Why is it so hot in Texas?


by By MICHAEL E. YOUNG / The Dallas Morning News


Posted on August 15, 2009 at 4:37 PM

Updated Monday, Oct 19 at 6:39 PM

It's hotter than the Sahara out there.

Well, not quite, but Texas and the African desert do have something in common when it comes to explaining the area's summer swelter: Latitude 30.

Latitude 30 north slices through the heart of Texas, a few miles south of Austin and a few miles north of Houston. Dallas is three degrees north, Brownsville three degrees south.

And globally, the 30th parallel, north or south, is an inhospitable place to be - like North Texas today when the high temperature was expected to hit triple digits again.

"If you look around the globe at 30 degrees latitude, mostly what you see is desert," said Dr. Kenneth Bowman, head of the Department of Atmospheric Studies at Texas A&M University.

Latitude 30 on a photo of Earth looks like a dusty trail jumping from continent to continent, desert to desert: Africa's Kalahari, South America's Atacama, the barren regions of Australia, the Sahara, the sand-strewn Middle East and, closer to home, the Sonoran and Chihuahuan deserts of Mexico and the American Southwest.

Conditions aren't so dire across much of Texas, though anyone who has driven to El Paso knows there's plenty of parched earth out there. But the eastern half of the state, and our neighbors through the South, can be downright sultry in the summer.

"The southeastern United States is different than a lot of places of the same latitude," Dr. Bowman said. "It isn't a desert because of the Gulf of Mexico."

The Gulf's impact stretches only so far, though. While Houston averages a reasonable 4.36 inches of rain in July, with an average high temperature of 93.6, Dallas, 240 miles to the north, is almost two degrees hotter with half as much rain.

The Dallas area sits right at a natural weather divide, which roughly follows the Interstate 35 corridor from Laredo to Minneapolis. On one side, you have the forested east, where rainfall and evaporation balance out, said Dr. Gerald North, a professor of atmospheric studies and oceanography at Texas A&M. And on the other stands the drier west, a place of grasslands and shrubs on a mostly treeless plain.

But summer is dry in both halves of Texas because high pressure reigns.

"For us, it's the Bermuda high," a sprawling ridge of dry air often anchored in the Atlantic and stretching to Texas and beyond, Dr. Bowman said.

"And because it's high pressure, it tends to be sinking air, which suppresses the convection you need for summer storms."

The meteorological process is called a Hadley Cell: The equator's heat forces moist air to rise, and it spreads toward the poles because of Earth's rotation. The moisture falls as rain over the tropics, and the suddenly dry air begins to sink around the 30th parallels.

In the Texas summer, though, the effect of the Hadley Cell pushes farther north and takes the storm track with it.

"Most summers here, we don't see a cold frontal passage in July or August. It's hard to fight that large-scale high pressure," Dr. Bowman said.

The western edge of the Bermuda high, which most affects Texas, can ebb and flow, allowing some storm fronts to slip through. But when high pressure drifts west and settles in, Texas temperatures soar.

"Right now, we're in the closing phase of a 'La Nina,' and that almost always means we're dry," Dr. North said.

"And when we're dry, it's hot.