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vicleecy
31st Jan 2005, 05:40
Hi flyboys,

I understand the relationship between Pressure, Density, and Height.
But sometimes I may have some funny questions in mind.

Temp high-->Density low-->Pressure low
Temp low-->Density high-->Pressure high
Alt high----> Density low-->Pressure low
Alt low----> Density high-->Pressure high

Let's look at
Temp high-->Density low
Alt High----->Density low

BUT
Alt High-->Temp low, the pressure is low according to above. Alt High-->Density Low, didn't we just say cold temp brings out higher density?? Also, what about Temp low-->Pressure high?? Sometimes I got confused here. Can anyone explain this? Is there anything relating to tropopause?


For instance today;s weather is 15'C and pressure is 1013, can we say it will always be 1013 at 15'C and then draw a temp-pressure graph? Say 20'C for 1000, 30'C for 950, just for example. Of course we know it isn't correct, so what other factors are affecting the temp-pressure relationship?

I understand it can be -10'C in US right now but also -10'C at high altitude.

Getting confused..

EvoIV
31st Jan 2005, 08:43
AIUI, air temperature relates almost exclusively to the qualities of the air mass that is being measured.

An air mass is a parcel of air sharing relatively uniform properties. The temperature of the air mass is mostly affected by where it came from, how far it has travelled and what it has travelled over - arctic air masses will start off cold but this can be moderated by passing south over warm seas for example.

In this instance, very small pressure differences (in the scheme of things) doesn't have a massive difference on the temperature of the air. The air isn't in a solid container so it can never be exposed to the compressing forces necessary to alter the temperature by any meaningful amount. Meteorologically speaking areas of high pressure or low pressure really are referring to airmasses which are either sinking or rising.

But I am by no means an expert so it's all probably rubbish.

ft
2nd Feb 2005, 08:06
While I’m not exactly sure about the source of your confusion, I think you are taking the relations a bit too far?

While low pressure works towards a higher density altitude, there’s nothing saying the pressure will be low if the density is low. You can have warm air at a high pressure and low density. The pressure depends more on the current weather pattern than anything else (except for altitude, obviously).

There’s also no direct relationship between altitude and temperature. Yes, the temperature of a rising parcel of air will drop with falling pressure (lapse rate) and the temperature will (typically) drop as altitude increases… but that’s only in one air mass at one time. Nothing says that it has to be colder at point A than at point B since point A is at a higher altitude. It is, however, perfectly possible and not uncommon to have a higher temperature at a higher altitude (inversion).

What is fixed is that the pressure will decrease with increasing altitude. The rate at which the pressure falls off with altitude is regulated by the air density though… which depends on temperature.

Perhaps it will be easier to sort out if you give us an idea of where you are going? What are you trying to sort out, what's the application?

Old Smokey
5th Feb 2005, 04:55
vicleecy,

Extracting 2 of your statements, with following comment which might ease your confusion -

.....didn't we just say cold temp brings out higher density?? Also, what about Temp low-->Pressure high?? Sometimes I got confused here. Can anyone explain this? Is there anything relating to tropopause?

Firstly, Density is in direct proportion to the Pressure divided by the Absolute Temperature. As Altitude increases, Pressure Decreases, and therefore Density decreases. Below the Tropopause, temperature also decreases with increasing Altitude, BUT AT A SLOWER RATE THAN DOES THE PRESSURE. Thus, below the Tropopause, there is a significant degree of "Density Recovery" (my term) due to decreasing temperature, such that the Density Lapse Rate is less than if there were no corresponding decreasing Temperature. Above the Tropopause, Pressure continues to fall, but Temperature is constant, and Density now decreases at a much higher rate with increasing Altitude, as there is no "Density Recovery". This is very significant to Jet aircraft operations, where performance falls off very quickly after the Tropopause.

So, to go back to your quote, NO, cold does not bring out higher density with increasing Altitude, it REDUCES it's rate of decrease. Yes, as stated, the Tropopause has a significant effect upon density with increasing Altitude, Density now declines MUCH faster.

I understand it can be -10'C in US right now but also -10'C at high altitude.

This is normal in both Cold and High surface temperatures. The lower layers of the atmosphere (say, up to 5000 feet) are significantly affected by surface temperature, whilst the higher levels (say, above 5000 feet) are in accordance with the properties of the "greater air mass". If, for example the Canadian Winter "greater air mass" was ISA-10°C at altitude, but -10°C at the surface, we would initially encounter -10°C at commencement of climb, but, as we climb away from the surface influence, the temperature will be more in line with the upper air mass. In my example, at 5000 feet and ISA-10°C, the temperature will now be -5°C, WARMER than the surface, and not return to surface temperature values until 7500 feet. Conversely, in Tropical Desert regions, we typically see surface temperatures around 45°C, i.e. ISA+30°C due to surface influence, but stabilising at ISA+15°C at higher levels away from the surface influence. If we were to draw a graph showing the Temperature Lapse Rates for several atmospheric temperature profiles, we would see a bulge at lower levels (due to surface influence) until stabilising at values typical of the "greater air mass" at higher levels away from surface influence.

The Tropopause is very high in the Tropical regions (about 55000 feet), and very low in the arctic regions (somewhere in the twenty thousands). As the atmosphere continues to cool all of the way to the Tropopause, temperatures at high level, say, 43000 feet (about the "max" for most airliners) are very low in the tropics, and performance is good. Conversely, high level temperatures in the Arctic are very high, and the same airliner would suffer considerable performance degradation there as compared to the tropics. I see this every day.

I hope that all of this helps to clarify the situation for you, and not muddy the waters.

Regards,

Old Smokey