Weather Watch

Does it rain more at night and on weekends?

Heavy rain at PG&E’s Energy Education Center in Avila Valley on Friday.
Heavy rain at PG&E’s Energy Education Center in Avila Valley on Friday.

At our house, like many others, you can hear rain hitting the roof especially well at night when all is quiet except for the wind howling through the trees and the occasional car tires rolling across the wet streets.

If it rains hard, you can hear the droplets overflowing our roof gutters and hitting the heater air vent on the side of the house.

So far this rain season, most of the rainfall in San Luis Obispo County has occurred during the night with most of the cold fronts passing during the early morning hours — leaving sparkling clear skies in their wake.

Many have asked whether it rains more at night than during the day.

A reliable source of answers to questions such as this is Diablo Canyon nuclear power plant’s weather monitoring data. After researching rainfall patterns over the past 15 years, I discovered that we do receive more rain during the night than during the day.

Of course, the nights are longer during our rainy season (November through April), with the first day of winter having the longest night at 14 hours and 13 minutes of darkness and only nine hours and 47 minutes of daylight in San Luis Obispo County.

As the winter progresses, the days become longer until the first day of spring, when daytime and nighttime are about equal, or about the date that the Central Coast transitions from the wet to dry season.

So from a statistical standpoint, rain is more likely simply because the nights are longer than the days. However, there are other reasons why we get more rain during the night.

One factor is the top of the clouds cool during the night, allowing the air mass to reach its dew point more readily and produce greater amounts of precipitation — drizzle, rain, hail or snow.

This also takes place during the summer, when the marine layer can often produce heavy drizzle, especially during night and early morning hours. There have been a few times I have actually measured up to .02 inches of precipitation during a summer night.

For reasons I can’t explain, there seems to be a greater occurrence of frontal rain passages over the Central Coast about 9 p.m., with another peak about three hours before dawn.

I’ve asked many meteorologists and climatologists for their opinion on this local phenomena and not one was able to provide me with a definitive reason.

This may have to be left as a weather mystery, along with why it mainly seems to rain during the weekends.

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The last major El Niño storm season in 1997-98 created widespread flooding and caused power outages impacting more than 1 million PG&E customers. PG&E urges its customers to be ready for natural disasters. That includes having a family emergency plan and keeping emergency kits for your home, your office and your vehicle. PG&E offers emergency-preparation tips on its website at www.pge.com/en/safety/preparedness/index.page.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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