Precipitation Definitions

Rain: Falling drops of water larger than 0.5mm in diameter. In forecasts, "rain" usually implies that the rain will fall steadily over a period of time. See "showers" below.

Light rain: Falls at the rate of 2.5mm or less an hour.

Moderate rain: Falls at the rate of 2.5mm to 7.5mm an hour.

Heavy rain: Falls at the rate of 7.5mm an hour or more.

Drizzle: Falling drops of water smaller than 0.5mm in diameter. They appear to float in air currents, but unlike fog, do fall to the ground.

Showers: Rain that falls intermittently over a small area. The rain from an individual shower can be heavy or light, but doesn't cover a large area or last more than an hour or so.

Snow pellets: White, opaque ice particles that form as ice crystals fall through cloud droplets that are below freezing but still liquid (supercooled). The cloud droplets freeze to the crystals forming a lumpy mass. Scientists call snow pellets "graupel." Such pellets falling from thunderstorms are often called "soft hail."

Hail: Falling ice in roughly round shapes at least 0.5cm in diameter. Hail comes from thunderstorms and is larger than sleet. Hailstones form when upward moving air -- updrafts -- in a thunderstorm keep pieces of graupel from falling. Drops of supercooled water hit and freeze to the graupel, causing it to grow. When the balls of ice become too heavy for the updrafts to continue supporting them, they fall as hailstones. Sleet, in contrast, consists of raindrops that freeze on the way down.

Thunderstorm: A rain or snow shower in which there is lightning. Thunder is always caused by lightning. In general, the upward and downward winds, updrafts and downdrafts, in thunderstorms are more violent than those in ordinary showers.

Severe thunderstorm: A thunderstorm with winds of 90 km/h or faster or hail more than 2cm in diameter reaching the ground. Severe thunderstorms can also produce tornadoes.

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