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