# Heat Reducton

1. Jul 18, 2013

### JMcGoo

If the hood of my at rest car is 106 degrees and I drive at 55 MPH, how much is the hood temparature reduced by the wind so created?

2. Jul 18, 2013

### ModusPwnd

Not even close to enough information to answer this theoretically. By far the easiest way to get this info would be to put a thermometer on the hood of your car and test it out.

3. Jul 19, 2013

### CWatters

What Moduspwnd said. Way too many variables to answer. It might even depend on why your hood reaches 106C with the car stationary (eg is it the heat from the engine or the sun?). If you drive for a long time at speed the hood temperature could well fall to air temperature because cooling air goes under as well as over the hood. So the length of time you spend driving may/will effect the results.

4. Jul 19, 2013

### JMcGoo

Sorry if I made the question too full of variables. Take a sheet of metal and place it on the grass in the sun. Blow a fan on it. Assuming the ambiant temp is unchanged, with no other variables such as moisture, is the temp of the metal effected downward by the breeze from the fan?

5. Jul 19, 2013

### Staff: Mentor

If the air is cooler than the metal, it will tend to remove heat from (cool) the metal as it flows by; and to add heat if it is warmer than the metal. The actual rate of heat exchange will depend on many variables: humidity, the intensity of the sun and the emissivity of the metal, the air speed, the size of the temperature difference, and lots more.

Google for "IR temp gun" - twenty dollars US will buy you the tool you need to answer your question.

6. Jul 19, 2013

### HallsofIvy

You said that the hood temperature at rest was 106 degrees. Does it not occur to you that the heat of the engine, driving at 55 mph, will heat the hood?

7. Jul 20, 2013

### CWatters

If the air is colder than the metal then heat will be conducted away into the air. The power flowing between metal and air will depend on the thermal resistance between metal and air and the temperature difference between them.

If you blow air over the metal the temperature difference (edit: "Gradient" is a better word) will be increased because the layer of air next to the metal will be colder. This will increase the rate at which power flows from metal to air.

What happens to the temperature of the metal depends on the balance of power going into and out of the metal. Presumably a heat source made the metal hot in the first place? If the power supplied by that heat source is greater than that lost to the air then the temperature will rise. If less then it will fall.

So if the heat source (the sun?) still exists the fan may or may not cause the metal to get colder. All you can say is that it won't be as hot as it would have been without the fan.

PS: Car radiators and the heat sinks in a computer have fans for a reason.

8. Jul 20, 2013

### SteamKing

Staff Emeritus
It's not clear if the hood temp is 106 F or 106 C. If the latter, I would expect serious burns from touching it.

9. Jul 20, 2013

### Staff: Mentor

Heat it MORE than sitting still at idle? It won't: a car engine produces only a moderate amount of power at cruise and cools itself vastly better.

So at 55mph, I'd expect the hood temperature to be only a little above ambient.