Weather Watch

Rain, drizzle, mist — what do we call this precipitation on the Central Coast?

A driver of an SUV takes a risk Friday crossing a flooded San Luis Creek in Avila Valley.
A driver of an SUV takes a risk Friday crossing a flooded San Luis Creek in Avila Valley. Special to The Tribune

So far this year — as of Saturday to be exact — the Diablo Canyon nuclear power plant rain gauge has recorded more than 18 inches.

That’s the most rain recorded through Jan. 21 since the 22 inches that fell during the 2004 rain season.

In all, the 2004 rain season created nearly 36 inches of precipitation. On average, Diablo Canyon receives about 22 inches of rain each season. All this wet stuff has led to numerous questions about the differences between mist, drizzle, rain showers and rain.

Liquid precipitation is classified by the size of its water drops and the rate at which they fall.

When the marine layer comes rolling in off the Pacific, it is composed of stratus clouds. When those clouds reach the ground, we refer to them as fog. Usually, the stratus clouds are relatively thin and have leisurely upward-moving air currents.

Under these conditions, water droplets have little time to grow. They become too heavy for the weak air currents to support, and they fall to the ground as drizzle. Drizzle is defined as water drops with diameters less than 0.02 inches. That’s very small. Mist water drops are so tiny they remain suspended in the atmosphere.

Heavy drizzle occurs when visibility is less than one-fourth of a mile. Moderate drizzle happens when visibility ranges between one-fourth and one-half mile. And light drizzle occurs when visibility is greater than one-half mile.

The marine layer is a crucial survival factor for many of the native plant species along our coastline. The blanket of coastal fog adds moisture to the plants via the condensation of water on the plants and soil. The low marine clouds also increase the humidity of the air, lowering the plants’ evaporation rate.

Rain is composed of water drops with diameters greater than 0.02 inches.

Rain falls along the Central Coast when cold fronts move down the California coastline, producing rapid, upward-moving air currents. These upward air currents keep the water droplets suspended in the air column, where they combine and grow. Raindrops can reach sizes of up to 0.25 inches before they fall to the ground.

The intensity of rain is based on the amount that falls in one hour. Light rain is classified as 0.1 inch or less per hour. Moderate rain ranges from 0.11 to 0.3 inches per hour. Heavy rain is greater than 0.3 inches per hour.

This leads to the question of the difference between rain and rain showers.

Rain is persistent precipitation over a large area associated with a warm or cold front. In other words, a rain episode lasts for a longer time.

On the other hand, rain showers occur in a much smaller area of coverage and usually have an abrupt start and end and are often associated with convective types of clouds caused by cold and unstable air that often filter in behind cold fronts.

Sunday’s storm potentially could be the strongest we’ve seen in years. Wind gusts could reach 60 mph along the coastline, with heavy rain throughout the day. These winds could topple over trees that are positioned in saturated soils and create power outages.

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Prepare for power outages: Keep a battery-operated flashlight and radio within easy reach. Ensure those items are always accessible and that your batteries are fresh. Listen for updates on storm conditions and power outages. Use safer LED candles. Wax candles are not recommended. Plan for another way to communicate rather than depending on a phone that requires electricity to communicate. Keep a standard handset or mobile phone ready as a backup. Store water-filled plastic containers in your freezer. You can use them as blocks of ice to prevent food from spoiling. Avoid flooded locations and areas with downed trees. Both are typical places for downed power lines to occur. Remember, call 911 first to report downed lines.

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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