New heat wave triggers Excessive Heat Warning for Phoenix

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Click to enlarge (Source: National Weather Service) Click to enlarge (Source: National Weather Service)

The National Weather Service has once again issued an Excessive Heat Warning for the Phoenix metro area and surrounding cities – the second such warning in just a few weeks.

We kicked off both the monsoon and the official start of summer with crazy heat – it was 119 degrees on the first day of summers -- and an Excessive Heat Warning that lasted several days.

This heat wave will be shorter and less extreme by a couple of degrees.

The warning went into effect at 10 a.m. Wednesday and runs through Friday evening.

[SPECIAL SECTION: Arizona's Extreme Heat]

Meteorologist April Warnecke says the mercury will climb to 113 Wednesday and to almost 116 by Friday.

“An Excessive Heat Warning is issued within 12 hours of the onset of extremely dangerous heat conditions,” according to the National Weather Service page about Heat Watches and Heat Warnings. “The general rule of thumb for this Warning is when the maximum heat index temperature is expected to be 105° or higher for at least 2 days and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas not used to extreme heat conditions. If you don't take precautions immediately when conditions are extreme, you may become seriously ill or even die.”

So, what is this “heat index”?

“The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature,” the NWS explains on

You know how Phoenicians regularly remind people who don’t live here that “It’s a dry heat”?

Most of us will take a dry heat over heat and humidity any day because we feel warmer since our bodies cannot cool as efficiently.

It’s all about evaporation. And biology.

“When the body gets too hot, it begins to perspire or sweat to cool itself off,” the NWS explains in its breakdown of the heat index. “If the perspiration is not able to evaporate, the body cannot regulate its temperature.  … When the atmospheric moisture content (i.e. relative humidity) is high, the rate of perspiration from the body decreases.  In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases.”

As an example, the NWS says a 100-degree day with a relative humidity of 55 percent will have a heat index of 124 degrees. Drop the relative humidity to 15 percent and that 100-degree day will feel like it’s 96.

There is another factor, of course. Are you in the shade or direct sunlight?

“If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F,” according to the NWS.

That’s why the agency advises people working or play outdoors during an Excessive Heat Watch or an Excessive Heat Warning to take plenty of shade breaks. It’s better to spend some time in air conditioning if you can.

The formula for determining the heat index is complicated, which is why the NWS has put together a chart and created a calculator that can use either dew point or relative humidity.

[WEATHER: Your forecast | Severe weather alerts]

Heat is a sneaky and silent killer.

By the time you realize you’re thirsty, you’re already dehydrated. Things can spiral quickly from there.

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Symptoms of mild to moderate dehydration

  • Dry or sticky mouth
  • Fatigue and lethargy
  • Decreased urine output
  • Dry skin
  • Headache
  • Constipation
  • Dizziness

Symptoms of severe dehydration

  • Extreme thirst
  • Irritability and confusion
  • Very dry mouth
  • Lack of sweating
  • Little or no urination
  • Sunken eyes
  • Dry skin that has lost its elasticity
  • Low blood pressure
  • Rapid heartbeat
  • Rapid breathing
  • Delirium or unconsciousness (in the worst cases)

Heat is the No. 1 weather-related killer in Arizona and the U.S., according to Will Humble, the director of the Division of Health Policy and Program Evaluation the University of Arizona’s Health Sciences Center. Heat kills more people than tornadoes, hurricanes, floods and wildfires -- combined.

"It’s hard to take a picture of heat so it gets less attention than things like floods, lightning, hurricanes and tropical storms," said Humble, former director of the Arizona Department of Health Services. "The Arizona heat is a lot more than a nuisance – it’s dangerous and lethal."

If you're going to be out and about in the heat -- whether for work or play -- there are several things you need to do.

Make sure you…

  • Hydrate before you head out
  • Have at least 16-32 ounces of water for every hour you're going to be out
  • Have your cell phone and be sure it's charged in case you need help
  • Take breaks to cool off, indoors if you can
  • Wear light colors and loose clothing
  • Wear a hat or use an umbrella
  • Wear sunscreen to protect your skin; you'll need to reapply it if you go swimming or sweat it off

What to do when heat-related illness sets in

  • Move the person to a cooler location
  • Remove or loosen tight clothing
  • Apply cool, wet towels
  • Fan the person
  • Give him or her small sips of cool water; do not let him or her drink too much too fast
  • If you suspect heatstroke
  • Call 911 immediately
  • Do everything listed above
  • For rapid cooling, apply ice or cold packs wrapped in cloth to wrists, ankles, groin, neck and armpits

Super complicated formula for those who enjoy maths

It has an error margin of plus or minus 1.3 degrees F.

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2

T - air temperature (F)
R - relative humidity (percentage)

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