This blog explores the history of Phoenix, Arizona and a little bit of Los Angeles and San Francisco, California

The heat island effect in Phoenix, Arizona


Territorial Phoenix didn't have air conditioning, but it had trees. From the 1860s until the 1920s, which is when air-conditioning started to become popular, the city was a forest of trees. The canals were lined with trees. Houses almost disappeared behind trees. Tall trees arched over roads casting deep shade. I'm not suggesting that the summers were even close to being comfortable in territorial Phoenix, but at least the city wasn't actually heating itself up. That started with the two things - air conditioning, and cars.

The Evans House in 1904, 11th Avenue and Washington. The house is still there, but the trees are gone.

If you've ever stood next to an air conditioner, or a car, you know that the vast majority of the energy that they consume is expelled as heat. And if you multiply that by millions of these heat-generating machines operating at the same time, you would see that it brings up the air temperature quite a bit. But that is just the beginning of the *heat island effect*. When cars became popular in Phoenix, in the late teens and early twenties, more paved roads had to be built. It may not rain often in Phoenix, but when it does, you don't want mud-clogged roads! And those paved roads reflect and hold in the heat generated by the machines traveling on them.

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The greater number of cars meant that roads had to be widened. And when a road widens, it means that the trees that were originally along the edges, and making shade, have to go. Take a look at Central Avenue between of Bethany Home Road and the Arizona Canal and you will see what streets in Phoenix used to look like, lined with trees.

Looking north on Central Avenue from Monroe in 1919. This photo was taken from the Heard Building.

The modern era came to Phoenix in the 1920s. And that spelled the end Phoenix as a city of trees. As new buildings and houses were built, a few decorative trees were planted, but the original function, which was shade, was lost, to be replaced by air conditioning. Old-timers who knew territorial Phoenix would have been shocked at how stripped bare their city had become after the 1920s.

Phoenix was once a city of trees, and hopefully that day will return.