Naked Science Forum
General Science => General Science => Topic started by: Lin Stuart on 03/03/2009 17:30:02
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Lin Stuart asked the Naked Scientists:
Hi naked Scientists
I have a question that I'm sure you will be able to answer.
When it is so hot and the humidity is thick and no matter what you do the heat wins - why is it that when you move the air about it is cool, for instance by using a fan?
Thank you
Lin
What do you think?
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The fan doesn't neccessarily effect the actual temperature, it's just that it helps your sweat evaporate by blowing air over your skin. The evaporation has a cooling effect.
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Yes. Also, the air flow continuously removes hotter air around your body (heated by it) and so you feel cool. Of course this work as long as air is colder than your body's external surface.
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If you have high temperatures and v. low humidity then you can achieve a lot of cooling by using evaporation. But a fan won't help you much if you are in humid, tropical conditions.
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And it should be noted that directing a fan at you is much more effective than just sucking in air from the outside and depositing it anywhere in the room. Your goal most likely is to get cool yourself. The room does not matter a whole lot. Moving air allows your sweat to function better. Wet T-shirts work great too.
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[^]Thank you all for your input, it makes sense now.
Bot leaves me with yet another thought...........If there is no one in the room does the fan actually cool anything at all?
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Nope
Well, it will act to bring anything that isn't room temperature to room temperature. So it'll cool a hot pie, warm a cold drink.
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If you include all the effects, a fan will warm the room. Energy is conserved, it takes energy to run the fan, and it will not be 100% efficient.
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Nope
Well, it will act to bring anything that isn't room temperature to room temperature. So it'll cool a hot pie, warm a cold drink.
sorry could you explain, i dont understand
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If you include all the effects, a fan will warm the room. Energy is conserved, it takes energy to run the fan, and it will not be 100% efficient.
yet if I leave a fan on in a hot room and no one is there when I return it is cooler than the room without a fan, why?
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The fan will add a few Watts of heat to the room with its motor. It may stir the air around and mix air from warmer and colder spots to give an illusion of temperature drop.
Of course, if you are judging the temperature of the room by how it feels, then the moving air will evaporate the sweat from your body. Is the door shut whilst you are out?
Try it with a thermometer in various places. Measurements rule, you know.
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Nope
Well, it will act to bring anything that isn't room temperature to room temperature. So it'll cool a hot pie, warm a cold drink.
sorry could you explain, i dont understand
If something is hotter than the room temperature then the air the fan blows over it will be cooler than the object. So it will cool it down. If something is colder than the room temperature then the air the fan blows over it will be warmer, so it will warm it.
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cos it does!
I'm helpful am i not.
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If something is hotter than the room temperature then the air the fan blows over it will be cooler than the object. So it will cool it down. If something is colder than the room temperature then the air the fan blows over it will be warmer, so it will warm it.
sorry I am having trouble with this one.
I dont know what the 'something' you are talking about is. I'm not trying to be rude just trying to understand.
If you have 2 rooms both identical/ air pressure, heat, etc etc...
Put a fan in one and not the other.
The room with a fan becomes cooker no matter where you place the therometer.
How can this be when you are using electicity and movement to creat coolness?
I just cant get my head around this.
Can someone explain it simply to me please?
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Your actual evidence for a cooler room being what? Did it involve a thermometer?
What was the temperature difference?
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Ok, when you're hot the effect is twofold. You have the heat radiation from your body and the sweat molecules that your body reacts with. There is also the question of what humidity the room is at.
On a hot day in Mumbai it may be at a 100 percent, presumably ;).
So you put on that fan to cool you down, if it is a still day, the air around your sweating body is already saturated with water vapour, just making you even wetter without any of your own sweat evaporating. that fan will work though as it does two things. It blows away your heat radiation and it cools you by stripping away your own sweats water and the city's water molecules, and the hottest water molecules will evaporate first. So you will feel a cooling effect, although your body won't get any cooler than the room temperature, then again, the work that is done by the wind created should 'steal' some energy from your body so maybe your body will be just a tiny bit cooler than the room? you can test the evaporation effect by putting a drop of alcohol near a drop of water on your skin. The alcohol evaporates faster so your skin will experience a stronger 'cold' than from the spot of water.
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Yes we know about the sweat factor
Can you tell me why the empty rooms differ in temperature when a fan is on in one of them.
(2-3 degrees)
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Firstly it isn't magic. There must be a reason. If you have been running a motor in there and everything else is equal, there must be a rise in temperature if you have increased its internal energy.
It is possible that your thermometer did not explore the temperatures in the whole of the room (high up and near the floor). One cool spot could upset your odd result.
OR, there could be some evaporation somewhere.
It is always helpful to state the units you are using; Fahrenheit or Celcius?
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In a normal room the air is not hermetically closed in. It is possible that you by putting on this fan get cooler air from outside that room to circulate in. Otherwise I say as SC. The fan will get warm and the room will heat up by it. In a desert where you have a hot sun and a strong wind you can die fast of dehydration as your bodies water evaporates.
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It sounds like there hasn't been sufficient experiement recorded on this.
A new building fully insulated and cooler than outside - 2 rooms just Closed. nothing inside except one room has a fan and it is turned on.
That room can be 1-2 deg C cooler than the exact same room without the fan.
There may be air drawn in from outside but the air is hotter outside than inside anyway.
Yet the fanned room is actually cooler [???]
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It sounds like there hasn't been sufficient experiement recorded on this.
A new building fully insulated and cooler than outside - 2 rooms just Closed. nothing inside except one room has a fan and it is turned on.
That room can be 1-2 deg C cooler than the exact same room without the fan.
There may be air drawn in from outside but the air is hotter outside than inside anyway.
Yet the fanned room is actually cooler [???]
Do you have any empirical evidence to support this? It contradicts known physics. A room will naturally have a temperature gradient in it, and if you are measuring in a region that is hotter, the fan will mix the air and reduce the temperature where the sensor is, even if the overall temperature is unchanged.
If you think otherwise, you need to present the results of an actual demonstration where the temperature has decreased. With thorough documentation. Assertion is not enough.
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If something is hotter than the room temperature then the air the fan blows over it will be cooler than the object. So it will cool it down. If something is colder than the room temperature then the air the fan blows over it will be warmer, so it will warm it.
sorry I am having trouble with this one.
I dont know what the 'something' you are talking about is. I'm not trying to be rude just trying to understand.
It doesn't matter what it is, but for example lets say you have two glasses, one of boiling hot water and one with ice cold water. If left in a room without the fan they will both come to room temperature, but with the fan it will happen faster, because you're getting more air flowing onto them and absorbing or giving them heat than you would without the fan.
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It sounds like there hasn't been sufficient experiement recorded on this.
There have been thousands and thousands of hours spent on "this". I suggest that you do your particular experiment again and measure more temperatures in more plces more carefully.
A proper Scientist doubts a one off odd result and investigates until he has eliminated all errors. Only when there are no more errors to eliminate do you look for a "NEW THEORY".
Do not imagine that you have found anything new.
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I think it is because air flow,which will remove hot air around our skin,and the air flow by fan cooler than nature air,so you will feel cool when you open the fan in summer...and I think it is of great importance to have a digital humidity meter in summer,because air become dry in summer,and a digital humidity meter have many different functions.
If you want one you can see my signature~Hope my suggestion can help you~ :D
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Simple reason the room with the fan running will feel cooler is because the fan moving the air will tend to blend the air more than the non fan room, and this blending will allow any heat leaks ( things like gaps, windows and thinner insulation around doors) to lose heat, while the fan less room will only have a slow convection to blend the air and get it to an average temperature.
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Yes a fan cools a hot room.
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Yes a fan cools a hot room.
No it doesn't. Did you actually read any of the thread above? Clearly not.
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Yes a fan cools a hot room.
No it doesn't. Did you actually read any of the thread above? Clearly not.
It may any it may not. Depending on the humidity in the room, the movement of air in the room may give rise to evaporative cooling. If the air is allowed to escape further evaporative cooling may take place as moist air is replaced with dry air