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Published: Sunday, Jul. 12, 2009

John Lindsey: Size of drops distinguishes drizzle from rain

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On a gloomy and drizzly morning last month — a classic “June gloom” condition — King Harris of KVEC asked about the difference between drizzle and rain.

Scientifically, they are both forms of precipitation and are classified by the size of the water drops and the rate at which they fall.

When the marine layer comes rolling in off the Pacific, it is generally comprised 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.

Heavy drizzle is defined as occurring when visibility is less than one-fourth of a mile. Moderate drizzle occurs 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 our 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,” said Sally Krenn, a PG&E biologist. “The marine low 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.

Generally, 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 0.10 inch or less per hour. Moderate rain ranges from 0.11 to 0.30 inches per hour. Heavy rain is greater than 0.30 inches per hour.

An upper-level trough passed over Diablo Canyon Power Plant on Feb. 16. Rainfall rates at one point reached 2.2 inches per hour. This high rate of rainfall only lasted for a few minutes. Consequently, the total rainfall for that day was only 0.67 inches.

On the other hand, Mumbai (formerly Bombay), India, recorded a record-breaking 37.1 inches of rain on July 26, 2005. The region received more rain on that date than we have received over the past three years combined because of our ongoing drought.

This week’s forecast

An upper-level low off the coast of Oregon pulled up moisture from the remnants of former tropical storm Blanca and produced scattered rain showers in the Bay Area on Friday.

A ridge of high pressure over our area will continue to produce above-normal temperatures, especially in the interior, through Monday.

Temperatures will range between the high 90s and the low 100s in our inland areas; the high 70s to the low 80s in the coastal valleys; and the mid- to high 60s at the beaches today.

The exceptions will be Avila Beach and Cayucos, which will reach the high 70s.

Most of the inland locations will reach the low 100s on Monday.

The marine layer will burn off rather rapidly from the coastal valleys later this morning, and from the beaches by the afternoon hours. The marine layer will redevelop along the shoreline this evening and will move into the coastal valleys by tonight. Little change in this pattern is expected through Monday.

The high-pressure ridge, responsible for the above-normal temperatures, breaks down on Tuesday. This condition will produce a minor cooling trend, decreasing winds, and a deeper and more persistent marine layer, along with morning drizzle, Tuesday through Thursday.

For the following week, just in time for the Mid-State Fair, another high pressure ridge is expected to produce hot inland temperatures.

Surf forecast

This morning’s 4- to 5-foot, 320-degree, deep-water sea/swell with a 5- to 9-second period will continue at this height and period through Monday.

Note: The sea/swell will be higher at the offshore buoys.

The wind fields will shift northward and a 3- to 4-foot northwesterly (300-degree deep-water) swell (with a 7- to 11-second period) will develop along our coastline on Tuesday and will continue at this height and period through Thursday.

Increasing northwesterly winds and sea/swell are forecast along our coastline on Friday and Saturday.

Arriving from the southern hemisphere: The longer-range models are still indicating vigorous storm activity in the southern hemisphere, but the wind fetches are mostly directed toward the coast of Central and South America, not California.

John Lindsey is a communications representative and meteorologist for Pacific Gas and Electric Company in San Luis Obispo. Visit www.wecandothis.com to learn energy-saving tips from PG&E. These tips will help protect the environment and your wallet. If you have a question, e-mail Lindsey at pgeweather@pge.com.