Air Pressure, Density, temp.

[EnRoute]

So I was sitting with another CFI candidate who I study with a couple of times a week. After teaching a lesson to each, we started to discuss pressure altitude and how density and temperature effect said pressure altitude. My first question to him was "sitting at home with a telephone in hand, how will he determine the current PA at the airport" He answered accordingly with acquiring the current altimeter reading and doing a little math. OK, great. So this is were we started getting a bit lost in space. I live in Washington state where we seldom have tripple didged temps. Last week we experienced record highs of 107. Our airport is about 25' msl. Our Density altitued was nearly 3,500' WOW! awsome.

Now as I understand it, presure varies directly with density and inversly with temperature. So if pressure and density are directly related, which stands to reason, the less densley packed the air, the lighter, hense less pressure there will be in that 1" column of air. So again, why when my density altitude was 3,500' and I'm at sea level is my altimiter setting not 26.42? Confuseing!!! :dunno:I hope I'm makeing sence

 

[tgrayson]

Now as I understand it, presure varies directly with density and inversly with temperature. So if pressure and density are directly related, which stands to reason, the less densley packed the air, the lighter, hense less pressure there will be in that 1" column of air. So again, why when my density altitude was 3,500' and I'm at sea level is my altimiter setting not 26.42?

The pressure on the surface is the result of the weight of the column of air above the point where the pressure is being measured. Changing the temperature of the air doesn't immediately change the quantity of air above the surface, hence the pressure will not change. Long-term, however, some air will start to flow out of this column into cooler air masses, which will reduce the weight of air above the surface and you will see the pressure fall.

 

[unclenobby]

Just imagine altimeter setting of 29.92 over the Poles vs the Equator. Over the Poles 29.92 is caused by a compressed column of cold air, while over the equator 29.92 is caused by expanded column of air - both colums create an equal pressure on the surface, but the density is different.

 

[EnRoute]

Just imagine 29.92 over the Poles vs the Equator. Over the Poles 29.92 is caused by a compressed column of cold air, while over the equator 29.92 is caused by expanded column of air - both colums create an equal pressure on the surface, but the density is different.

Is the atmosphere of less vertical altitude over the poles? and if not, which I suspect, what happens to the weight of the air above the compressed column of air over the poles? Is it non existent?

 

[unclenobby]

Is the atmosphere of less vertical altitude over the poles? and if not, which I suspect, what happens to the weight of the air above the compressed column of air over the poles? Is it non existent?

The tropopause is at its lowest at the poles and highest at the equator. The pressure levels at the top of each column of air are more or less equal (like the surface pressure)

 

[jynxyjoe]

I love these threads.

 

[EnRoute]

The tropopause is at its lowest at the poles and highest at the equator. The pressure levels at the top of each column of air are more or less equal (like the surface pressure)

Thank you. You da man! :rawk: