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That's false. TLDR: The air gets cooler but still can't cool you. Your body produces heat that has nowhere to go and you still die. Then the walls of the cella

That's false. TLDR: The air gets cooler but still can't cool you. Your body produces heat that has nowhere to go and you still die. Then the walls of the cellar heat up immediately, because cooling 100% humid air requires sinking a lot of energy. If you found yourself in conditions of an actual heat wave as described, you would despair at the fact the air can get cooler in your cellar but it doesn't cool you at all. Which is counterintuitive but unfortunately that's how physics works. First, humidity does not have to be very close to 100%. In fact, in most cases it isn't and the reported waves of 35C WBT are almost all between 42 and 44C in absolute temperature. This means the air has to cool down 7-9 degrees before it even reaches dew point. 7-9 degrees is a huge temperature difference when temperature is over 40C. Unfortunately the problem isn't the air temperature. The problem is your body's ability to remove heat. And if you take air and cool it down the temperature will drop but humidity will rise. The wet bulb temperature is preserved unless you can somehow remove water from air. And with wet bulb temperature being same 35C your body still doesn't have any way to remove heat and you are still going to die. Only after the air cools down by 7-9 degrees celsius you can start seeing any effects, but don't be too happy. Because physics has a nasty trick to play on you. The effects of water condensing on walls of your cellar is going to be your cellar walls heating up immediately. Yes, water cools stuff efficiently when it evaporates but just as efficiently it heats it up when it condenses. Conservation of energy is a bitch. Please, learn some physics, it is really fun. And sometimes terrifying. So your cellar is very likely to have very limited ability to cool down very hot humid air and condense water out of it.

f154hfds 5y ago
This is all fascinating stuff to me, I'd never really heard of wet bulb before. Quick question though: how does the pressure difference play into this in a basement? As far as I'm aware below ground pressure and voids under basements tend to cause pressure differentials in even non-AC basements (causing radon issues where I'm from)? I assume this would help a cellar situation in the event of power outage as air even slightly below ground tends to be much cooler than above ground air?
- lmilcin 5y ago
  The pressure difference between your backyard and your basement is for all practical purposes null. It is also orders of magnitude smaller than those caused by gusts of wind and changing weather.
  - f154hfds 5y ago
    I'm not sure this is so simple. Clearly your answer is correct with a largely uninsulated house with a basement only partially below ground (perhaps most of the developing world to be fair). Any semi-sealed home will have heat naturally accumulate (and dissipate) at the top. Apparently the movement of air upward - known as the stack effect - creates a (slight) vacuum pull at the bottom. Of course it's a small effect in normal temperatures but in extreme reverse stack-effect scenarios like you're describing the ground temperature below 3 - 5 feet should result in a basement being lower temperature than the backyard as available air gets sucked through the cooler ground to get into the basement? I'm sure this varies widely by region (ground temps vary in 3 - 5 ft depths) but everywhere this temperature will be cooler than the outside in such weather extremes.
bumby 5y ago
>The effects of water condensing on walls of your cellar is going to be your cellar walls heating up immediately. While true about the conservation of energy, the drastic differences in both mass and heat capacity of air/earth mean the cellar walls will probably have no perceptible change in temperature.
Aeolun 5y ago
> TLDR: The air gets cooler but still can't cool you. Evaporative cooling is not the only kind of cooling that exists. If this were the case you’d die of heat death standing in the middle of 100% humidity antarctica. If your cellar is cold enough you’ll be in a much better situation than before. You seem to assume that the moment you open your cellar door, all humidity and temperature will immediately equalize with the outside.
- lmilcin 5y ago
  Again, completely wrong. 100% humid 0C air is still 0C wet bulb temperature -- perfectly good for cooling you down.
  - bumby 5y ago
    >Again, completely wrong. You are saying the same thing as the OP. There are generally two type of heat in this scenario sensible and latent. Think about when you heat up water on the stove. If you were to graph the temperature T against the heat energy added Q, you would see the temperature gradually rise as the amount of heat was added. This is the 'sensible' heat. But at a certain point the water would hit boiling temperature. You're still adding heat but the temperature is no longer increasing so what gives? The energy is now going towards phase changing the water from a liquid to a gas. This is the 'latent' heat. The OP is just saying there are more paths than just latent heat transfer, which is why they said you won't overheat in Antarctica if it's 100% RH. Your statement here is that evaporative (latent) cooling won't happen at 100% humidity but sensible cooling can still occur. You're both on the same page.

f154hfds 5y ago

This is all fascinating stuff to me, I'd never really heard of wet bulb before. Quick question though: how does the pressure difference play into this in a basement? As far as I'm aware below ground pressure and voids under basements tend to cause pressure differentials in even non-AC basements (causing radon issues where I'm from)? I assume this would help a cellar situation in the event of power outage as air even slightly below ground tends to be much cooler than above ground air?

lmilcin 5y ago
The pressure difference between your backyard and your basement is for all practical purposes null. It is also orders of magnitude smaller than those caused by gusts of wind and changing weather.
- f154hfds 5y ago
  I'm not sure this is so simple. Clearly your answer is correct with a largely uninsulated house with a basement only partially below ground (perhaps most of the developing world to be fair). Any semi-sealed home will have heat naturally accumulate (and dissipate) at the top. Apparently the movement of air upward - known as the stack effect - creates a (slight) vacuum pull at the bottom. Of course it's a small effect in normal temperatures but in extreme reverse stack-effect scenarios like you're describing the ground temperature below 3 - 5 feet should result in a basement being lower temperature than the backyard as available air gets sucked through the cooler ground to get into the basement? I'm sure this varies widely by region (ground temps vary in 3 - 5 ft depths) but everywhere this temperature will be cooler than the outside in such weather extremes.

bumby 5y ago

>The effects of water condensing on walls of your cellar is going to be your cellar walls heating up immediately. While true about the conservation of energy, the drastic differences in both mass and heat capacity of air/earth mean the cellar walls will probably have no perceptible change in temperature.

Aeolun 5y ago

> TLDR: The air gets cooler but still can't cool you. Evaporative cooling is not the only kind of cooling that exists. If this were the case you’d die of heat death standing in the middle of 100% humidity antarctica. If your cellar is cold enough you’ll be in a much better situation than before. You seem to assume that the moment you open your cellar door, all humidity and temperature will immediately equalize with the outside.

lmilcin 5y ago
Again, completely wrong. 100% humid 0C air is still 0C wet bulb temperature -- perfectly good for cooling you down.
- bumby 5y ago
  >Again, completely wrong. You are saying the same thing as the OP. There are generally two type of heat in this scenario sensible and latent. Think about when you heat up water on the stove. If you were to graph the temperature T against the heat energy added Q, you would see the temperature gradually rise as the amount of heat was added. This is the 'sensible' heat. But at a certain point the water would hit boiling temperature. You're still adding heat but the temperature is no longer increasing so what gives? The energy is now going towards phase changing the water from a liquid to a gas. This is the 'latent' heat. The OP is just saying there are more paths than just latent heat transfer, which is why they said you won't overheat in Antarctica if it's 100% RH. Your statement here is that evaporative (latent) cooling won't happen at 100% humidity but sensible cooling can still occur. You're both on the same page.