When your favorite FOX Weather meteorologist says, "Expect moderate rain over D.C. tonight," what do they really mean? Do you need a hoodie, an umbrella or a boat? Light, moderate and heavy rain has precise definitions.
First, rain has to be a droplet more than 0.0196 inches in diameter. Any smaller and that is defined as drizzle.
Rain intensity is the rate of rain per hour as measured by a rain gauge which can be as simple as measuring rain collected in a cylinder per 6 minutes or per hour.
- Light Rain: 0.01 to 0.10 inches per hour or 0.10 inches in 6 minutes
- Moderate Rain: 0.11 to 0.30 inches per hour or 0.30 inches in 6 minutes
- Heavy Rain: 0.30 inches or more per hour or in 6 minutes
Keep in mind, just because the rain is described as light doesn’t keep it from being dangerous. Light rain will wet the detergents deposited from gasoline and cause the accumulated oil from the roads to float, making for a slippery surface for vehicles to travel. Visibility also reduces the heavier the rain is. That’s why many states have laws requiring low-beam headlights in the rain.
Even moderate rain can cause ponding on the roadway if sewers are clogged or the ground is already saturated from previous rains.