A hot time in the old town tonight
"Yes, but it's a dry heat." Standard statement in the Southwest. Contrast this with "It's not the heat; it's the humidity," which one hears in Florida. Yes, it is hot here in the summer, and it is dry. Not dry like Phoenix or Tucson dry, but less uncomfortable than it might be otherwise.
Summer days in Austin range in temperature from 75-80 up to about 105-110. (The high this summer was 112, but mercifully we were out of town for that baking.) In the morning, the low temperature of the day, just before dawn, plunges all the way down to maybe 80 (26-ish, for those of the modern Celsius persuasion). In the summer it is never cloudy, so the sun heats up the landscape continuously through the day. The heat of the day comes at the end of the day, 5 o'clock or so, not at mid-afternoon as we were accustomed to in the North. For four to six weeks, the peak temperature is at least 90, usually 95 to 105 (35 to 41 C).
For the last many decades, when I left work in the evening, at six or seven or eight, the heat of the day was well past and the air and the car were both cool. Well, cool relative to the maximum. This summer was different. Leave work at seven, and the heat slaps you in the face before the door closes behind you. The air is hot but not burning. It feels thick and hard to breathe. There's always a wind, too, dry and sometimes dusty. It dries out the eyes and contact lenses quickly so that sunglasses are essential. If I leave at seven, it's still light, of course, so sunglasses aren't that peculiar. But if I leave at nine or ten, it's dark, but the heat and the wind are still there, so I need light sunglasses or plain-glass glasses (which I had anyway from years of convertibles and sunroofs).
In this heat in the middle of the summer, you don't' sweat. Or you think you don't sweat. Ms T and I have both had days when we had horrible headaches and nearly dropped from dehydration. One afternoon, we were out looking at garden ponds, and it was about 105 that day. We had four bottles of water with us, and drank them all, but I collapsed just the same, and slept for hours. And I *never* nap.
In any case, the heat of the day feels very dry. But the mornings don't. Well, let's see. Relative humidity is what makes air feel wet or dry. Relative to what? Relative to its carrying capacity. The ability of air to hold water as vapor in suspension increases with temperature. The same air that feels humid at eighty degrees feels dry at a hundred. For example, air with 85% relative humidity at 80 degrees F (27 C) has only 53% relative humidity at 90 degrees F (35 C), 45% at 100 F (38 C), and only 40% at 105 (40 C).
So what I think happens is that the amount of (absolute) humidity in the air doesn't change much during the day, and maybe not for days at a time. This is a semi-arid area in the summer. It doesn't rain for weeks on end in July and August. Every day, the temperature goes up and down, the humidity goes down and up. And after weeks of this, you can tell when the summer heat breaks and fall is a-comin'. One night the temperature falls low enough to hit the dew point and the grass is wet in the morning.
You get used to the heat, to the point where, on the first day under 90 you find yourself using words like "reasonable" and even "cool." Both of which are nonsense in any objective view. But you *can* sit on the patio with your beer or lemonade or iced tea and not want to run back to the air conditioning for at least a few minutes. And, by the way, even though you may not sweat visibly, your glass of cold drink does. No matter how low the humidity -- within reason, I mean, we're not talking Death Valley 3% here -- the dew point is still well above your average ice cube's comfort zone.
Copyright (C) 2000, 2001, 2002, Richard Landau. All rights reserved.
2000-10-01
