Basically title, you always hear “windchill only affects humans” but we use fans to cool things all the time?
In: Physics
If an object is not already at ambient temperature, then moving air will hasten its return to ambient temperature. You are not usually at ambient temperature. Your car after some use is not at ambient temperature. In these cases airflow is important in understanding how quickly things heat and cool.
But when your car’s been sitting outside unused for an hour? Or the railing on your balcony? The door handle at the store? Those are going to be at normal temperatures to begin with.
Unfortunately, wind chill as a “temperature” number isn’t very helpful because it depends on the temperature of the thing that’s cooling off. Humans all tend to be around the same temperature, so it makes a bit of sense to choose one “feels like” temperature for wind chill, but the same cannot be said for a warm car or an uninsulated window. At this point the answer just becomes “it’s complicated” and any further analysis becomes mathematical.
You use fans to cool things that are hot. The more air that passes over it the more heat the air captures and removes from the object.
Windchill does not affect an object that’s already at the ambient air temperature.
For example, if my car is sitting outside in -30 for two days, my engine will be -30 because that’s what the ambient air temperature is. Adding wind to the equation will not cool my engine colder than -30. That’s not possible. There is no heat to remove from the engine.
My engine cannot be colder than the ambient air temperature no matter how much air you blast at it.
What “wind chill” actually means is that if you stood outside in naked skin, the effects on your skin/body are equivalent to being in a room with a temperature of WIND_CHILL.
It does this because air is not “nothing.” Wind Chill does “effect” objects, in that if those objects are in high winds for an extended period of time AND have chemical interactions with the air, then a higher wind would increase the wear on that object — just like the wind increases the “wear” on the human body.
This isn’t perfect, but should get you most of the way there.
Our bodies generate heat and use evaporation to cool us down. Fans blow the hot air away from things to let cooler air take it’s place, and the hot object them spreads some of it’s heat to the now cooler air. Then the fan blows THAT hot air away and the cycle continues.
Evaporation is another factor for humans, your skin always has some amount of moisture. That will evaporate faster if the air is moving. That’s an important reason to use fans. Evaporating something takes heat away.
In addition to what everyone else has said, fans arent *just* used to cool things by blowing cold air over them, they can also be used to blow hot air away from the thing.
Wind can affect objects. That’s why you blow on food. If something is hotter or colder than the air around it, then blowing on it helps bring it closer to the temperature of the air. Blowing on cold things warms them up and blowing on warm things cools them down. But if you go outside and there is a rock sitting on the ground the same temperature as the air, then making a fan blow on it won’t do anything. If you blow 40 degree air against a 40 degree rock then both will just stay 40 degrees.
Fans/wind will cool things that are hotter than their environment, but not things the same temperature as the environment
Imagine you have a bowl of hot soup, if you stir it, it can help it cool by exposing more or the hot soup to the cooler air around it, so it’ll cool faster. The soup at the center of the bowl will stay hot for a while without stirring because it’s surrounded by more hot soup rather than cool air
Now take a bowl of room temperature water, does stirring cook it down? No, because it’s all the same temperature as the environment anyways
So wind/fans are like stirring, and humans are like hot soup, wind chill is like stirring your hot soup to make it cool faster
If you feel hot and need cooling, then fans help you cool by providing a continuous change of the layer of air that is next to your skin. This takes advantage of two ways to cool something: convective heat loss, where heat is conducted from your skin to the air, and evaporative heat loss, which uses evaporation of sweat from your skin.
Convective heat loss is just a transfer of heat from your skin to the air (or to the water, if you are in water), and if you are continuously changing the warm air that is right next to your skin (warm because it has absorbed heat from your hot skin), with cooler air from elsewhere in the room, then convective heat loss will be more rapid. A bigger temperature difference between your hot skin and the layer of air around it means faster heat transfer.
Evaporative heat loss is extremely efficient, because the phase change from liquid water (skin) to water vapor (evaporated water), which is called evaporation, requires lots of energy, and this energy is taken from your skin in the form of heat. You can experience this by putting a sweaty arm in front of a fan, and it will instantly feel cooler, even if that air is warm, such as air from a window fan on a hot day.
Wind chill does affect objects, in a way. When it is windy, objects lose heat by the convective heat loss mechanism more rapidly than with no wind, but they can never get colder than the air around them. Heat spontaneously flows from a warmer thing to a colder thing, never the other way.
A little breeze on a hot day produces wind chill as well, especially if you are sweating, but we welcome it.
The wind affects at what rate heat is transferred from an object, it does not affect what temperature an object that does not produce its own heat ends up at. So the time for temperature to change, not the final temperature.
If an object activity tries to keep a specific temperature like a human does wind has an effect of the rate heat has to be generated. Enough can be generated it determine what temperature the object ends up at.
For something that generates heat at a constant rate like for example a computer CPU at a constant load, a car’s engine at a constant load the wind will determine what temperature it reaches.
Let’s first look at an object that does not produce any heat.
If the wind cools something down it has to be cooler the the object. Cold is not a thing it is just less hot. It is the object that heats up the air, it will reduce the temperature of the object and increase the temperature of the air. The result is both end up at a temperature in between what they started at. Air is not still but moves and transfers heat to other cooler air, the result is the temperature of the object will after some time be the same as the air temperature.
What wind does is move away the air that has been heated up and replace it with cooler air. The rate at which energy is transferred depends on the temperature diffrence. The result is wind results in the time it takes for the object to reach the same temperature as the air is shorter, but it does not affect what temperature the object ends up at.
A human on the other hand needs to be in a quite small temperature range to function. The extremities can handle larger temperature diffrence than our internal organs. We can cool ourselves down by sweating or wake ourselves up by using the calories in our food. Moving around gets lots of heat and is an efficient what to stay warm.
Wind will just like other objects result in heat being transferred faster from our bodies. We need to replace that hat to keep the body temperature. We need to generate more heat if there is a wind. This is why windchill matters for humans, we try to keep a constant temperature.
Windchill that is measured/calculated for humans in some specific clothes. The face is bar and the person walks into the wind. It shows how much cooler the air would be if there is no wind to remove heat at the same rate as the air with wind. If you were naked the effect would be larger. It is a tool to help you determine the amount of clothes you need when the effect of wind it included.
It will be relevant for another object that tries to keep a constant temperature like a house we heat up. Bot to what degree differs. If you are out you likely have the face exposed that is warm, and the clothes are not airtight. A house on the other hand is covered all over, no part hat is at room temperature is on the outside. The outer shell is hopefully airtight, but that is not always the case for our clothes. So wind will increase the heating required for a house a small bit.
For something that generates a constant amount of heat like the CPU or a car engine wind will once again change the rate heat is removed. If the heat generated in constant but the amount that is removed changes with the wind the result is the object’s temperature depends on the amount of wind. There will be an equilibrium temperature where heat is removed and heat added is the same.
There are models for heating too,m it is called heat index. They will include air humidity because the rate of heat is removed when we sweat depends on the air humidity. We increase the humidity of the air around us and replace it will dryer air resulting in we bring able to cold down faster.
When the air is warmer your body wind will heat you up faster. When the air heats you it gets cooler and the wind replaces it will air that you have not cold down and you get warm faster.
Latest Answers