Phoenix Dry Heat: About the Heat Index

bottle of water in the desert
The Power of Forever Photography E+ / Getty Images

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 Phoenix humidity levels are lower than many other regions of the country, 100 degrees F may not feel as horrible or suffocating in the Valley of the Sun as it does when temperatures rise to triple digits in parts of the country that have higher levels of humidity. When considering the temperature, it is also important to keep the Heat Index in mind.

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. 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. 

Dangers of a High Heat Index 

People can be affected by heat even when the temperatures aren't that high, but certainly, when the Heat Index goes over 105 F, there is a higher risk of heat exhaustion or heat stroke.

Temperature vs. Relative Humidity: Heat Index

°F 90% 80% 70% 60% 50% 40%
80 85 84 82 81 80 79
85 101 96 92 90 86 84
90 121 113 105 99 94 90
95 133 122 113 105 98
100 142 129 118 109
105 148 133 121
110 135

Heat Index Chart provided courtesy of the National Weather Service.

Humidity Levels in the Summer in Phoenix

When it is 100 degrees F or higher, the humidity level recorded for the past hundred years was in the neighborhood of 45 percent. Usually, it's significantly lower than that except during Monsoon Season. At this time of the summer, the moisture levels can increase as monsoon conditions develop.

The National Weather Service established June 15 as the first day of Monsoon Season in Arizona and September 30 as the last day of the state's monsoon season, alerting visitors and residents alike who should be concerned with monsoon safety. Monsoons can create damaging micro-bursts and huge thunderstorms called Haboobs. In addition, once-dry river washes can quickly fill up when it rains and create dangerous conditions. 

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.

Frying Eggs on the Sidewalk

There are few days that reach 115 degrees F or higher but it does happen. When it hits triple digit temps in Phoenix, people start talking about frying an egg on the sidewalk. It may be possible. At a minimum, the frying temperature for eggs is 130 degrees. Concrete can make things as much as 50 degrees hotter. So at 115 degrees, the concrete could register 165 degrees—on paper, enough to fry an egg. However, most people in the Valley of the Sun who have tried this say it doesn't work.​

Was this page helpful?