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Maricopa County Cooperative Extension Home Horticulture:
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Gardening & Landscaping in the Low Desert

Heat in the Valley of the Sun


When Does It Hit 100 degrees In The Valley?

This chart shows the dates that Sky Harbor Airport reached the first official 100 degree temperature from 1980 to 1996. For comparison, the chart shows the long term average 100 degree date. The earliest first 100 degree temperature was March 26, 1988. The latest was June 18, 1913. As the graph shows, only four of the last 16 years has the first 100 degree temperature equal to, or later than, the long term average.

In 1995, the first 100 degree day didn't occur until May 31. This was the latest first 100 degrees in over 15 years.

May 15, the average date of the first 100 degree day 1896-1996

Heat Wave Safety Rules

In our desert climate, heat causes more health problems than any other weather factor. Our soaring summer temperatures and low humidity can be a dangerous combination. Follow these rules to stay safe in the Arizona desert.

Slow down. Your body can't do its best at high temperatures and humidities. Don't dry out. Drink plenty of water while the hot spell lasts.
Heed your body's early warnings that heat syndrome is on the way. Reduce your activities immediately and get into a cooler environment.Avoid thermal shock. Acclimatize yourself gradually to warmer weather. Treat yourself gently for those first two or three hot days.

Dress for summer. Light colored and lightweight clothing reflects heat and sunlight.

Vary your thermal environment. Physical stress increases with exposure time in heat wave weather. Try to get out of the heat for at least a few hours each day.
Put less fuel on your inner fires. Foods like proteins, that increase metabolic heat production, also increase water loss.

Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.

Heat Stress

The heat wave of 1980 killed more than 1250 people in the United States. In the 40 years from 1936-1975, nearly 20,000 people have died from the effects of excessive heat. The only other weather related phenomena that kills more on the average each year is lightning.

The effects of heat on the body are known as heat stress. Many factors contribute to heat stress, but the most important elements influencing heat stress and comfort are temperature and humidity. As the chart indicates, the combination of high temperature and humidity greatly increases the threat for heat stress. The Apparent Temperature or Heat Stress Index assumes a very light breeze and you being in the shade. Of course, how hot "it feels" varies from one person to another, but this index seems to give a good idea of what the hot weather "feels like".

To find out what the air "feels like", use the table below. For example, with the air temperature of 90 degrees, and relative humidity of 60%, move down to where the columns meet and find the Heat Stress Index of 100 degrees.

Heat Stress Index

9588909193949698101104107110114119124 130 136
10093959799101104107110115120126132 138 144
10597100102105109113118123129 135 142 149
110102105108112117123 130 137 143 150
115107111115120127 135 143 151
120111116123130 139 148
125116123 131141
130122 131

    Sun Awareness in Arizona

    Arizona is famous for its sunshine, and experts generally agree that the sun can be beneficial. It can provide relief from acne, aching joints and psoriasis, and it is also a source of Vitamin D. Experts also generally agree that the hazards from too much sun outweigh the benefits. Arizona has the highest rate of skin cancer among the 50 states and one of the highest rates in the world!

    Prolonged overexposure to the sun is the primary cause of skin cancer. Skin cancer is reported in 500,000 Americans each year and 6,500 to 7,500 cases result in death. That is more deaths per year than all other weather factors combined, including tornadoes, hurricanes, heat waves and winter storms. Fortunately, 95% of skin cancer patients are free of the disease once treated. The cure rate would approach 100% if everyone with symptoms sought prompt medical attention.

    How to Prevent Skin Cancer

    1. Minimize sun exposure, especially during the hours of 10 a.m. and 2 p.m., when the sun's rays are the most intense;
    2. Use a sunscreen. Apply often while in the sun. Use a sunscreen with a sun protection factor (SPF) of 15 or higher;
    3. Cover up. Wear a hat, long-sleeved shirts and pants when out in the sun;
    4. Be aware of reflecting surfaces. Sand, water, pool decks, and snow can reflect up to 85% of the sun's damaging rays;
    5. Remember that damaging ultraviolet rays can penetrate clouds and are stronger in the thinner air of higher altitudes. Don't forget the sunscreen in these conditions.
    6. Avoid tanning salons and sunlamps. Ultraviolet rays from these sources are the same as in sunlight and can cause sunburn, premature aging and increased skin damage.

    Skin Cancer Warning Signs

    See your doctor if you see any of these or other changes on your skin you can't explain or which last for than 30 days.

    1. A sore that doesn't heal;
    2. Appearance of a new blue, black, brown, or pink area on you skin;
    3. A mole that changes in color, shape, size or texture;
    4. A mole that changes in sensation.

    For More Information on Skin Cancer:

    The Arizona Cancer Center
    University of Arizona
    Health Sciences Center
    Tucson, AZ 85724
    (520) 626-6044

    Arizona Sun Awareness Project
    Tucson, AZ 85724
    (520) 626-7935

    Cancer Cope Line 1-800-622-COPE

    ® Copyright 1997 by Ed Phillips/KTAR Radio. Information provided courtesy of KTAR via Ed Phillips Arizona Almanac.

    To Home Horticulture in Maricopa County, AZ

    Heat in the Valley of the Sun
    visitors since April 10, 1998
    Last Updated July 22, 1998
    Author: Cathy Rymer, Administrative Secretary, University of Arizona Cooperative Extension, Maricopa County
    Reviewed by: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
    © 1997 The University of Arizona, College of Agriculture, Cooperative Extension, in Maricopa County
    Comments to Cathy 4341 E. Broadway Rd., Phoenix, AZ 85040
    (602) 470-8086 ext. 308