Temperature and Pressure

[redblue]

So I read in a text book that the Poles tend to have higher pressure due to the colder air that sinks, and the equator tend to have lower pressure due to the hotter air that rises. Okay that part I understand.

Now, isn't it also that a hotter region with an expanded taller column of air will give you a higher pressure at sea level, compared to when it is not as hot (like during night time cooling)? Afterall, altimeter under-reads at higher temperature and we have to increase the altimeter setting, which means we are flying in a higher pressure region when we enter a higher temperature region.

So does hotter temperature produce lower or higher pressure?

[Mad (Flt) Scientist]

In the atmosphere, the answer is "it depends", because you can't just look at one section and treat it as an enclosed volume. So the simple increased pressure=increased temperature model doesn't apply.

[bookworm]

Try this mental model:

Imagine 3 columns of air that start off identical. The surface pressure is the weight of air (per unit area) of the entire column pressing down on the surface. When the middle part of the surface heats up, the column of air above it heats up and expands upwards. If the pressure at the surface stayed the same, the column would therefore be higher pressure at upper levels, and thus the air spills out into the neighbouring columns. Thus the surface pressure in the hot column decreases.


|+++|+++|+++|
| | | |
|+++|+++|+++|

|+++|
|+++| |+++|
| | | |
|+++|HOT|+++|

<-|+++|-> spills out
|+++| |+++|
| | | |
|+++|+++|+++|

|+++|+++|+++|
| | | |
| HI|LOW| HI|

[redblue]

Hello,

Um... once in a while I am told that I can't just look at one section of the model. What I find in aviation a lot of times, is that you really have to just take it from the text and just leave it at that. One can't really ask for any prove or derivation, much like what you'd do in a physics or math class. Coming from a heavy science background, I find this process to have to place a lot of trust on the author of the book, or the person instructing. Does anyone know of a good book where they'd actually provide good math models on aviation related issues (ie how slip stream would like to circulate around the fuselage on its own, and the temperature / pressure we just talked about)?