Sunday, June 26, 2011

[californiadisasters] Summer On the Coast Begins With a Cool Note

Summer on the coast begins with a cool note

Published: Sunday, Jun. 26, 2011

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.

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