A Large High-Pressure Umbrella Is Keeping Heat On in Southwest : Atmosphere: System is closer to California than normal and is forcing hot air down. The torrid temperatures also surprised air pollution officials by blasting smog out of the L.A. Basin.

The enormous mixing bowl of gases and dust that we call our atmosphere is playing tricks on the people of the Southwestern United States.

And, hanging from the lips of nearly every resident of states from Kansas to California is the same question:

“Why is it so damn hot?”

The answer is that an enormous high-pressure system has formed over the Southwest, closer to California than normal for this time of year, and it is forcing hot air down, resulting in record temperatures. The torrid air also blasted the smog out of the Los Angeles basin, to the surprise of air pollution officials.

Basically, a huge high-pressure system has built up over the southern Rockies, and “it covers all of the southwestern United States,” said Arthur Lessard, meteorologist in charge of the National Weather Service’s Los Angeles office.

The high-pressure system acts “sort of like an umbrella, and underneath it, everything is just frying, baking,” he said.

What makes this high-pressure system a little different is that it is very large, and it is farther west than normal for this time of year. Furthermore, it happens to be occurring at the time of the year when the days are the longest, providing the greatest opportunity for the sun to heat the atmosphere.

But why is this happening in June?

Across the land, weather experts struggled to explain the complex mechanisms that have turned much of western North America into an inferno. But somehow, as people camped out next to the refrigerator and hoped for a better tomorrow, the answers just weren’t good enough.

“People ask why,” lamented meteorologist Bill Hibbert, “and you come up with an answer. And then they say, ‘but what is the real reason?’ ”

“People aren’t being punished,” he said, hoping to dispel any fears of devine retribution.

The fears may not have been real Wednesday, but the temperatures were.

“A lot of places are shattering their all-time records,” said Hibbert, who is with WeatherData Inc. of Wichita, Kan.

Nevertheless, the blistering temperatures that stretched all the way into central Canada brought a little good news to Southern California. High temperatures literally poked holes in the inversion layer over the Los Angeles Basin, allowing most of the smog to escape.

“The lid has been broken,” said Joe Cassmassi, senior meteorologist with the South Coast Air Quality Management District.

That breath of fresh air may not have been apparent throughout the Los Angeles Basin, however. Some residents reported seeing pockets of smog. Some areas were dead calm while others were buffeted with fierce winds. Temperatures varied widely across the basin.

But June is supposed to bring coastal fog, not searing temperatures that have knocked the record books off the shelves.

“The pattern shifted,” a weary Hibbert said. “There’s not much else you can say.”

“This is a pattern that is by no means unusual for late August or early September, but it is highly unusual for June,” said Morton Wurtele, an atmospheric scientist at UCLA.

“As for why it’s happening at this time, that’s hard to answer,” he added.

In the complex world of meteorology, it started far away. California is blistering because a high-pressure system formed over the Southwest that resulted from a strong low-pressure system over the Gulf of Alaska, said Harry Woolford, a meteorologist with WeatherData.

A high-pressure system can be described as a huge block of air that has too much air in it. A low-pressure system, by contrast, has too little air in it. If you could back off and see the Earth’s atmosphere, a high-pressure system would bulge up, and a low-pressure system would appear as a dent.

When a low-pressure system forms, Woolford said, “it creates a rising motion as air flows around it.” The rising air, swept horizontally by the jet stream and local storm systems, eventually forms a high-pressure system.

The high-pressure system pushes air down in a process called “subsidence.” The sinking air is heated as it is forced down, causing temperatures to rise.

What makes the June weather pattern unusual is that the low-pressure system over the Gulf of Alaska was quite large, and “it allowed the high to build for about a week,” Woolford said.

That created an enormous high-pressure system that simply doesn’t want to go away.

Unfortunately, the system settled into place when the days are long enough for the sun to bombard the lower atmosphere “for about 14 hours a day,” he added.

The lower atmosphere heats more quickly than air at higher elevations, and the air near the ground “became very warm and buoyant” Wednesday, said Cassmassi of the air quality district. That caused “atmospheric mixing” and generated winds throughout much of the basin, including strong breezes out of the north that held the marine air offshore.

The hot, lower air rose so quickly that it blasted through an inversion layer. Air pollution officials had expected the inversion layer to trap smog throughout much of the basin on Wednesday.

“We had not anticipated the excessive heating,” Cassmassi said. “We thought the inversion would remain intact.”

However, the inversion layer, Cassmassi said, “was broken in a huge number of areas.” That gave the basin crystal clear, but very hot, skies.

At this point, it doesn’t appear that relief is just around the corner.

The high-pressure system has become “very strong now,” said meteorologist Woolford. It shows some indication of moving off toward the northeast, but that slow movement should result in continued hot weather at least through the end of this week.

But the forces that drive weather systems are unpredictable. As any meteorologist will tell you, don’t put any money on it.

RECORD HEAT

Why it’s hot 1. As the Jet Stream moves clockwise over the U.S. a cone of hot sinking air comes spiraling down. 2. High pressure system creates an inversion layer and forces hot air to stay down. 3. Sun heats the ground and causes air to rise quickly, piercing the inversion layer and allowing the smog to scape.

Northridge: 108 Degrees Pasadena: 110 Degrees Death Valley: 118 Degrees Ontario: 112 Degrees San Bernardino: 107 Degrees Palm Springs: 119 Degrees Needles: 116 Degrees Thousand Oaks: 112 Degrees Ventura: 85 Degrees Long Beach: 106 Degrees Santa Ana: 101 Degrees San Clemente: 86 Degrees Riverside: 108 Degrees El Cajon: 106 Degrees San Diego: 91 Degrees Source: National Weather Service, Weather Data Inc.