Homeless in Arizona

2) On the average Phoenix is actually slightly cooler than equatorial cities. The average annual temperature in Phoenix is about 75°F, whereas places like Lagos, Singapore, Mombasa and other equatorial cities near sea level tend to average about 80°F. The difference is that we have more seasonal variation in temperature. Those equatorial places don't get over 100°F, but they also don't get down to 45°F, like we do on winter mornings. Latitude is part of this, as the sun is much higher in the sky in June here than it is in December. In the equatorial region the temperature pretty much stays the same all year long.

3) Lack of water is a major reason our summer afternoons get hotter than those of many locations at the same latitude. Places that get a lot of rain have an evaporative cooling factor. New Orleans and Atlanta, for example, are close to our latitude, but they don't usually get over 100°F on summer afternoons. That's because they get a lot of rain, and their soil has a lot of water to evaporate. Evaporative cooling prevents it from getting extremely hot. That's also why in India spring is hotter than summer. Before the southwest monsoon arrives, the soil is dry, so the sun heats up the ground, but when the rains come, evaporative cooling keeps the afternoons from getting hot. This is also a factor with the equatorial regions, as this region is rainy most of the year.

4) Coastal southern California also has very little rain in summer and is at our latitude, but they don't get as hot as we do, because they are close to an ocean with a cool current. Even in the summer the Pacific water off southern California is only about 70°F, because the current is coming from the north, so the sea breeze from that cool current keeps the coastal region from getting extremely hot.

5) There are some places in the world that get about as hot as we do for similar reasons. The Sahara Desert, Saudi Arabia, most of Iraq, parts of Syria and Jordan, parts of Iran and Pakistan, interior Australia, and parts of southern Africa get about as hot as we do in summer. They are at about our latitude and get little rain in summer.

--Kevin Walsh

Source

Why is Phoenix so hot?

Weldon B. Johnson, The Republic | azcentral.com 9:07 p.m. MST June 17, 2016

120 degrees, but yeah, it's a dry heat

The urban heat island, high pressure and a phenomenon called Hadley Cells contribute to the Valley's hot climate.

So yeah, we know it gets really hot in Phoenix.

It's a given. Our kind of thing. It's basically what the city is known for.

The question of the day is why?

What factors combine to create the annual furnace that cranks up in May and doesn't relent until, sadly, October? (Yes, it's true. In 2015, the triple-digit temperatures lasted until Oct. 14.)

Forget that folks who have lived here for any length of time just accept it. Or that newcomers are told they’ll get used to it (which may just be a way for Valley veterans to let newcomers know no one wants to hear them complaining).

Why? Why is it so hot here in Phoenix?

Here are answers to this burning question.

First, consider our latitude

Arizona is relatively close to the equator. Note we said "relatively" -- or about 2,300 miles, according to most official calculations. Thus, it receives a lot of the sun’s energy, particularly at this time of year. We’re approaching the summer solstice (June 21), the longest day of the year in the Northern Hemisphere.

Days are longer during summer in the Northern Hemisphere as a result of the tilt of the Earth on its axis as it rotates around the sun. Longer days create more opportunity for things to heat up, particularly near the equator. Now, you might ask, "But other cities far closer to the equator, like say, Kuala Lumpur, Malaysia, aren't nearly so hot. What gives?"

Well, relative latitude is just one factor. Read on.

A big heat engine

Weather and, over a longer time, climate, can be explained on a very basic level as an attempt by the Earth to achieve balance.

The sun’s energy heats things up along the equator, and weather forces spread some of that energy toward the North and South poles, which don’t get nearly as much sun. This energy movement is accomplished with high- and low-pressure systems, evaporating and condensing water, ocean currents and winds.

It’s all a big heat engine, according to University of Arizona climatologist Mike Crimmins.

Hadley Cell circulation is part of that engine.

“Imagine all that energy at the equator is producing rising air,” Crimmins said. “There are thunderstorms occurring around the globe, basically following the sun’s energy. All that rising air has to come down somewhere. The rising air reaches the top of the troposphere and spreads out north and south and sinks somewhere in the Northern and Southern hemisphere. That’s the description of a Hadley Cell.”

Arizona happens to be near one of the places in the Northern Hemisphere where all that sinking air winds up.

Under (high) pressure

Folks who look at weather maps regularly will notice that there is often a big H near or directly over Arizona. That H is how meteorologists represent areas of high pressure.

High pressure often creates clear, dry conditions that offer a greater opportunity for things to heat up. Arizona is under the influence of a persistent area of high pressure. Some persistent highs have catchy names like the Bermuda High that hangs around the East Coast of the United States. Our high is known as the Eastern Pacific subtropical high-pressure system.

The system isn’t always right overhead, but it never goes too far.

“On the weather time scale it gets shoved out of the way,” Crimmins said. “It builds north, gets shoved south, wanders off to the east or whatever. Climatologically, that high-pressure system is really close to Arizona just about all year long. We’re always under the influence of this sinking air.”

The system's wandering plays a big role in monsoon conditions. When it moves far enough east it helps bring moisture into the area from the Gulf of California, which fuels summer storms.

The system is also a factor in the heat wave we're expecting this weekend. High-pressure systems can vary in strength and the persistent high over the Southwest is expected to intensify in the coming days. High temperatures in Phoenix could top 115 degrees Sunday and could reach 120 Monday.

It's a dry heat

The arid conditions that persist over much of the Southwest contribute to heat buildup because it takes less solar energy to heat dry air than moist air.

Paul Iñiguez, a meteorologist with the National Weather Service in Phoenix, explains.

“If I have a pound of air and a pound of water, takes a lot more energy to warm up that water,” Iñiguez said. “If you put water vapor in the air, you have to put more energy in it to warm the water vapor up. That’s why it takes a long time for your pool to warm up in the spring after the air starts to get hot.”

Elevation

Two other factors make the heat more intense in the Phoenix area compared to some other parts of the state.

Elevation plays a role. Other parts of the state have dry conditions and some are even farther south than the Valley, but higher elevations keep those places cooler.

“The Phoenix area is at roughly 1,000 feet (elevation),” Iñiguez said. “You go someplace like maybe Benson, and it's around (3,600) feet. It usually doesn’t get to 100 degrees even though it's further south.”

Heat island

The urban heat island effect also plays a role. Sprawling development and less natural vegetation in the past 30 years has caused temperatures, particularly overnight lows, to increase. It doesn't cool down at night like it used to.

In 2015, the average overnight low for Phoenix was 65.4 degrees. That tied the record for warmest low temperature set the previous year and was 1.9 degrees above normal.