Weather Watch

Frost forms in fair conditions

The clear and still nights over the past week produced near perfect stargazing conditions. After the bright glare of last week’s moon descended below the horizon, the Milky Way resembled a backbone stretching across the night sky.

This dry December weather pattern has also produced frigid nights over the past week. Farmer Bill at Windrose Farm near Creston reported temperatures as low as 15 degrees.

Ironically, the atmospheric conditions responsible for these cold temperatures during winter are usually associated with large high-pressure systems that produce fair weather.

Closer to the ocean, overnight lows have remained mostly above the freezing level, but frost has still developed. On these clear and calm nights, roofs, cars and other things near the Earth’s surface cool rapidly by emitting infrared radiation and actually become colder than the air surrounding them. In other words, your thermometer may indicate air temperatures above freezing, but your car windshield is shrouded in ice as you get ready to drive to work or school.

As the warmer air comes in contact with these cold surfaces, like blades of grass or leaves on a tree, it eventually cools to its dew-point temperature. If the temperature of the air immediately surrounding the object should drop to 32 degrees or below, the dew will freeze, becoming tiny beads of ice called frost.

The dew point is the temperature at which air becomes saturated. At that point, the air can no longer hold all of its water vapor. Some of it condenses into water or ice, as dew or frost. Thus dew point is simply the temperature at which dew forms.

Usually, the air temperature can never be colder than its dew-point temperature. When the dew-point temperature and air temperature are the same, the relative humidity is at 100 percent. The higher the dew-point temperature or the greater the relative humidity at freezing or below, the greater the amount of frost that will accumulate.

Frost is more likely to develop on clear and calm nights than on nights that are windy and cloudy.

When conditions are calm, denser cold air flows downward along mountain slopes and accumulates in the valleys. Even when the land is only gently contoured, the cold air will accumulate in the low-lying areas. Often valley floors will be much colder than the surrounding hillsides. The long and clear nights this week have allowed more of the atmosphere’s heat at the surface to radiate out in space.

On the other hand, if the skies are overcast, the clouds act like a blanket keeping the temperatures warmer. If it’s not calm, the winds will mix the cold air at the surface with the warmer air above, giving warmer temperatures.

Recent cold temperatures have been especially difficult for the homeless. If the overnight temperature drops to 32 degrees or below in San Luis Obispo, the Prado Day Center opens as a shelter for the homeless from the overnight cold.

On Dec. 21, the Homeless Services Oversight Council of San Luis Obispo County will mark National Homeless Persons’ Memorial Day with a candlelight vigil from 5:30 to 6:30 p.m. at Mission Plaza in San Luis Obispo on the longest night of the year and the beginning of winter. United Way of San Luis Obispo County, the fiscal agent for Homeless Services Oversight Council, is one of the sponsors of this event.

Nearly 3,800 people are homeless in San Luis Obispo County and of these about 630 are children. It’s been estimated that 17 percent of homeless people are veterans.

Today’s forecast

Low clouds, fog and a few areas of drizzle will develop along the coastline this morning.

A weak 1,008-millibar cutoff low-pressure system is forecast to bring light rain to Northern California this morning. This cutoff low is expected to track southward near the California coast and is then retrograde off our coast tonight before moving into Southern California on Monday. This system is looking weaker with each additional model run. However, it still looks like scattered rain showers are possible this evening into Monday along the Central Coast.

Total rainfall amounts should be less than a quarter of an inch. Many locations in San Luis Obispo County may not receive any measurable rain. This system is relatively cold, and snow levels over the southern Sierras and Tehachapis are expected to drop to nearly 3,500 feet Monday, although snow accumulations will be light.

The last time we had a dry December was 1989. The average rainfall total for December is 3.79 inches at Cal Poly.

Dry conditions are forecast for Tuesday and Wednesday, followed by chances for more precipitation Thursday and Friday. Clear and cool weather is expected next weekend.

Today’s surf report

Today’s 5- to 7-foot northwesterly (295-degree deep-water) swell (with 15- to 17-second period) will remain at this height but with a gradually shorter period Monday through Tuesday.

This northwesterly (295-degree deep-water) swell will decrease to 3- to 5-feet (with an 11- to 13-second period) Wednesday.Fresh to strong (19- to 31-mph) northwesterly winds should generate a 6- to 8-foot northwesterly (310-degree deep-water) sea and swell Thursday through Friday.

Preliminary analysis

A series of storms along the midlatitudes of the northern Pacific Ocean will produce a number of medium-high, longer-period swell trains beginning Dec. 19 and continuing through Christmas.

Seawater temperatures

Seawater temperatures will range between 54 and 56 degrees through Friday.

This community, more than any other that I have experienced in my lifetime, has a purpose of service to others.

I was proud to see that service to others firsthand when more than 100 PG&E employees, members of the San Luis Obispo Sheriff’s Office, KaBOOM, Boys and Girls Club of Oceano, the Farm Bureau, Arroyo Grande Rotary Club, Victory Outreach Church of 5 Cities and local politicians came together recently and built a magnificent playground for the kids of Oceano. I was also very appreciative of the Kiwanis of Greater Pismo Beach, Me & Z restaurant and Burger King for donating meals to a bunch of hard-working folks during construction of the playground.

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 nearly 25 years. To subscribe to his daily weather forecast or ask him a question, email