A live cross-section of the United States’ two coasts. Air blows across the terrain, rises, cools, builds clouds and drops rain, then sinks warm and dry on the far side. Drag sideways to spin the model; switch coasts to see the difference.
Gray line: path of the air. White band: cloud. Blue bars: where rain falls.
Across the middle latitudes, the prevailing winds blow from west to east. On the West Coast that means ocean air slams straight into the mountains. On the East Coast the same wind blows from the continent out to sea, so the mountains get dry inland air, and the moist Atlantic air is mostly pushed away.
The Sierra Nevada and Cascades climb to around 14,000 feet and are geologically young and steep. The Appalachians top out at 6,684 feet (Mount Mitchell) and are hundreds of millions of years old, worn down into rounded ridges. Less lift means less cooling, which means less rain squeezed out.
West Coast ranges sit roughly 50 to 150 miles from the Pacific. The Appalachians sit behind a wide, flat coastal plain and the Piedmont, often 150 to 250 miles from the Atlantic. Even when ocean air does come inland, it rises slowly and gradually instead of all at once.
Eastern rain comes from passing storm systems, Atlantic nor’easters, and humid air flowing north from the Gulf of Mexico. Because it arrives from different sides at different times, no single slope is always the wet one, so rain spreads out fairly evenly at about 40 to 50 inches a year.
info It’s not zero in the East. The western slopes of the Alleghenies catch extra rain and snow, the Shenandoah Valley sits in a mild shadow behind them, and the southern Blue Ridge, where Gulf moisture runs uphill from the south, is among the wettest places in the eastern United States. The effect is real, just small and local compared with the West.
| Place | Side of the mountains | Roughly |
|---|---|---|
| Hoh Rain Forest, Olympic Mountains, Washington | Windward | 140+ inches |
| Sequim, Washington (about 60 miles away) | Rain shadow | about 16 inches |
| Western Sierra Nevada mid-slopes, California | Windward | 50 to 70 inches |
| Reno, Nevada | Rain shadow | about 7 inches |
| Death Valley, California | Deep rain shadow | about 2 inches |
| Washington, DC | East of the Appalachians | about 40 inches |
| Shenandoah Valley, Virginia | Mild shadow | about 35 inches |
| Southern Blue Ridge, North Carolina | Upslope from the Gulf | 80+ inches in spots |
The model above is a simplified teaching tool. It lifts one stream of air over an idealized terrain profile using standard cooling rates (about 9.8°C per kilometer for clear air, about 6°C per kilometer inside cloud) and real water vapor physics. Rain amounts in the model are relative, not forecasts. Real-world figures are rounded long-term averages.