As this air strikes the Cascade Range, just east of the city, it rises and cools. The moisture condenses on cloud droplets. The droplets grow and fall. The air continues over the Cascades. But now it's drier.
Much less rain falls on the other side of the mountains.
In Atlanta, a broiling summer sun heats up humid air near the ground, causing it to rise straight up. This rising column of hot air forms billowing cumulus clouds that may suddenly unleash heavy, local showers. Often, of course, rain occurs over a whole region. These storms are the result of a clash between large air masses. When warm and cold air masses collide, you can get a forced lifting of air over a large region, says Peggy LeMone.