What Is an Atmospheric River?
An atmospheric river (AR) is a long, narrow corridor of moisture in the atmosphere, typically 400-600 km wide and over 2,000 km long. It carries water vapour from the tropics toward the poles, often producing extreme rainfall when it makes landfall.
How Much Water?
A strong atmospheric river can transport 7.5-15 times the average flow of the Mississippi River as water vapour. When this moisture hits mountains and is forced upward, it condenses into rain or snow — sometimes dumping 50-100mm in a single day.
The AR Scale
| Category | Strength | Impact |
|---|---|---|
| AR-1 | Weak | Beneficial rain |
| AR-2 | Moderate | Mostly beneficial, some risk |
| AR-3 | Strong | Balance of benefit and hazard |
| AR-4 | Extreme | Primarily hazardous |
| AR-5 | Exceptional | Catastrophic flooding risk |
Where They Hit
- Western North America: The "Pineapple Express" brings tropical Pacific moisture to California, Oregon, and British Columbia
- Western Europe: Atlantic atmospheric rivers bring heavy rain to the UK, Portugal, and Norway
- East Asia: Similar features affect Japan and Korea
Beneficial vs Destructive
Atmospheric rivers provide 30-50% of California's annual precipitation and are essential for filling reservoirs and building snowpack. But the strongest events cause devastating floods, landslides, and billions in damage.
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