Heat Wave Physics

| August 12, 2007 @ 3:14 pm | 6 Replies

The heat wave that we are experiencing may be related to many factors. However, a couple that really stand out are the following:

1) The drought over the past few months has left the soil extremely dry over Alabama. When energy from the sun comes in, it either goes into evaporating water in the ground (latent heat – similar to the effect of the cooling you feel even on a hot day if you get out of the water and the wind blows); or it goes directly into heating the ground (senisble heat), and subsequently the air above the ground. If you take a look below at the NAM forecast latent and sensible heat fluxes for this afternoon at 4 pm (21 UTC), compare Birmingham, AL with locations in NE Texas, at a similar latitude. Here, where the ground is dry, our latent heat flux is only about 175 W, whereas it is near 400 W where the ground is wetter in Texas. However, the sensible heat flux in BHM is around 200 W, whereas in NE Texas it is around 100 W. They both total about 500 W, it’s just that a lot more of the sun energy here is going into heating the ground, not evaporating water. This causes hotter temperatures.


The MPR instrument at UAH remotely detects temperatures in the atmosphere, and produces a new “sounding” every minute. For real-time data from MIPS MPR, go to https://vortex.nsstc.uah.edu/mips/data/current/mpr, then click on Current MPR Skew-T. If you are looking at this in the daytime, note the rapid decrease in temperature with height near the surface. Normally, this decrease is never larger than about 5.5 degrees per 1000 feet, but with the dry ground, it is more like 9 degrees per 1000 feet, very rare.

2) The air is moving very little. Around here, especially in the Summer, more heat energy comes in from the sun every day than we radiate back out into space. So, unless we get flow from the Gulf (note the cooler temperatures near the Gulf coast), thunderstorms, or something else to cool us down, the air mass tends to stay the same or get a little hotter every day. This is my concern, since with low rain chances and little air movement, the air mass may keep slowly warming up, with highs near 105 in BHM by Wednesday, with some places maybe approaching 109 or 110.

At least with the very dry air, the heat index is not much higher, if any, than the temperature. But, this is dangerous, and we need to remember to drink plenty of water (even if not thirsty), check on any elderly family or neighbors, and bring the pets inside or give them plenty of water and shade if that’s not possible. If you are involved in football or band practices, consider outdoor practices only before 11 am or after 8 pm, and force fluids.


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