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Central Alabama 7 Day Forecast

Heat Wave Physics

| 3:14 pm August 12, 2007 | Comments (6)

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.

heat-flux.JPG

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 http://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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  1. Joe says:

    If they would not start school so early in August (I remember when school started after Labor Day), then we would not have to worry about school activities during this heat wave. And think of the saving in electrical cost.

  2. David says:

    Thanks for the info. I have a dumb question. What is going to happen to all this hot air? I was watching the Weather Channel and they show the temps going down by next weekend. It’s like this big heat bubble gets smaller and then the above 100 temps just go away. I maybe could understand this if there were some low clouds around, but how can this much really hot air cool off by 5 or 10 degrees with the sun beating down on it every day?

  3. Fred Gossage says:

    It’s not all from just the sun itself. The high pressure area over us also causes the air to sink….and that subsidence causes the air to warm.

  4. Alison M says:

    I agree with Joe that school starts too early. The energy demand is heavy. When they get out in May (now barely past mid) the pools are still too cool to swim in! I’ll be as glad as anyone when it is cooler, but I really think it is not such a good idea to start school so early in the middle of summer.

  5. [...] SOIL MOISTURE: I strongly recommend Tim Coleman’s excellent post on the “physics of the heat wave”… no doubt if we had adequate soil moisture in [...]

  6. Chris L says:

    I’m curious about this. Oklahoma City’s temperatures are running about a degree or two below what we are running but they have had quite a bit of rain this year and thus, a high soil moisture content. Would they be hotter due to their geographic location?

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