Summer in Phoenix – It’s a Dry Heat

Why summers in Phoenix aren’t so bad…

This article is presented for general informational purposes only. If you have a question please consult with the appropriate professional before implementing any of the following suggestions.

Air conditions can be quickly characterized by using a special graph called a psychometric chart. Properties on the chart include dry-bulb and wet-bulb temperatures, relative humidity, humidity ratio, specific volume, dew point temperature, and enthalpy (see Figure 1).

Figure 1:  A Psychometric Chart

Only two properties are needed to characterize air because the point of intersection of any two property lines defines the state-point of air on a psychometric chart. Once this point is located on the chart, the other air properties can be read directly. Air properties at 29.92 in. Hg. (standard) atmospheric pressure can be found using the enclosed psychometric chart. Put a laminated version of this chart on a wall at your facility for handy reference.

Air Properties

Dry-bulb temperature, which is usually referred to as simply air temperature, is the air property that is most familiar. Dry-bulb temperature, Tdb, can be measured using a standard thermometer or more sophisticated sensors. This temperature is an indicator of heat content and is shown along the bottom axis of the psychometric chart. The vertical lines extending upward from this axis are constant-temperature lines.

Relative humidity, RH, is the ratio of the actual water vapor pressure, Pv, to the vapor pressure of saturated air at the same temperature, Pvs, expressed as a percentage. Relative humidity is a relative measure, because the moisture-holding capacity of air increases as air is warmed. In practice, relative humidity indicates the moisture level of the air compared to the airs moisture-holding capacity. Relative humidity lines are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air (RH = 100%) is the uppermost curved line on the chart.

Wet-bulb temperature, Twb, represents how much moisture the air can evaporate. This temperature is often measured with a common mercury thermometer that has the bulb covered with a water-moistened wick and with a known air velocity passing over the wick. On the chart, the wet-bulb lines slope a little upward to the left, and this temperature is read at the saturation line.

Dew point, Tdp, is the temperature at which water vapor starts to condense out of air that is cooling. Above this temperature, the moisture stays in the air. This temperature is read by following a horizontal line from the state-point (found earlier) to the saturation line.

Specific volume represents the space occupied by a unit weight of dry air, in ft3/lb, and is equal to 1/air density. Specific volume is shown along the bottom axis of a psychometric chart, with constant-volume lines slanting upward to the left.

Humidity ratio, w, is the dry-basis moisture content of air expressed as the weight of water vapor per unit weight of dry air. Humidity ratio is indicated along the right-hand axis of a psychometric chart.

Enthalpy, h, is the measure of airs energy content per unit weight (Btu/lbda). Wet-bulb temperature and enthalpy are related intuitively. So, enthalpy is read from where the appropriate wet-bulb line crosses the diagonal scale above the saturation curve.

The Midwest and East Coast are just as hot as Phoenix, despite what thermometer readings would lead you to believe. The Deep South and Gulf Coast are even hotter than Phoenix. A dry 115 feels like a humid 95. A dry 105 feels like a humid 85. If anything, a dry 115 is more comfortable than a humid 95 because when you sweat, it actually evaporates and cools you off. With high humidity, you just get wet and smelly.

If the relative humidity in New York City or Washington DC were to drop to 10%, the temperature would most likely rise to 120 F.  If the air in Phoenix were to become saturated, it would only get up to 95 max. Don’t be fooled by thermometer readings alone.  In fact, when it’s extraordinarily bone dry (with single digit dew points), even 80 degrees can feel a little chilly in Phoenix.  There’s some truth to the cliché’s, “yes, but it’s a dry heat,” and “it’s not the heat, it’s the humidity.”

I speak from experience. I have been in Phoenix for more than 30 years. The most miserable summer I ever spent was in Rochester, New York where I spent the first 23 years of my life.  If you sit outside for 5 minutes in Rochester, in the middle of summer, you’re drenched with sweat.  In Phoenix, you have to work up a sweat.  It would have to be 122 everyday all summer long for me to feel as bad here as I did there.  People coming from the other direction have the wrong mind set.  They figure, “It’s 95 and I feel terrible.  I can’t imagine what 115 must feel like.”  In actuality, they’re comparing apples to oranges.

The only places that are actually cooler are the West Coast and at high elevation (cities like Denver, Salt Lake City, etc.).  However, it gets cold and snowy in the Mountains.  It gets cold, rainy and foggy in the Pacific Northwest.  You have fires, floods, earthquakes and mud slides in Southern California.  The cost of living is high in Hawaii.  All this makes the Sonoran Desert the best place to live in the U.S.

To top it off, cars don’t rust in Phoenix and I love my cars.

I love this place. However, for those of you winged individuals who must leave this wonderful place in May (AKA snowbirds), there are a few tips from a veteran of over 30 years of Phoenix summers (and a quite a few weeks in San Diego):

  • Have your air conditioner or heat pumps serviced and don’t turn them off. It’s important for the units to run to keep bearings lubricated and seals from drying out. Keeping the thermostat at about 88 F is a good balance between preserving your equipment and saving on energy costs.
  • Turn off all ceiling fans. An unattended fan that breaks while operating can wreak havoc on a home and cause a fire.
  • Have a friend or property manager enter the home about once every two weeks to flush toilets, turn on faucets, run the dishwasher, use the washing machine, etc. This will keep your valve seals from drying out and keep water in the waste line P-traps, the neat invention that keeps sewer gas and its accompanying foul odor from entering the house.  I strongly recommend that the water lines to the washing machine be turned off at the valves. The typical rubber hoses that are used can burst under pressure and result in flooding your home.  Steel braided feed lines are great protection against this and easy to change out.
  • If you brought a lot of grandma’s antique furniture from the East Coast to Phoenix, expect it to dry out very quickly in the summer. To combat this problem, locate that rocking chair and bureau in a room with an uncovered 5 gallon bucket full of water on the floor and close the door.  Make sure your friend or relative keeps the bucket full of water.  If small children will be in the house at any time do not do this as a child can fall into a bucket of water and drown.
  • If you have a security system make sure it’s turned on and monitored remotely by a security company.   Make sure they have your contact information so that you can be easily reached via cell phone or internet if there’s security breach at your home. Also, a trusted friend or relative who lives near the house should be listed as a contact. Some systems can be monitored over the internet.  If an alarm permit is required by our local police department or building official be sure to get one as a false alarm and subsequent visit by the police can result in a heavy fine.If you have a fire sprinkler system it is vital that you leave it on and that it is wired to your security system.
  • Notify the Post Office to redirect mail to a temporary location. Do not let UPS or FedEx and all such packages be left at the front door. One way a lot of service companies market their businesses is to fill a little plastic bag with stones, attach a business card or flyer to it, and throw it in the vicinity of your front door. Some leave flyers stuck to your front door knob. Thieves look for these signs as evidence that no one is home.  Again, a trusted friend or relative is vital to keep your house having that lived in appearance.
  • Verify that your garage door openers are turned off or unplugged. Thieves have ways of decoding these openers and stealing stored items out of the garage or entering your house through the garage. Ask your security company if the garage door is monitored as well as all the other entry points into the house.
  • The yard irrigation system needs to be re-programmed for summer use. Remember that plants in the summer heat cannot go without water for more than a few days. If you have a lawn make sure is kept neat and trimmed.
  • Secure all pet doors located in exterior walls.
  • If you have a pool, make sure that all side gates and pool fence gates are self-closing and self-latching. A pool service company is vital to keeping your pool healthy during the summer as a lack of proper chemicals in the water can ruin your pool plaster in a very short amount of time. During a summer storm pools can fill up with dust and plant debris clogging the filtering system quickly. Also, if you have an automatic water leveler keep it on as a pool can lose 2 inches of water every day during the summer. Never let the water level drop below the tile line. Exposed plaster will crack and require very costly repairs.
  • If you have a water softener, place it in a “no service” mode or simply unplug it.  Service the water softener when you return home and check the salt levels.  Break up any salt bridges that may have formed in the water softener.  If you have an RO filter under your kitchen sink, be sure to turn all valves off to it and have it sanitized when you arrive back home.   It’s also a good idea to check the air pressure in the holding tank.
  • It is strongly recommended that you turn off the natural gas to the house. Call the local gas utility and they will usually do this for you at no charge. It is especially important to turn the valves off to all gas appliances such as the water heater, clothes dryer, stove, furnaces, fireplaces, pool and spa heaters, barbecue, fire pits, yard torches, etc.
  • Turn off the electricity at the main electrical panel to all non-essential circuits such as to the water heater, stove, most room outlets, etc. Computers, monitors and all peripheral equipment should be turned off as well. Keep the electrical panel locked. It is important to keep the heat pump or air conditioning systems, security system, fire sprinkler system, irrigation system timers, and pool and spa filtering equipment turned on.
  • If you are leaving a vehicle in the garage, have a friend start it up once every few weeks. Make sure they do this outside and not in the garage as carbon monoxide is potentially fatal. If a friend can’t do this have a mechanic physically disconnect the battery. If you are unsure about this check with the car manufacturer as to the long term effects of disconnecting the car battery.
  • Instruct your friend or relative to always visit the house after a heavy rain storm. During the monsoon season, it would not be uncommon for one inch of rain to fall within a few minutes. It is important that if there are any roof leaks that they be attended to immediately.
  • All windows and doors must be locked.
  • Turn off all light fixtures in closet and attic areas.
  • Finally, to keep the effects of the sun from fading furniture, carpets, etc. provide exterior shading (sun screens) at all south and west facing windows and keep the curtains and shutters drawn.


Copyright 2007 – Schembri Engineers, Inc., All Rights Reserved. No reproduction of any kind is permitted without express written consent.