A bit embarrassing but doesn’t click in my head. I have a BS in mechanical engineering and did pretty well in thermodynamics, aerodynamics, and heat transfer. I understand the pressure and temperature relationship of gases.
But when it comes to the weather, why does high atmospheric pressure cause hot temperatures? I understand the vice versa because gases expand (and raise to the top) when they are hot. So warm weather results in higher atmospheric pressure.
But why does high pressure cause hot temperatures? I would think hot air raises therefore the hot air should (almost always) be at the top of the atmosphere, leaving nice cool air for us at the bottom.
Also, it’s cooler the higher you go in altitude. But the air gets “thinner” as you go up.
P.S. I hate the heat.
In: Planetary Science
You’re actually pretty close, the key is that pressure temperature relationship you mentioned. Yes warm air rises, but the pressure drops as the air rises, which means the air will expand as it rises. Expanding air cools, which means air will cool from expansion as it rises. This means the air higher up, even if it is colder in an absolute temperature sense, could still be warmer than the lower down air once you lift that lower down air up.
E.g. you have an air parcel at the surface with a temperature of 20°C and the air aloft is 10°C. When you lift that air parcel to the same level as the 10°C air, it has cooled to 5°C, so it is colder than the 10°C air and sinks back down (numbers made up, couldn’t be bothered digging out the actual lapse rate and putting heights on it)
You can use the idea of potential temperature to assess if an air parcel would rise or sink when it’s moved up or down. It accounts for the pressure changes and gives air parcels a single temperature you can directly compare regardless of pressure
The reason high temperatures are associated with high pressure is that there is a lot of sinking air associated with high pressure. This means that the air is getting compressed as it sinks and warms up from that same pressure temperature relationship. Plus you tend to have clearer skies and sunnier days which helps a lot
The PV=T you are familiar with is for closed systems and idealized scenarios.
The air at altitude isn’t less dense because its expanded by heat (although heated air will rise up and join it). Its less dense because it isnt being compressed. The air near the ground is being compressed by the air sitting on top of it.
You’ve also got the weather pressures reversed. Cold air masses are high pressure, hot air masses are low pressure.
Here’s a link that covers the subject as it relates to weather.
https://www.weather.gov/lmk/basic-fronts
The simplest explanation is that if you throw a ball upwards in the atmosphere it slows down. Temperature is a measure of kinetic energy, so it drops.
Mathematically energy is thermal energy (specific heat*temperature=29.19 J/mol/K*T) plus potential energy (m*g*z~0.029 kg/mol *9.8 m/s^2) -g/c_p is then roughly -0.01 K/m or -10 K/km.
The pressure explanation is also correct but harder to work out mathematically.
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