CLIMATE OF PHOENIX: PART 2B

FREEZE AND GROWING SEASON DATA
MEAN DATES OF LAST 32 DEGREES IN SPRING AND FIRST 32 DEGREES IN FALL IN THE GREATER PHOENIX AREA

STATIONLASTLENGTH OF GROWING SEASON [DAYS]FIRST
Alhambra 2 NEMarch 10258November 23
BuckeyeMarch 6260November 21
CamelbackMarch 8260November 23
Deer ValleyMarch 7263November 25
Falcon Field-MesaApril 3234November 23
LaveenFebruary 21279November 27
Litchfield ParkFebruary 26270November 23
MarienetteFebruary 25271November 23
Mesa Experiment StationMarch 3266November 24
Phoenix Sky HarborFebruary 7309December 12
TempeMarch 2265November 22
Tempe Citrus StationMarch 14253November 22

PROBABILITY OF LOW TEMPERATURES
SPRING

STATIONTEMP90%75%50%25%10%
Litchfield Park40MAR 22MAR 30APR 8APR 17APR 25
36MAR 3MAR 13MAR 25APR 6APR 16
32FEB 1FEB 13FEB 26MAR 11MAR 23
28DEC 29JAN 14FEB 3FEB 23MAR 12
24JAN 25FEB 12
20

FALL

10%25%50%75%90%
40OCT 21OCT 27NOV 3NOV 10NOV 16
36OCT 29NOV 6NOV 15NOV 24DEC 2
32NOV 4NOV 13NOV 23DEC 3DEC 12
28NOV 16NOV 28DEC 11DEC 24JAN 5
24DEC 20JAN 4
20


ARIZONA INDIAN PROVERB:"When small water snakes leave the sand in low damp lands, frosts may be expected in three days".


In these probability of low temperature tables, the 50% level gives the "mean" or "average" as well as the "median" of occurrence of each threshold. Another way of expressing the same result is to state that, on the average, the first temperature as low as 40 degrees at Litchfield Park will occur by November 3 in one half of the years (or, for example, in 5 years out of 10).

By the same line of reasoning, there is a 90% probability that the first temperature as low as 40 degrees will occur by November 16. Again, it can be stated that 40 degrees will occur by November 16 in 9 years out of 10, in the long run.

PROBABILITY OF LOW TEMPERATURES
SPRING

STATIONTEMP90%75%50%25%10%
Mesa Experiment Station40MAR 27APR 4APR 13APR 22APR 30
36FEB 23MAR 7MAR 19MAR 31APR 12
32JAN 27FEB 13MAR 3MAR 21APR 7
28JAN 20FEB 8FEB 25MAR 13
24JAN 24FEB 10
20JAN 19

FALL

10%25%50%75%90%
40OCT 17OCT 25NOV 2NOV 10NOV 18
36OCT 25NOV 2NOV 11NOV 20NOV 28
32NOV 2NOV 13NOV 24DEC 5DEC 16
28NOV 8NOV 25DEC 14JAN 1
24DEC 13DEC 26
20JAN 10

GROWING SEASON MEAN LENGTH (DAYS)


TempLitchfield ParkMesa Experiment Station
40209203
36235237
32270266
28311309
24365365
20365365


Source: ARIZONA CLIMATE, SUPPLEMENT NO. II, PROBABILITY OF LOW TEMPERATURES OCCURRING IN ARIZONA, April 1967, University of Arizona Press.

EXTREMES OF FREEZE AND GROWING SEASON DATA
1896-1990

LATEST DATE OF FREEZE IN SPRING (32 Degrees or Lower)
CityOfficeMarch 31 1897
AirportOfficeApril 4 1945
EARLIEST DATE OF FREEZE IN AUTUMN (32 Degrees or Lower)
CityOfficeNovember 5 1922
AirportOfficeNovember 3 1946
AirportOfficeNovember 4 1956
LONGEST FREEZE FREE PERIOD (Above 32 Degrees)
City Office 368 DaysFebruary 19 1952 through February 21 1953
AirportOffice 693 DaysJanuary 13 1977 through December 6 1978
AirportOffice 1897 DaysNovember 23 1979 through January 31 1985
SHORTEST FREEZE FREE PERIOD (Above 32 Degrees)
CityOffice 222 DaysMarch 31 1897 through November 8 1897
AirportOffice 231 DaysApril 4 1945 through November 21 1945
AirportOffice 236 DaysMarch 13 1956 through November 4 1956

FREEZE AND GROWING SEASON DATA -- "THE COLDER SECTIONS OF THE VALLEY"

The climate of any urban area is continuously changing as housing developments are created from open lots and fields, as vegetation such as citrus groves are added or taken away, as streets are paved, and even, in the extreme, as the natural topography is altered.

Each home actually creates its own microclimate, absorbing heat during the day and radiating it at night. The temperature range can vary 5 to 10 degrees in relatively short distances. Groups of homes, prominent geographical features such as the Papago Buttes, Camelback and Mummy Mountain, South Mountains and Squaw Peak, large buildings such as the Central Avenue groups, broad streets and parking lots, and even a concentration of heat-producing automobile engines actually create what are known as "heat islands" that are separate, but nevertheless important, factors in the climate of the Phoenix area.

In general, however, the farther a particular location is away from these "heat islands", the more temperatures are likely to be colder than at other locations in the valley. Tolleson, on the west side of Phoenix, surrounded by agricultural land, is frequently 3 to 5 degrees colder than most locations in the city. However, there are some locations inside the city that are as much as 5 to 10 degrees colder than temperatures reported at Sky Harbor Airport. These locations are unique only because of their individual relationships to the drainage of cold air and their distance from "heat islands".

At night, cold air generally flows like water toward lowest sections and often "pools" when it can go no farther or is "dammed" by a natural or unnatural obstruction. On still nights, the ground loses heat more rapidly than the air itself, and a temperature inversion forms where the air close to the ground is much colder than the air a short distance above the ground. At times there may be as much as 5 to 6 degrees difference between the temperature at ground level and the 5-foot level, where standard temperatures are measured. Hence, frost may appear on the grass and low vegetation when temperatures above freezing are reported.

In summary, the temperature structure of the area is quite complex, varying from point to point in the valley and is constantly changing with the time of the day and with the growth of the community. Most people can, with a little study, determine the relationship of their local minimum to the forecast low temperature in the colder sections of the valley.

NOTE: The lowest temperature of the day usually occurs within one hour before or after sunrise; while the highest temperature usually occurs about two to three hours before sunset.