As summer thunderstorms breakout in the southwest US, you frequently hear them described as “monsoons.” And the image is of heavy rains – the type that are common in Asian monsoons. In truth, “monsoon” refers to a seasonal wind pattern. The name, derived from an Arabic word meaning season, was first applied to the winds over the Arabian sea. Six months of the year, the winds come from the northeast – the other six months, the winds come from the southwest. The primary cause is that large land areas heat more rapidly than the nearby oceans. During the winter, it is the oceans that retain the heat. These temperature gradients fuel the wind, shifting direction as the warmest region shifts. In the Southwest US, monsoon season usually runs from June to October.
Mary Knapp, Weather Data Library