[chris (OP)]

Why is Death Valley so hot and dry?

[Tim the Plumber]

It's very low. Below sea level.

Temperature and pressure are sort of the flip side of the same coin. That is if the pressure increases the temperature of the gas will increase.

The temperature of the top of Venus's atmosphere is not all that hot but because the pressure is very high at the bottom it's extremely hot down there.

[alancalverd]

Elevation (or depression) has little to do with it. The dry adiabatic lapse rate is about 2°C per 1000 ft (no apology for mixed units - it's standard aviation terminology!) so teh lowest point of Death Valley will only be about 0.2°C hotter than its equivalent at sea level.

Death Valley is uniquely devoid of cloud, recording an average of 400 hours of sunshine in June-July, protected from the cooling ocean wind by mountains, and as dry as Antarctica, hence temperatures are determined only by radiation balance.

The fact that the highest temperature ever recorded was 100 years ago, and the average January temperature is pretty close to that of the UK, suggests that carbon dioxide has little to do with it either. Another blow to the AGW lobby!