Is temperature different at altitude?

[calgarian]

Is temperature "different" at altitude?

I know the boiling point of water is lower at higher altitude. But a friend of mine says that temperature in general is different at altitude. So, if the cooking instructions say to bake something at 425, he says you should set the temperature of the oven higher to adjust for higher altitude. I say that temperature is an absolute that does not change with altitude. So an oven set to 425 at sea level has the same heating effect as an oven set to 425 at 4000 ft. This assumes we preheat the oven before placing the food in it. So, who's right?

 

[russ_watters]

Temperature is temperature, but what you are doing with the oven is trying to transfer heat to an object. If there is less air, there is less heat transfer.

 

[HallsofIvy]

Air temperature tends to be lower as you go higher but that has nothing to do with the temperature of a pan on a stove where you have set the desired temperature. Also, if I understand russ watters' post, the density of the air has nothing to do with transfer of hear from, say, an electric burner to a metal pan.

 

[D H]

I know the boiling point of water is lower at higher altitude. But a friend of mine says that temperature in general is different at altitude. So, if the cooking instructions say to bake something at 425, he says you should set the temperature of the oven higher to adjust for higher altitude. I say that temperature is an absolute that does not change with altitude. So an oven set to 425 at sea level has the same heating effect as an oven set to 425 at 4000 ft. This assumes we preheat the oven before placing the food in it. So, who's right?

Yea and no. If your food was dry you wouldn't have to compensate the cooking temperature or cooking time. Your food isn't dry, so you do need to compensate, and do so in many ways. You may need to increase the water, decrease leavening ingredients, and increase the temperature, just as a starter.

 

[russ_watters]

[To Halls]
Yes: an electric stove transfers heat through conduction, an oven mostly through convection.

 

[russ_watters]

Yea and no. If your food was dry you wouldn't have to compensate the cooking temperature or cooking time.

Why?

 

[calgarian]

Temperature is temperature, but what you are doing with the oven is trying to transfer heat to an object. If there is less air, there is less heat transfer.

I see what you're saying. But I would think that at equal temperatures the fewer air molecules at higher altitude are moving faster than the larger number of slower moving air molecules at lower altitude. If this is true, why would net heat transfer be different?

 

[Drakkith]

I see what you're saying. But I would think that at equal temperatures the fewer air molecules at higher altitude are moving faster than the larger number of slower moving air molecules at lower altitude. If this is true, why would net heat transfer be different?

I don't believe this is the case. As far as I know the temperature of a gas is usually taken to be the average kinetic energy of it's particles. If so, then fewer molecules means fewer interactions per second between the air and the object to carry heat away, which would lower the heat transfer.

 

[D H]

I see what you're saying. But I would think that at equal temperatures the fewer air molecules at higher altitude are moving faster than the larger number of slower moving air molecules at lower altitude. If this is true, why would net heat transfer be different?

It's not true, at least not for an ideal gas. For an ideal gas, the distribution of velocity is a function of temperature only. Diatomic oxygen is pretty close in behavior to an ideal gas, and diatomic nitrogen is even closer yet.

 

[pa5tabear]

I know the boiling point of water is lower at higher altitude. But a friend of mine says that temperature in general is different at altitude. So, if the cooking instructions say to bake something at 425, he says you should set the temperature of the oven higher to adjust for higher altitude. I say that temperature is an absolute that does not change with altitude. So an oven set to 425 at sea level has the same heating effect as an oven set to 425 at 4000 ft. This assumes we preheat the oven before placing the food in it. So, who's right?

Temperature is temperature, you're right. But he is correct that the cooking behavior will still be different.

The key difference is the pressure (and probably density as well) of the air being lower, which results in a lower convectivity coefficient of the air. This means that the heat transfer from the air to your food is slower.

In order to achieve the same amount of total energy transfer to your food, you must increase the cooking time or increase the temperature.

 

[calgarian]

Interesting stuff. Thanks for all the replies.

So this must mean that the same temperature feels warmer to us at sea level compared to high altitude. I'm curious if anyone knows by how much. For example, if someone accustomed to sea level moved to Denver (5000 ft), what temperature would an 80 degree day feel like to him?

 

[russ_watters]

It isn't really that simple -- and you have it backwards. Our bodies dissipate heat via convection, radiation and evaporation. Even the convective part is highly dependent on the wind. So being in Denver will cause your body to dissipate less heat -- and thus feel hotter -- than being at sea level. But not enough to make a noticeable difference vs a slight breeze.

It does, however, have a significant impact on heating and air conditioning systems, which have fixed airflows and intended design temperatures. It can make a difference of 20% for Denver, which is enough to eat-up a typical safety factor.

 

[Drakkith]

Russ, do you know if sweating is more or less effective thanks to the reduced pressure?

 

[fluidistic]

Russ, do you know if sweating is more or less effective thanks to the reduced pressure?

May I add the factor "air humidity" which is very important when considering sweating.

 

[russ_watters]

Russ, do you know if sweating is more or less effective thanks to the reduced pressure?

That's a toughie. The air will have less capacity to carry moisture away from you, but I think the evaporation itself would be faster due to the lower pressure.