In my neck of the woods, we’re having a bit of a heat wave. The high temperatures will be flirting with 100°F the rest of this week and into next week. Much of the rest of the country is hot this week, too. Thankfully, I live in an area with enough recent rain and without enough trees to catch on fire. Yesterday afternoon, I took my daughter to play outside in the shade. We lasted about five minutes. I’m actually pretty okay with melting sometimes, but it seemed like a bad idea to keep the kid outside very long.
Why wasn’t it warm in the shade with a gentle breeze blowing? I thought we’d be okay for a while, but no. What I didn’t realize was that the actual temperature right then was probably 101°F. Not only is that really hot, but it means that the breeze doesn’t really cool us.
Generally, wind feels cold because the air is cooler than we are. If it’s an eighty degree day, then the air is about twenty degrees cooler than me. I’ll cool off slowly if there’s no wind. If there is wind, the air moving past me makes my temperature lower to eighty degrees faster–we don’t actually cool down to eighty because we’re little furnaces making heat constantly. On a fifty degree day, I cool a bit faster with no wind, and any wind increases the cooling even more, since there’s a greater difference between my temperature and the air temperature.
On a 100°F day, the air is actually warmer than me. I’m making heat, but the air is already hotter than my body (which should be 98-99°F). Besides making heat, I’m also absorbing heat from the air. A breeze speeds up the heat transfer, which makes me hotter faster. That’s why even today’s breeze wasn’t a refreshing, cool breeze.
There is an advantage to wind on a hot day, though–it helps our sweat evaporate faster. When sweat evaporates, it takes a little heat from your body with it, cooling you down. The more air (wind) available, the faster the sweat evaporates. Thus, when you’re sweating, the breeze might cool you down a little.
The limitation of sweating in some parts of the country is that, as the relative humidity approaches 100%, not very much sweat will evaporate into nearly saturated air. That’s why “dry heat,” such as in much of the southwestern US, is a bit more bearable at 100°F than in humid areas, such as the southeastern US.
Slate published an interesting article about sweating last week. At least, I found it fascinating, so I think everyone should read it.