Temperature and Pressure
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.. |
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. |
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? |
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? 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. 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 |
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