Weather Watch

When taken collectively, raindrops pack a punch

Raindrops are tiny — typically less than 5 millimeters in diameter — and come in a lot of different sizes and shapes, but collectively they can deliver a huge amount of water.

As they fall from the sky, their journey has only begun. Typically, as they rush through the air they often flatten into a saucer shape and later contour into an upside-down bowl profile before bursting into smaller droplets.

If a storm produces just 1 inch of rain over 1 square mile, that adds up to about 17.4 million gallons of water. The same inch of rain delivered over the entire County of San Luis Obispo would total about 63 billion gallons. Now consider the storm that occurred on March 9 and 10, 1995, which produced recorded 24-hour rainfall totals ranging from 3.4 inches in Pismo Beach, to 11.6 inches in Santa Margarita. That’s a lot of water indeed!

So where does all this water go? If the rainfall is light, say less than a tenth of an inch, most of it evaporates back into the atmosphere, from which it can return another day.

Heavier amounts of rain will soak into the ground, where it’s absorbed by plants and then transported back into the atmosphere by transpiration. An amazing 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.

During heavier storms, a lot of the water is absorbed by the ground and goes through a process called infiltration. This filtered water recharges aquifers, which can later be pumped from wells and used for industrial, agricultural and residential use. This water also seeps into creeks and streams and provides a base flow. However, impervious surfaces such as paved highways, parking lots, and roof tops, even during light rain events, can produce storm runoff which can carry oils and other pollutants into creeks, lakes and eventually the ocean.

If the ground is saturated after a series of storms, much of the new rain runs off, adding to the flow of streams.

In hydrology there is a concept called “time of concentration.” Basically, it is the time needed for a rain drop to fall on the highest point in the watershed and make its way to the watershed outlet.

Jill Ogren, a registered civil engineer with Hydraulic Planning of San Luis Obispo County Public Works Department, calculated that a raindrop falling in the San Luis Creek headwaters at the top of Cuesta Grade would take about 3.6 hours to reach Avila Beach. The time of concentration in Arroyo Grande Creek for a raindrop to go from the furthest point in the Lopez Watershed through the Lopez Reservoir and then down the creek to the ocean would be approximately 10.5 hours. This information is vital in determining the time for a creek or stream to reach its peak flow and aids in flood preparation efforts in the county.

According to the United States Geological Survey, an average of some 70 percent of the annual precipitation returns to the atmosphere by evaporation from land and water surfaces and by transpiration from vegetation. The remaining 30 percent eventually reaches a stream, lake, or ocean.

Today’s weather report:

An upper-level trough will produce mostly cloudy conditions with areas of fog and drizzle this morning. Skies will turn partly cloudy this afternoon with temperatures mostly in the low 60s throughout San Luis Obispo County.

Another very weak cold front, like many others this year, will move down the California coastline tonight into Monday. This system will produce increasing clouds with areas of drizzle and a couple of light scattered rain showers late tonight through Monday afternoon.

High pressure rebuilds into the Central Coast on Monday night and strengthens through the week. This condition will produce offshore winds and warm and dry weather, especially by the latter part of the week. The coastal valleys could see temperatures approach the high 70s by Wednesday and Thursday.

Moderate gale to fresh gale force (32- to 46-mph) northwesterly winds along the coastline next weekend will produce cooler temperatures along with night and morning low clouds and fog.

Today’s surf report:

Today’s 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 15-second period) will decrease to 7 to 9 feet (with a 7- to 14-second period) tonight.

A 6- to 8-foot northwesterly (300-degree deep-water) swell (with an 11- to 14-second period) is forecast along our coastline Monday, decreasing to 5 to 7 feet (with an 11- to 13-second period) Tuesday.

Another 6- to 8-foot northwesterly (310-degree deep-water) swell (with an 11- to 17-second period) is forecast along our coastline Wednesday and remaining at this height but with the gradually shorter period through Friday.

Moderate gale to fresh gale force (32- to 46-mph) northwesterly winds will generate a 10- to 12-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) Saturday through next Sunday.

The National Weather Service will conduct Weather Spotter Training at the PG&E Energy Education Center this Thursday at 6:30 p.m.

The National Weather Service Storm Spotter Program is a volunteer organization of people with an interest in the weather. Whenever significant weather is occurring, weather spotters call in a report to the National Weather Service. These reports provide important ground truth for forecasters.

This training session is free and open to the public. Anyone interested in volunteering to become a storm spotter for the National Weather Service is welcome to attend.

John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, send him an email at