Weather Watch

Guessing it rains more at night? You’re right

At our house, like many others, you can hear rain hitting the roof, especially 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 rain 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 dark hours of the night, with most of the cold fronts passing during the early morning hours, leaving behind 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 the Pacific Gas and Electric Company’s Diablo Canyon nuclear power plant weather monitoring data.

After researching rainfall patterns over the last 10 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 nighttime and only nine hours and 47 minutes of daylight at our latitude.

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 than the day that can be explained and not.

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 months when the marine layer can often produce heavy drizzle, especially during the night and early morning hours.

There have been a few times I have actually measured up to 2⁄100 of an inch 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 near 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 none 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.

This week’s forecast

A weak cold front moving down the California coast passed Diablo Canyon at 3 p.m. Saturday and produced light rain showers.

Most locations reported rainfall totals around 1⁄10 of an inch, except for the coastal mountains near Cambria, Cayucos and Morro Bay, which received a little over a third of an inch of precipitation.

Windy and clear weather is on tap today with high temperatures ranging between the mid- to high 50s throughout San Luis Obispo County.

A 1,027-millibar Eastern Pacific High will take a position about 500 miles to the west of San Luis Obispo later today.

This area of high pressure will produce moderate to fresh (13 and 24 mph) with higher gusting northeasterly (offshore) winds, especially in the eastern regions of San Luis Obispo, Cal Poly and the coastal canyons and passes Monday through Tuesday.

This offshore flow will give dry and clear weather with plenty of sunshine. However, the North County will once again have very cold mornings with temperatures reaching the low 20s. The northeasterly (offshore) winds should keep areas on the south side of the Cuesta Grade above freezing.

Wednesday will be fair with warmer temperatures.

A very wet weather pattern is forecast to develop for Oregon and Northern California as the Gulf of Alaska’s low pressure system taps the tropical air north of Hawaii.

Most of the activity will remain north of San Luis Obispo. However, the tail end of the first system moving through Northern California will produce increasing clouds and a chance of rain Thursday.

Another system will produce rain over Northern California on Friday with another chance of rain reaching our area next weekend.

Surf and sea report

Strong to gale force (25 and 38 mph) northwesterly winds will generate an 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 12-second period) today.

As the winds shift out of the northeast (offshore) Monday, the northwesterly sea and swell will decrease to 5- to 7 -feet, further lowering to 3 to 4 feet (with an 8- to 11-second period) Tuesday.

Increasing northwesterly winds will generate 3- to 5-foot northwesterly (320-degree deep-water) sea and swell (with a 4- to 11-second period) Wednesday.

A roughly 6 foot northwesterly (300-degree deep-water) swell (with a 9- to 12-second period) is forecast along our coastline Thursday through Friday.

This morning’s long-range charts are still not indicating any high-energy swell events over an extended period. Seawater temperatures will range between 53 and 55 degrees through Monday, increasing to 54 and 56 degrees Tuesday and remaining at this range through Friday.

Conservation tip

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John Lindsey is a media relations representative for PG&E. He’s a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail