Heat Index, Explained

Is There Such a Thing as a Dry Heat?

It's a Dry Heat!
••• It's a Dry Heat!. The Power of Forever Photography E+ / Getty Images

Phoenix: Is It Really a Dry Heat?

You've certainly heard the phrase, "It's a Dry Heat." Some people actually think that this is the Phoenix city motto! You'll even find that phrase on tee shirts around town. The truth is that because our humidity levels are lower than many other regions of the country, 100°F may not feel as terrible here as it does when temperatures rise to triple digits in parts of the country that have higher levels of humidity.

Learn more about our desert heat.

When considering the temperature, it is also important to keep the Heat Index in mind.

What is the Heat Index?

The Heat Index is the temperature the body feels when humidity is taken into account. The concept is similar to wind chill factor, only on the opposite end of the temperature scale.

Why Does it Feel Hotter When it Is More Humid?

When the humidity is high, sweat doesn't evaporate as much, and so our body loses some of the cooling effect that the evaporation of sweat provides.

Is a High Heat Index Dangerous?

People can be affected by heat even when the temperatures aren't that high, but certainly when the Heat Index gets into the blue zone in the chart below, there is a higher risk of heat exhaustion or heat stroke.

Temperature vs. Relative Humidity: Heat Index
°F90%80%70%60%50%40%
80858482818079
851019692908684
90121113105999490
95 13312211310598
100  142129118109
105   148133121
110     135

 

How High Does the Humidity Get in the Summer in Phoenix?

When it is 100°F or higher, the highest humidity level for the past hundred years was about 45%. Usually, it's significantly lower than that.

Is Humidity Rising in Phoenix?

Many people think that because the population of Phoenix has increased so rapidly, and there are more lawns and more pools, that humidity levels are also rising.

Studies have shown that actually the opposite is true. More pavement and related urbanization have meant that in recent years humidity levels have actually decreased.

So 110°F Doesn't Feel So Bad, Right?

I saw this comment in an online forum, and I thought it answered this question admirably:

The phrase "yeah, but it's a dry heat" is most often muttered by people who have never actually spent a summer at 115 +. Eggs frying on the sidewalk is only a myth because chickens are too smart to be outside during the summer in Arizona. "Yeah, but it's a dry heat." So is the surface of the sun, but I ain't about to move there either.

Seriously, though, there are few days that reach 115°F but it does happen. Here is some triple digit trivia for you to enjoy! Also, you might be curious — can you really fry an egg on the sidewalk on those hot Phoenix days? I tried it!

Heat Index Chart provided courtesy of the National Weather Service.