What is the difference between drizzle and rain? It’s all in the drops

“She awaits the rain like a writer embraces metaphors, A drizzle isn’t for the child who dances in the storm. Of rain that washes away the petrichor it brings, A downpour of a hail of bullets, and she calls it spring.” ― Sanhita Baruah

This spring has not only seen well above average amounts of rain, but also many mornings of drizzle.

Last Monday morning, April 13, the rain gauge at Diablo Canyon Power Plant recorded 0.05 of an inch of drizzle, the most I’ve ever seen. Last Thursday, Dann Cianca, chief meteorologist at KION-TV in Monterey County, posted drizzle totals. Carmel reported 0.03 of an inch.

Earlier this month, while on top of the Santa Lucia mountain range near the Cuesta Grade, gusty Santa Lucia (northeasterly) winds produced clear skies along the shoreline, but clouds, fog, and heavy drizzle along the ridgeline. I became drenched while checking a weather station on Black Butte.

In the Pacific Northwest, especially in the coastal regions, winter seems to be one long drizzle. This often leads to the question, “What is 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 it 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 a 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.

Even though it may be just fog or drizzle, 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.

The marine low clouds also increase the humidity of the air, lowering the plants’ evaporation rate.

A fantastic amount of water transpires from the leaves of plants. A single large oak tree in our county can move 100 gallons or more of water into the air in just one day during the summer; an acre of corn gives off about 3,000 to 4,000 gallons of water each day.

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 February 16, 2016. Rainfall rates at one point reached 2.2 inches per hour; however, this high rate of rainfall only lasted for a few minutes. Consequently, the total rainfall for that day was only 0.67 inches.

If a storm produces just 1 inch of rain over 1 square mile, that adds up to about 17.4 million gallons of water.

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 in one day than many Central Coast locations typically get over the course of two to three years.

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