Why Does the Southwest Get So Much Hotter Than the Rest of the Country?

Phoenix just broke its own all-time March temperature record. Martinez Lake, Arizona, hit 110 degrees in the middle of spring. Las Vegas set a new record for its hottest March day ever. Cities that typically do not reach 100 degrees until May are already hitting 100+ degrees in mid-March.

But even in a normal summer, the Southwest operates on a different temperature scale than the rest of the country. Atlanta and Phoenix sit at roughly the same latitude, yet Phoenix regularly hits 110 to 115 degrees while Atlanta peaks in the low 100s at most. What is going on?

Moisture is the Missing Ingredient

The single biggest driver of extreme Southwest heat is the near-complete absence of atmospheric moisture. When the sun heats the ground, the energy goes somewhere. In humid regions, a large portion of that energy is absorbed by evaporating water, a process that actually cools the surface. The eastern half of the country is full of water vapor, forests, wetlands, and soil moisture, all of which consume solar energy without raising air temperature.