National Weather Service United States Department of Commerce

Heat in the Southwest


Excessive Heat Watches/Warnings

Excessive Heat Outlook Heads up! Extreme heat is possible three to five days from now.

Excessive Heat Watch Watch out! Extreme heat is expected within the next two to three days.

Excessive Heat Warning Warning! Extreme heat will occur today or tomorrow.

During Arizona's hottest months, the National Weather Service (NWS) Warning and Forecast Office (WFO) in Phoenix issues special products to alert the public when unusually hot weather (by Arizona standards) is expected. These products are intended to raise the public's awareness to prevent heat illnesses from occurring. When the NWS WFO Phoenix issues one of its heat products, it should serve as a signal that on that day outdoor activities are not "business as usual."

Studies have shown that our bodies have a greater ability to tolerate heat as the summer wears on. For example, a temperature of 105 degrees in May will seem hot, whereas the same temperature in June or July will not seem as hot because our bodies have acclimated to the heat. Hence, there is not one single, constant temperature used by the NWS WFO Phoenix to determine when a heat product will be issued.

If significantly hot weather is forecast, the NWS WFO Phoenix will issue an Excessive Heat Watch generally two to three days in advance. An Excessive Heat Watch is a way to give the public and emergency officials a "heads up" that extreme temperatures are expected. If significantly hot temperatures remain in the forecast for today or tomorrow, the Excessive Heat Watch will be upgraded to an Excessive Heat Warning, indicating that extreme heat has either arrived or is expected shortly.

Safety Information

The negative effects of excessive heat can be easily avoided. Some simple steps you can take include:

  • Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
  • Dress for summer. Lightweight light-colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
  • Put less fuel on your inner fires. Foods (like proteins) that increase metabolic heat production also increase water loss.
  • Drink plenty of water or other non-alcohol fluids. Your body needs water to keep cool. Drink plenty of fluids even if you don't feel thirsty. Persons who (1) have epilepsy or heart, kidney, or liver disease, (2) are on fluid restrictive diets or (3) have a problem with fluid retention should consult a physician before increasing their consumption of fluids.
  • Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day (during hot weather) in an air conditioned environment affords some protection.
  • Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.

For additional safety information...
Center for Disease Control and Prevention - Extreme Heat
Environmental Protection Agency - Extreme Heat
NOAA - Heat Waves: A Major Summer Killer
Arizona Dept. of Health Services - Protecting Yourself from Arizona's Heat
State of California Heat Safety

Statistics on Heat

Contrary to common perception, heat is the number one weather-related killed in the United States. According to data from the Center for Disease Control and Prevention (CDC)[source], heat-related deaths numbered 3,442 in the United States from 1999 to 2003. On average, more people are killed by heat in the U.S. than are by tornadoes, hurricanes, floods, and lightning combined (see graphic below).

In Arizona, heat-related deaths are by far the number one weather-related killer. The CDC reports that between 1993 and 2003, 253 deaths in Arizona were heat-related[source]. The rate of deaths for those 25 years old or greater was three to seven times higher in Arizona as compared to the entire U.S.

The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. The table to the right is a quick reference as to what the heat index is based on current temperature and relative humidity. Click on the image for a full size version you can print for your own use!

Heat index table.
Bar chart indicating that most weather-related fatalities are due to heat.

Climatology of Heat in the Southwest

The southwest United States is one of the hottest areas of the United States. Temperatures in the triple-digits are common for several months of the year. In addition, the rapid expansion of major urban areas in Phoenix has caused a significant urban heat island (UHI) to develop - causing low temperatures to be abnormally high. The table below compares the frequency of extreme temperatures at select locations in the NWS WFO Phoenix County Warning Area.

  Phoenix Casa Grande Parker El Centro Yuma Globe
Average Number of Days w/High of 100+ °F per Year # 110 121 110 115 118 19
Average Number of Days w/High of 110+ °F per Year # 18 30 29 20 26 0
Average Number of Days w/Low of 80+ °F per Year # 67 10 37 29 59 0
All-Time Record High Temperature 122 °F
123 °F
127 °F
122 °F
124 °F
113 °F
All-Time Record High Low Temperature 96 °F
96 °F
100 °F
98 °F
94 °F
86 °F
# Based on 1978-2007 data. * And previous years.