Why is there always low pressure over Las Vegas, but they don't get any rain during the summer?

Question

Whenever summer comes around, I always see a low pressure over Las Vegas. But how come there isn't rain right there all during that time? Or why isn't there rain in California?

Answer 1

A low pressure trough often forms from the Las Vegas area and down along the Colorado River Valley and ends at the gulf. This is called a thermal trough and is created by the surface heating in the desert. As the surface heats it heats the air about it and raising air causes low pressure. The opposite is sinking air creates high pressure. Two things are needed to creat rain. One is moisture and two is instability. You have one in the low pressure and raising air (unstable) but you don't have the moisture. When the air is moving in from the west it is very dry as the moisture is removed as it passes over the mountain chains between LV and the California coast. When the air is coming from the east (usually associated with the rotation around a high pressure ares) it is moist gulf air and you will get thunderstorms around the Vegas Valley. Moisture plus raising and unstable air causes this.