WEATHER

Arizona weather: What causes summer heat waves?

Weldon B. Johnson
The Republic | azcentral.com
July 5 was the hottest day of 2018 so far.

Hot summers are the price we pay for pleasant weather the rest of the year in Arizona.

We grudgingly put up with triple-digit temperatures for a few months to get to the payoff in the winter. But there usually are a few days when even the most seasoned Arizonans gripe about how unbearably hot it is.

The average temperature during July — the hottest month in Phoenix — is 106.1 degrees but every year a few days are at least 10 degrees hotter than that.

Why is that?

Several factors play a role.

Long days and dry heat

Long summer days allow the sun’s rays to heat us up more than in the winter.

The longest day of the year (known as the summer solstice) comes on June 21 or 22, depending on the year. On that day we have about 14 hours and 20 minutes of daylight.

From that point, each day gets a little shorter until the winter solstice in December.

That famous dry heat? It plays a role in our heat waves, too, because it takes less of the sun’s energy to heat dry air than it does moist air.

Climate change

The urban heat-island effect (where developed areas with abundant concrete and asphalt retain heat more readily than undeveloped areas) along with global and regional warming also contribute to the steady rise in temperatures overall in the Southwest.

Overnight lows are creeping up as a result of the urban heat-island effect and all three issues contribute to the area experiencing a longer-lasting heat season than in years past.

In a 2016 study, National Weather Service meteorologist Paul Iñiguez looked at the average first and last date for hot temperatures in Phoenix.

The average date for the last 110-degree day of the year is about a month later now than a century ago, going from late July to early September. The research cited global, regional and localized warming as playing a role.

In recent years Phoenix has experienced 100-degree highs as late as Oct. 27.

High pressure is key

But the major factor for those exceptional Phoenix heat waves is high pressure in the atmosphere.

An area of high pressure generally floats around the Southwest most of the year. Winds flow around that high-pressure area in a clockwise fashion, so the position of the center of that high goes a long way toward determining what kind of weather we’ll have.

When that high is centered to the northeast of Phoenix, in the Four Corners area, we get monsoon conditions as it pulls up moist air from the south. When that high is centered to the south, we get dry, westerly winds.

But when it’s centered right over the state, we get heat.

Chris Kuhlman, a meteorologist with the National Weather Service in Phoenix, explains.

“Sometimes the high actually moves,” Kuhlman said. “Next week it’s going to shift further west and set up right over Arizona. That means stronger higher pressure over us and higher temperatures.

"If we would keep monsoon moisture the highs would stay at about 109 or 110, but because we’re going to dry out, the drier air will allow the temperatures to warm up.”