On average, the hottest day of the year comes later in coastal California than in most parts of the country.
Average Temp In San Diego In September
As scorching heat blanketed the nation from the desert Southwest to the Northeast this weekend, Californians can't afford to be complacent. The hottest part of the season in the Southland is usually yet to come, with dry and fire weather conditions leading the way.
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The National Weather Service said a strengthening high pressure system stretching nearly coast to coast is expected to trigger extreme heat warnings across the country for the next few days. At this time of year the jet stream has usually moved far north for the season, near or beyond the Canadian border. This allows the most dangerous summer heat to occur, on average, from mid-July to early August, below that part of the contiguous United States to the south.
But California is a somewhat different story, largely dependent on its distance from the Pacific Ocean, thanks to its permanent seabed. This gray layer of stratus clouds is produced by the cold coastal waters of the California Current that flows south of Alaska.
As the sun rose over the Sierra Nevada foothills, it looked orange in the overcast sky in Mariposa County.
In Southern California, the interior valleys are warmest in mid-August, slightly later than the rest of the country. Near the coast, the warmest period from late August to Sept. 1 is just a fraction behind, said Eric Bolt, a meteorologist with the National Weather Service in Oxnard. Some of the hottest temperatures occurred in September, he said, because of Santa Ana winds from the coast and downwind heat.
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Other parts of coastal California, especially north of Point Conception, don't get their warmest average temperatures until later. For example, Half Moon Bay sees its warmest average maximum temperatures in the second half of September and the first half of October, with almost all of its record readings in the low 90s during those months. .
But why don't all the hottest days occur when the sun is most directly in the northern hemisphere? Solar radiation peaks with the summer solstice on June 21. The reason for the delay in most of the United States is that the rate of daily heat input from the sun exceeds nighttime cooling for several weeks after the solstice, until temperatures begin to cool. End of July and beginning of August.
An increase in catastrophic wildfires has reduced California's tree cover by 6.7 percent since 1985, and researchers fear the lost trees may never grow back.
Notable exceptions are Arizona and New Mexico, where the hottest weather occurs in June, before the North American monsoon clouds usually gather. Thunderstorms usually bring cooler temperatures, and the monsoon, which peaks in July and early August, provides about half of the annual rainfall for parts of the region. But in 2020, when the monsoon was essentially unreported, the region suffered record heat.
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During the summer, the West Coast is dominated by a Pacific coastal high and a thermal low over the continent, said John Null, a veteran Bay Area meteorologist with Golden Gate Weather Services. Air moves from high to low pressure, creating moist northwesterly coastal winds that move down the California coast, promoting stratospheric clouds.
As we move into fall and winter, the pattern changes, Noel said. The ocean surface weakens and moves southward, while at the same time the polar jet stream begins to descend southward, bringing cold air masses and high pressure over the Great Basin.
This leads to less protective ocean layer clouds for the coast and temperatures begin to rise. The southern jet stream is a harbinger of cold winter weather to come. But the build-up of high pressure in the Great Basin also signals the start of Southern California's Santa Ana season, when a cooler layer of ocean dissipates and coastal temperatures rise.
In fall and winter, the Great Basin, surrounded by mountain ranges, can be a huge bathtub of powerful, high-pressure air, all eager to move from intensity to low pressure. This low pressure can be found near sea level off the coast of California. To get there, high-pressure air will travel downhill and through passes and valleys. The atmosphere is constantly trying to rebalance, and wind is related to these short-term changes in air pressure. So, once again, high pressure flows into low pressure, only this time it flows offshore, driving the infamous Northeast Santa Ana winds in Southern California.
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The valleys of the Great Basin range in elevation from 2,000 to 6,000 feet. As this air descends toward the coast, it is heated and dried by compression. Sinking air increases in temperature by 5.5 degrees Fahrenheit for every 1,000 feet of descent. This is called the break rate. Air flowing from a 4,500-foot mountain, for example, toward the ocean will be warmed by about 25 degrees.
In addition to this pressure difference, or pressure gradient, winds accelerate as they make their way through the canyons, creating a venturi effect—much like a nozzle on a garden hose.
Santa Ana onshore winds, when at their strongest, can reach the coast and even Santa Catalina Island. They can carve out waves, much to the delight of surfers. But even weaker tidal currents can limit the sea floor. Now densely populated areas near the coast - which earlier in the season could have been protected from the heat under sea clouds - are recording some of the highest temperatures.
Before the Santa Ana winds kicked in, coastal Southern California began to see its oceanic crust erode. Overnight minimum temperatures peak from mid-July to mid-September at their highest average, with many of LA's warmest temperatures on record occurring in the low 80s in September. This means an uncomfortable sleeping position for those who cannot afford air conditioning. Those who can afford it put pressure on the power grid by running their air conditioners around the clock.
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In addition, sea surface temperatures off Southern California on average peak in August and September, which increases humidity, said Alex Tardy of the weather service's San Diego office. This, along with the possibility of a moist monsoon flow from the southeast, could keep the cloudier day and night.
Although ocean temperature clouds can provide protection from the sun for the coast, while many other places enjoy moderate temperatures, they usually bring only rain, but California's persistent drought There is no serious relief from In a sense, with California's Mediterranean climate and months without rain , they save some of California's worst heat for the end of the dry season when vegetation is driest and fire-prone.
Every year it's a race to see which comes first, the start of the rainy season or the Santa Ana winds, as meteorologist Bill Pacert, formerly of NASA's Jet Propulsion Laboratory, likes to point out.
La Nina, a global climate pattern that typically results in dry winters in the American Southwest, will continue for the third year in a row. A decrease is expected from July to September before increasing again in the fall and winter, the National Weather Service's Climate Prediction Center said earlier this month. So the chances look good that climate conditions of drought and fire are likely to persist - and worsen - until 2023.
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Paul Doginski is a graphics and data visualization journalist. He joined the Los Angeles Times in 1996. A native of Minnesota, he earned a bachelor's degree in English from Morehead State University. The first month of fall, September, is also a moderately warm month in San Diego, California, with an average temperature between a low of 65.1°F and a high of 76.6°F. Temperatures in San Diego recorded an average high of 76.6°F for September, roughly mirroring the previous month's climate. San Diego averages 65.1°F during September nights. Humidity In San Diego, the average relative humidity in September is 68%. Precipitation in San Diego In September, an average of 0.39" of rain accumulates during the 4.5 rainy days. San Diego experiences up to 61.8 days of rain during the year and accumulates up to 8.7" of rain. Ocean Temperature In San Diego, the average sea water temperature in September is 66.7 °F.
Note: Swimming is not a pleasant activity for most people when the water temperature is 66.7°F. As the water temperature drops from 69.8°F to 59°F, it becomes difficult to control breathing. Daylight in San Diego, the average length of the day in September is 12 hours and 22 minutes.
On the first day of September, sunrise is at 6:23 a.m. and sunset is at 7:13 p.m. On the last day of the month, sunrise is at 6:42 AM and sunset is at 6:35 PM PDT. The average sunny time in San Diego in September is 10.3 hours. UV Index July is the month with the highest UV index in San Diego, California.
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