To rephrase, if a rock sits outside in 10F weather with -10F windchill, is the rock’s surface temperature 10F or -10F?
In: 4775
Seeing a lot of great responses here; would like to offer another take from the perspective of near-body phase-transfer mechanics.
Your body is always heating up the air around you, creating an extremely thin layer of air just above your skin that is permanently warmer than the actual air. This is called your epiclimate; think of the area in front of a heater vs the rest of the room. Your body is necessarily a heater.
Therefore, when we experience outdoor weather, we feel that the temperature is much warmer than it actually is, because our bodies are providing a very thin, warmer barrier as we lose heat to the atmosphere.
Wind, however, removes this barrier and exposes us to the true ambient temperature. Technically, it’s not 10°F and feels like -10°F with wind chill; 10°F feels like 30°F without wind.
Think of it like this:
If you are in air or water colder than you will cool down. The colder it is the faster you cool down and the colder you will get.
If wind or water moves past you it will cool you down faster and the colder you will get.
In both cases you, as hot bodied mammal, will not reach ambient temperature, somewhere above ambient.
So a ‘colder’ -10 deg room and a 5 deg room that has wind instead, can both make you cool to the same point (7deg) despite being different ambient temperatures. (No one’s blood is freezing normally when it’s snowing)
Without internal heating, a rock would just cool to ambient temperature. However! The wind would cool it faster, and while it’s cooling down to ambient, the rock may feel like it’s in a colder environment due to the rapid rate of cooling – but it stops at ambient.
“Wow, I am a rock and my temperature is dropping super fast, I must be in a very cold environment” (but actually is just wind cooling it fast)
I wanted to add something about *why* wind causes things to cool down faster.
Without wind, your body warms up the air that is touching it which reduces the difference in temperature. The greater the difference, the more heat your body loses. The lower the difference, the less heat you lose. Wind blows away that pocket of heated air, exposing your body to a greater temperature difference. The former situation is an example of conductive heating (or cooling I guess), while the latter is an example of convection.
Ever blow on coffee or soup to cool it faster?
“Air” is made from molecules with a certain amount of heat. When they bump into others with more or less, energy tries to equalize.
More airflow leads to more opportunities for the molecules to bump, leading to a faster change in temp.
That being said, the temp of an object won’t get colder than the wind blowing, as energy would transfer the other way.
To make a long story short, if the temperature of the rock is already at 10 F, then the windchill has absolutely no impact on it. It doesn’t matter if the windchill is 0F or -70F, it’s all the same.
Where things change, however, is when that rock starts out significantly hotter (or colder!) than the ambient temperature. If it starts out at, say, 20 F, then a -10 F windchill will cool it down significantly faster than if there is no windchill – but in both cases, it will eventually stabilize to 10 F. Interestingly, if the rock starts out at -20 F, then it will warm up to the 10 F ambient temperature *faster* if there is a -10 F windchill than if there is no wind.
Here’s the important thing to consider: heat is transferred between the rock and the air through conduction – it’s just like putting a pot of water on a hotplate. The key thing to keep in mind is that heat transfers more quickly when there is a greater difference in the temperature of the two items – in this case, the rock and the air. If the air sits stagnant, the air around the rock will warm up, slowing the rate of heat loss from the rock. If air is constantly blown across the rock, there is always a fresh supply if cold air. If the rock is colder than the air, the reverse happens. This is why heatsinks in electronics are often equipped with fans.
The reason that a wind chill makes it feel that much colder is that the internal temperature of humans is almost always significantly higher than the ambient temperature.
Have you ever blown on hot food to cool it down quicker? That’s wind chill.
If you took 2 rocks from your house and one was in the wind and the other was protected from it, the rock in the wind would cool down to 10 F faster. That’s what wind chill is, speeding up the rate of heat transfer by moving the air. It’s the same principle as a convection oven (or air fryer). But they would both end up at 10 F eventually.
If something is wet then the drier the air is the more the wet thing cools off. This can make the wet thing cooler than the surrounding air as it takes heat to evaporate water. This is why your body can maintain a constant temperature even when the air is over 98 F. It is also true that this affect is greater when the wind is blowing as the water molecules are rapidly moved away allowing more to evaporate faster.
So back to the rocks, if they were wet they could temporarily get colder than the surrounding air until the rock dried off. (Note: it gets more complicated because you chose 10 F, but imagine you chose 35 F to simplify things by not worrying about freezing.) the amount and rate the water evaporates and causes cooling would also be affected by the wind.
In any event, moving the air simply affects the rate at which the heat is transferred, not the amount.
The windchill effect is the same as sitting in front of a fan on a hot summer day. Say the room’s air temperature and everything in the room (including the rock) is at 85 deg F. Your body temperature is at 98 deg F. You have a “bubble” of stationary 98 deg F air surrounding your body, making you feel miserable. Now, you turn on a fan which blow 85 deg F air at you, which also blows away your 98 deg F bubble. You feel cooler with 85 deg F air hitting your skin, but the 85 deg F rock feels no temperature change with moving (vs. stationary) 85 deg F air around it.
Wind blows the heat away.
You are warmer that the wind and get cold faster.
The rock already is cold and thus doesn’t get colder by the wind
Wind chill is how cold it FEELS
If there’s no wind, you wont be feeling the 10F even on bare skin, because your body warms itself.
With wind, you will cool down faster because wind will transfer that cold air faster
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