the thermometer, but it cuts to the bone.

  3. Meggie, it's because at higher humidity our sweat does not evaporate as easily into nearly saturated air. We feel uncomfortable, and hotter, as we are not cooling down as easily as in a dryer atmosphere.


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