why do fall winds always warm as they descend the lee side of a mountain
Downslope winds occur on the leeward side of mountain barriers when a stable air mass is carried over the mountain by strong winds that increase in strength with height. Moisture is removed and latent heat released as the air mass is orographically lifted. As the air mass descends, it is compression heated. The warm foehn wind, locally known as the Chinook wind, Bergwind or Diablo wind or Nor'wester depending on the region, provide examples of this type of wind, and are driven in part by latent heat released by orographic-lifting-induced precipitation.
Jab -- please post the source of your information.
The phenomenon of fall winds warming as they descend the lee side of a mountain is known as the Foehn effect, or simply, Foehn winds. These winds occur when moist air is forced to rise up one side of the mountain, cools, and then descends down the other side, warming in the process.
The primary reason for this warming is adiabatic heating. As the air is forced to rise up the windward side of the mountain, it is subjected to lower atmospheric pressure at higher altitudes. As the air moves up, it expands and cools due to the decrease in pressure. This process is called adiabatic cooling.
When the cooled air reaches the top of the mountain, it has already lost much of its moisture in the form of precipitation. As the dry air begins to descend down the leeward side, it gets compressed due to increasing atmospheric pressure. As the air pressure increases, it also gains heat energy, causing it to warm up. This process is called adiabatic heating.
The combination of adiabatic cooling and adiabatic heating leads to a net warming effect for the air as it descends the lee side of the mountain. The warming effect can vary depending on the initial moisture content of the air, the height and shape of the mountain range, and other local meteorological conditions.
So, to summarize, fall winds warm as they descend the lee side of a mountain due to adiabatic heating, which occurs as the compressed air gains heat energy while moving from higher to lower atmospheric pressure.