# Temp. and air pressure

1. Aug 26, 2004

### 7bear

Higher the temperature, lower is the air pressure.
Lower the temperature, higher is the air pressure.

Are they right? If yes, why?

2. Aug 26, 2004

### cepheid

Staff Emeritus
In general, I think that this is correct. A warm air mass will rise and expand. When this is happening over you, the local atmospheric pressure is therefore lower. I'm guessing it's simply because you have a less dense air mass over you. Therefore it weighs less and exerts less pressure. Come to think of it, it's like suddenly having fewer air molecules per unit volume. Just as if this were happening in a container, the pressure would decrease.

Likewise, a cooler air mass will sink and contract, becoming more dense. I'm quite sure that when that happens in your area, it's referred to as a high pressure system.

Is my assessment correct?

3. Aug 27, 2004

### expscv

is this in a test tube, or the whole world?

you should give more deatail , casue this will leads to several answers

4. Aug 27, 2004

### ArmoSkater87

Actually, the exact opposite is true about what you said. First of all, pressure increases with temperature, and increasing pressure or temperature causes density to go down. Hence why hotter air rises and cooler air sinks. In fact a "high pressure system" that they refer to in forcasts always have higher temperature, and low pressure systems always have lower temperature. According to the Ideal Gas Law....

$$PV = nRT$$

Where P = pressure, V = volume, n = moles, R = ideal gas constant, and T = temperature
As you can see, when pressure increases if volume and moles is kept constant, then temperature must increase as well.

You can manipulate the equation to see the effects on density...

$$n = \frac{m}{MM}$$ Where m = mass, and MM = molar mass

$$PV = \frac{mRT}{MM}$$

$$P = \frac{DRT}{MM}$$ Where D = density

$$D = \frac{P(MM)}{RT}$$

From this, it is clearly seen that if pressure is kept constant, increasing temperature must make density decrease

Actually, it is much easier to see the relationship between P, V, and T by using the Combined Gas law...

$$\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}$$

See for yourself by pluging in numbers.

Last edited: Aug 27, 2004
5. Aug 27, 2004

Actually, if you look at weather forecasts, its the other way around.

6. Aug 27, 2004

### cepheid

Staff Emeritus
Yeah, actually. I'm really confused about this after reading everyone's answers. I admit I must have been wrong, but what I'd like to know is what's meant by high pressure and low pressure weather systems.

Classroom explanations are kind of weird because they talk of an "air mass" being warmed, expanding, rising, and cooling. I'd also like to know: if we're surrounded by a sea of air, what differentiates one particular air mass, and how can it "expand" without displacing air around it?

7. Aug 27, 2004

### K.J.Healey

Thats the thing, it DOES displace those around it, hence "wind".

Imagine the air as a bunch of balloons filled up that can be compressed or inflated, heated (wont melt), but lets keep the AMOUNT of air particles "n" the same. We then have idea gas law :
PV ~ T

So if you increase the temp of one balloon, for its pressure to remain at equillibrium it increases its volume. This forces all the surronding balloons to shrink, which make them warmer, because as V drops, P increases and so does T. See, everything is trying to be in equillibrium, heat and pressure, so it balances itself out. By transfering heat to a colder balloon, the pressure in the hot balloon drops. Really its fluid mechanics of the changes of pressure, where if you dont know applied mathematics for fluid dynamics its hard to understand.

Also remember wind is mainly from bernoulli's principle, that theres a force normal to a pressure gradient. This also means theres a force(in air) to a temperature gradient. Basic thermodynamics.