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What is ‘Phoenix Heat Index’ & Why Do Residents Care?

It’s a familiar routine for Phoenicians: Wake up, check the news and ask, “What is the phoenix heat index today?” If you have ever followed that up with, “What’s a heat index?” this article is for you!

Phoenix is often in the news for our high temperatures and low air quality. The heat index is a reflection of those two issues.

A Little Science

The heat index relates air temperature to relative humidity, so if you do not understand those two concepts, you cannot understand the heat index and its numbers.



ACE Home Services - hot and humid

Air temperature is not measured randomly. If it were, nobody would care that the measurement on the neighborhood bank’s big sign was 20 degrees hotter than the measurement in your own backyard, neither of which agree with the “official” temperature. Scientists, such as meteorologists, measure air temperature from four to six feet off the ground, in shade, on grass or dirt, with good airflow.

Relative humidity is not identical to humidity – it is a measure of how close to saturated air the current air conditions are. Saturation is a physics concept, the same as when you dissolve sugar in water until no more sugar can dissolve. At that point, you have reached the saturation point.

For air, hotter air can hold more moisture (be more saturated) than colder air. The relative humidity compares the actual amount of moisture in the air to the amount it could hold at a given temperature. This gives a percentage, the relative humidity.

Here is a simple example: At 68 degrees Fahrenheit, air can hold 17.3 grams of water per cubic meter. If Phoenix’s air at 68 degrees is only holding 10 grams of water per cubic meter, that is a ratio of 10/17.3, or 0.578, otherwise known as 57.8 percent relative humidity.

You need never do any of these calculations, though, because the heat index builds in both the air temperature and the relative humidity in one easy-to-understand number.

What is ‘The Heat Index’?


ACE Home Services Phoenix - What is the Heat Index?

As the National Weather Service (NWS) puts it, the heat index “is what the temperature feels like to the human body when relative humidity is combined with the air temperature.” The NWS is primarily interested in weather as it relates to people, but anything with skin is affected by the heat index. This means, conversely, that machinery, buildings and, most especially, your own central air conditioner is not affected by the heat index – they cannot sweat.

See also  DIY Checkup for Air Conditioner Maintenance

How Humans Cool Off

The easiest, fastest way in Phoenix to cool off is to step into some comforting, invigorating air conditioning. Before AC, however, humans cooled off by perspiring, so that moving air passing over liquid on our skin took with it some of the body’s heat. That evaporative cooling lets us regulate our body temperatures so in hot (or cold) weather we keep the same internal temperature.

If the air temperature is high, but the air has a lot of moisture in it (is very humid), perspiration on our skin cannot evaporate into the air, and we feel uncomfortable. We feel warmer in humid weather than in dry weather.

Direct Relationship

As air temperature and relative humidity rise, the heat index rises. Think of it as a comfort index: As the heat index climbs, we get more and more miserable.

This is why Arizonans famously say, “Yes, but it’s a dry heat.” If you had a daytime temperature of 100 degrees and relative humidity of a damp 55 percent, the heat index would read 124 degrees and you would feel miserable. But if the air is dryer, as it often is in Phoenix, then the 100 degrees may only “feel” like it is 96 degrees! Not a cool autumn night, surely, but not anywhere close to the dangers of 124 degrees.

Made in the Shade


Woman feeling warm in home with broken air compressor

The heat index is also not intended to address direct sunlight – it only refers to the air temperature and is measured (as air temperature is) in the shade. This is actually crucial and can mean a life-or-death issue:

  • Heat index numbers of 103 degrees and higher are dangerous;
  • Add 15 degrees to the heat index for work under full sun.

This means, if the NWS reports a Phoenix heat index of 90 degrees but you plan to work outside under the noonday sun, an additional 15 degrees puts your particular heat index at 105 degrees, hot enough to cause heat cramps, heat exhaustion and possible heat stroke!

Fuzzballs and Scales

The same heat index humans use to determine risk also applies to pets we routinely enjoy around Phoenix. Cats and dogs (and any other mammals) are as sensitive to heat and humidity as we are, and they require the same safeguards. On high heat index days, do not expect your puppy to romp in the yard for hours on end; keep animals cool and hydrated, just as you do for your children and yourself.

See also  6 Fun Facts About the History of Air Conditioning

Reptiles adapt differently to Phoenix’s hottest days, since they are cold-blooded. They can get too hot, though, which is why keeping them indoors in a controlled environment is best. The little heat rock or lamp you provide them in your air-conditioned home is a small sacrifice you pay for your reptiles’ health. The animal will regulate its temperature by moving between your provided heat source and cool spots in its enclosure.

Those Planes!

Recent news stories mentioned Phoenix’s high heat preventing airplanes from taking off or landing. Those surface temperatures had nothing to do with the heat index; some planes are not designed to perform efficiently or safely in air temperatures as high as Phoenix Sky Harbor Airport reported (118 degrees). With air temperatures that high, tarmac surface temperatures exceed 140 degrees, weakening airplanes’ tire rubber.

That Air Quality!

Although Phoenix is often in the news for poor air quality, the heat index is only a small part of the issue with breathability. More critical is particulate pollution and ozone levels. Particle pollution does increase with warmer air temperatures, but Phoenix geography (living in a valley as we do) contributes as well.

Central Air Conditioning

For Phoenicians who like their comfort, and particularly for elderly and very young Arizonans, central air conditioning is essential. The basic physics of AC are easy: Heat from inside your home is pumped out into the already hot outside air, lowering the inside temperature and reducing humidity.

The equipment to achieve that cool comfort is marvelously efficient. Today’s central air conditioners use less electricity, require fewer air conditioner repair calls and produce more cooling power (measured in cooling tons) than systems of just a decade ago.

If your central air conditioner is not keeping up with the high temperatures Phoenix routinely reports, a new, more efficient central air conditioning system may be a wise investment. High heat index days, and especially many clustered together, can tax an older, less efficient system, making it work harder or quit unexpectedly.

See also  Energy Saving Air Conditioning Tips

How to Use AC on High Phoenix Heat Index Days


ACE Home Services: AC air

You can and should use your central air conditioner when the heat index is high, but be smart about it:

  1. Avoid running heat-producing equipment like clothes dryers, dishwashers and the oven when the AC is on;
  2. Keep your thermostat at 76 or 78 degrees to avoid high energy bills;
  3. Use ceiling fans in rooms with people or pets, but turn them off in empty rooms;
  4. Consider cooking outdoors (everybody enjoys a barbeque or grill night!).

If you have elderly, sickly or very young family members, keep them quiet and still on high heat index days. You can increase the perceived comfort in your home by using cool cloths on skin to help with evaporative cooling. Try to avoid closing off rooms, since your whole house is balanced for a certain volume of supply and return air through your ductwork.

If no one in the home is affected by particulates in the outside air, check the forecast to ensure the outside temperature overnight will drop, then open up your house and turn off the AC to let the cool desert air in. Close your home early in the morning to keep cool air inside.

Service and Safety First with ACE Home Services!

Properly tuned up and operating at peak efficiency, central air conditioning equipment can not only keep humidity levels low in your home, it can remove particulates from the air, lower the temperature to make everyone comfortable and less irritable and keep pets happy.

Although pre-season air conditioning tune-ups are ideal, even during the height of summer usage your system can benefit from a professional, complete inspection and service visit. The technician will clean your system, check it for safety, replace disposable air filters and ensure it is operating at ideal efficiency.

Ensuring the long life of your equipment is important because a midsummer breakdown can become an emergency. The safety of your home’s central air conditioner, family and pets depend on prompt, attentive service by your local HVAC professional.

ACE Home Services offers summertime savings on tune-ups and routine AC work. Contact ACE today to schedule an AC tune-up service call!

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