Monsoons occur in the deserts of the American Southwest from early-July to mid-September. Moisture flows into this region from the Gulf of Mexico, Gulf of California, and the eastern Pacific. Although clear skies may prevail in the morning, surface heating pushes the moisture aloft, creating towering cumulous clouds and severe thunderstorms by early afternoon. These storms begin to dissipate at sunset, but some can persist until late evening. Sometimes there is enough moisture left on the surface to create early morning fog.