One way your body cools off is by evaporation. When you sweat water forms on your skin. Turning that layer of water into vapor by evaporation takes energy and that energy loss cools you off. That's only half the story. At the same time that evaporation is occuring, the reverse process of condensation is also happening. Water vapor in the air condenses into liquid on your skin and the heat gained by the water warms you up. Most of the time the cooling from evaporation is much greater than warming by condensation, so the net effect is nearly always cooling.
Not so today! To see the cooling power of air take a look at the difference between the air temperature and the dew point temperature. Perspiration cools most when there's a big difference. This is why misters are used for outdoor cooling in the southwest. When the air is saturated, the air and dew point temperatures are equal, and cooling by evaporation is exactly matched by heating by condensation. That's what's happening today. By eleven this morning, the dew point jumped to 75 degrees, while the air temperature was 78. The air's got no cooling power and it feels totally yucky to be outside.
What about the rest of the weekend? Here's the math: Humidity + heat + a stationary front = craploads of rain! The Weather Service is expecting 1-3 inches of rain by Monday. Clouds, showers and thunder showers like what we had this morning are going to pop up pretty much any time over the next few days. It'll cool off from the mid-80s today to the high-70s on Sunday. Don't despair, Monday and Tuesday look gorgeous.
Weekend rain graphic from the NWS digital forecast database.
