If It’s Hot, Then You Must Be in San Juan : Temperatures: Geography and seasonal factors often conspire to heat up this historic community.

When the breeze blows out to sea and the fog and haze stay parked off the coast, even Death Valley can look cool compared to this mission city, a normally temperate community less than a mile from the ocean.

Take Thanksgiving week last year, for instance: While the desert floor topped out the day at 82 degrees on Nov. 20, San Juan Capistrano’s temperatures soared to 95. The locals opened windows, turned on air conditioners and swarmed to local watering holes, where bartenders reported doing fire-sale business on cold doubles.

On that day, San Juan Capistrano registered the nation’s highest recorded temperature, a distinction that the seaside community holds with surprising frequency. And as summer rolls in, residents and weather forecasters agree that while inland areas such as Santa Ana or Rancho Santa Margarita may register more record-breaking days, even the proximity of the ocean won’t keep San Juan from racking up some blistering afternoons.

“I guess we’re so well known that the good Lord has paid us a little special attention by making us get some real hot days,” said City Councilman Lawrence F. Buchheim, who has seen his share of warm afternoons during his 63 summers as a San Juan Capistrano resident. “It just adds to the pleasant feeling of living here.”

Indeed, tourists seem not to mind it, as they flock to the city year after year. The locals show few signs of annoyance , hot days notwithstanding. And workers at a nursery in east San Juan credit the weather with giving them bountiful crops.

So if it is strikingly hot, no one is doing much complaining.

Nor are they doing much explaining. Neither Buchheim nor other San Juan residents could quite put their fingers on why the town seems to attract such hot days. Meteorologists are a different story, though. They attribute it to a precise and unusual mixture of several different factors.

Chief among them is San Juan Capistrano’s proximity to the mountains that run through Camp Pendleton and on up through the eastern edge of Orange County. These mountains whose ridges include the familiar Santiago and Modjeska peaks, come within a few miles of the shore as they run to the east of San Juan.

It is also significant that while San Juan Capistrano’s western border comes within less than a mile of the water, the city actually has no coastline, so a light on-shore breeze can sometimes die in Dana Point, leaving San Juan sweltering.

Those geographic facts, when combined with the right wind currents, can bring hot, dry air bearing down on the city.

“You have that high terrain off to your east and northeast,” said Marty McKewon, a meteorologist with WeatherData, which provides forecasts for The Times. “Whenever there’s a high-pressure system over Utah or out there to your east, that brings an offshore flow that pushes down into the San Juan area.”

As the breeze blows offshore, it drops air down out of the mountains, some of which are more than 4,000 feet high. And as that air drops, it heats up, gaining more than five degrees for each 1,000 feet that it plummets, McKewon said.

And when the breeze hits the valley on the eastern edge of San Juan Capistrano, the wind dies and the air bakes--along with the people in it.

“Back in the summer of ‘38, it was hot for weeks,” Buchheim recalled. “We had a September that was so hot that everyone from the east side of the valley headed down for the beach. It seems like the whole town just slept on the beach night after night.”

Oddly, though, even when San Juan Capistrano shows up as the nation’s hot spot, that can say as much about the way temperatures are reported as it does about the wind coming off the Santa Ana Mountains.

Farther north in Orange County, other areas sit in weather zones that can produce some of the same toasty effects. Anaheim Hills, for instance, lies at the base of the mountains, and a mild Santa Ana wind can draw waves of hot dust down into the area, forcing temperatures well into the uncomfortable zone.

The thing is, Anaheim Hills doesn’t have a National Weather Service temperature station. The nearest one is in the flatlands of east Anaheim, at the intersection of Tustin and Miraloma avenues. So when the temperatures get recorded and passed along to the wire services for the evening newscast, they routinely miss some areas.

Still, even the weather service’s oversights can’t disguise the obvious: San Juan Capistrano, especially on its eastern border, can be just plain hot.

One result of the confluence of weather patterns is that San Juan can sometimes seem to be a city straddling two climate zones. On the town’s eastern edge, temperatures soar in oppressive stillness. But just a few miles away, closer to the ocean, a light onshore breeze can keep the valley heat at bay.

THE HEAT IS ON

Warm air sweeps into San Juan Capistrano when high pressure systems develop to the east, over Arizona or Utah. That pushes air toward the ocean, causing hot, offshore breezes to hold ocean-cooled air off the coast. The result: San Juan, which otherwise boasts a mild climate, can sometimes be the unlikely holder of the nation’s hottest-place honors.

As dry air drops in altitude, it increases in temperature, usually adding about 5.5 degrees Fahrenheit for every 1,000 feet.

Take a moderately warm summer breeze, of say 75 degrees, blowing off the top of 4,510-foot summit of Los Pinos Peak in South Orange County. By the time it settles into San Juan Capistrano, just a few feet above sea level, that same mass of air would be a scorching afternoon of nearly 100 degrees.