My son asked me why June wasn’t the hottest month of the year since it has the longest days. I told him: Our atmosphere is a lot like a large freight train; it takes a lot of energy to get it going.
Summer began Tuesday as the sun reached its highest point in the Northern Hemisphere and produced the longest day of the year. However, the warmest months in San Luis Obispo will not occur until August and September, when the average mean temperature reaches about 66 degrees.
Historically, September has the warmest maximum average temperature of 79.5 degrees. In fact, October has a warmer average maximum temperature than June, well past the warmest months for most of the country.
This summertime lag in temperatures in our region is due to the enormous volume of water in the Pacific and the northwesterly (onshore) winds that blow strongest during spring and early summer.
The northwesterly (onshore) winds produce upwelling along the coast, which brings cold, subsurface water to the surface along the immediate shoreline. The overlying air then becomes chilled.
It takes a much greater amount of the sun’s energy to heat the ocean than land. Water has a much higher specific heat capacity. Try holding a burning match to a balloon filled with air; it pops immediately, but a balloon filled with water will not.
The increase in solar energy during summer slowly raises the seawater temperature along our coastline, which in turn slowly warms the atmosphere. As we move toward fall, the seawater along our coastline continues to warm, allowing for more comfortable conditions for the surfers and scuba divers. October has the warmest seawater temperatures at about 57.7 degrees while June only averages about 53.3 degrees.
These warmer seawater temperatures, combined with prevailing winds, have a profound effect on temperatures along the Central Coast. For example, San Luis Obispo and Kitty Hawk, N.C., are near the same line of latitude and near the coast, yet San Luis Obispo is much warmer in the winter and cooler in the summer.
Part of the reason is the winds generally flow from west to east at our latitude. Thus air that flows over San Luis Obispo comes from the ocean while Kitty Hawk’s air mostly approaches from over land.
During summer, amazing temperature gradients can develop in our own county. It’s not uncommon to experience triple-digit temperatures in the North County while the meteorological tower at the Diablo Canyon nuclear power plant on the coast reports temperatures in the high 50s under a blanket of dense fog.
As the Central Coast moves into fall, the northwesterly onshore winds tend to relax, allowing for less upwelling and warmer seawater temperatures. In fact, we often get northeasterly (offshore) winds that can push temperatures well into the 100s along the beaches.
This occurred a few years ago at the Pops by the Sea concert in Avila Beach during September, when temperatures soared to well over 100 degrees.
Water not only affects our local temperatures but also moderates global temperatures. The Earth is actually farther away from the sun during our summer, but the Earth’s average temperature is higher in July. In January, the Earth is closer to the sun but is cooler. The reason for this condition is the Southern Hemisphere contains much more ocean and less land than the Northern Hemisphere.
This week’s forecast
Across the Central Coat this morning, low clouds and fog blanket the beaches, coastal valleys and parts of the North County with clear skies far inland.
As the marine clouds burn off through the morning hours, temperatures in the 50s along the coast will warm into the low 60s along the northwesterly facing beaches (Montaña de Oro, Los Osos and Morro Bay) and the low 70s along the southwesterly facing beaches (Cayucos and Avila Beach).
The coastal valleys (San Luis Obispo) will warm to the mid-70s and the North County (Paso Robles) will reach the low 90s.
The typical early summer northwesterly (onshore) winds will gradually increase to fresh to strong (19- to 31-mph) levels this afternoon along the coast and through the Templeton Gap, with low coastal clouds returning tonight.
After similar to slightly warmer conditions Monday, an unseasonably cool 1,001-millibar low-pressure system will move south from the Gulf of Alaska toward the Central Coast on Tuesday with increasing mid to high-level clouds.
The associated cold front will pass our region Wednesday morning with gentle (8- to 12-mph) southerly winds and drizzle or light rain showers. The farther north in San Luis Obispo County you are, the greater chance of receiving some measurable precipitation.
After the system exits to the east late Wednesday, dry and warmer conditions are expected Thursday through the Independence Day holiday weekend.
Surf and sea report
Today’s fresh to strong (19- to 31-mph) northwesterly (onshore) winds will generate a 5- to 7-foot northwesterly (315-degree deep-water) swell (with a 5- to 9-second period) through today, decreasing to 4 to 6 feet (with a 5- to 8-second period) Monday.
A 3- to 5-foot northwesterly (290-degree deep-water) swell (with an 8- to 10-second period) will develop along our coastline Tuesday, decreasing to 2 to 4 feet Wednesday.
Increasing northwesterly winds will produce a 3- to 5-foot northwesterly (315-degree deep-water) swell (with a 5- to 9-second period) Thursday, increasing to 4 to 6 feet (with a 7- to 14-second period) Friday and Saturday.
Arriving from the Southern Hemisphere: Today’s 2- to 3-foot Southern Hemisphere (225-degree deep-water) swell (with a 15- to 17-second period) will remain at this height but with a gradually shorter period through Tuesday.
Seawater temperatures will range between 53 and 55 degrees through Friday.
Summer is here, and PG&E urges customers to be aware of summertime hazards that come with hot, dry weather and certain outdoor activities. Careful planning and preparation are needed to prevent electrical hazards, heat-related illness, wildfires and recreational dangers.
1) Look up and live! Use caution and keep fireworks, balloons, kites and toys — such as high-powered water guns or remote-control aircraft — away from overhead electric lines. Contact with lines can lead to serious injury, fires and outages.
2) Never attempt to retrieve any type of balloon, kite or toy that becomes caught in a power line. Leave it alone and immediately call PG&E at 800-743-5000 to report the problem.
3) Observe local laws. Contact your local police or fire department to make sure fireworks are allowed in your area. If so, keep a bucket of water or a water hose close by.
4) Never climb trees growing near or touching overhead power lines.
5) Avoid strenuous activities in hot, direct sunlight.
6) Drink plenty of water and avoid alcohol or caffeine when the weather is hot.
7) Pay attention to your body. Muscle cramps, dizziness and nausea may be signs of a heat-related illness.
John Lindsey is a media relations representative for PG&E and local weather expert. To subscribe to his daily weather forecast, email him at pgeweather@ pge.com.