When it rains

Has it seemed lately like it always starts pouring around five or six o'clock?

This past June saw more rainfall than any June on record in Philadelphia. And July is starting off with even more precipitation. So we wondered: When is it most likely to rain in this city, and at what time of day?

The Philadelphia Water Department maintains a network of 24 rain gages spread throughout the city. We analyzed data collected by each gage and looked at the total amount of rain for each hour of each month, from 1990 to 2013.

PWD Rain Gage Network

As it turns out, the hours with the most rain are during the summer in the late afternoon and into the early evening.

Why is this the case? According to Patrick O’Hara, a meteorologist with the National Weather Service in Mount Holly, New Jersey, thunderstorms, which often bring heavy rain, are more common in the summer months due to warmer and more humid air rising from the ground throughout the day to form thunderstorm clouds.

Thunderstorms usually last for about 30 to 45 minutes and span four to six miles in diameter. According to NOAA’s National Severe Storms Laboratory, as many as 40,000 thunderstorms can occur each day around the world.

Note: During times of snowfall, the precipitation amounts recorded by the rain gages do not represent the amount of snowfall. Once the snow melts, the rain gages record the amount of melted snow in the gage.