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Arizona Climate Temperature Two characteristics define Arizona in terms of its temperature regimes: a wide variation in temperature across the state due to its great elevational differences and extremely high summertime temperatures in its deserts. Whereas in many other states daytime high temperatures may vary by 10 F or 20 F across the state, Arizona commonly records differences of 40 F to 45 F. In late June it is common to have temperatures of 115F in Bullhead City on the same day that Greer or Alpine record a temperature of only 75 F. This large difference in temperature across the state is due largely to differences in elevation. As elevation increases atmospheric temperature decreases due to a decrease in atmospheric pressure. Lower atmospheric pressure means that there are fewer air molecules in a given volume of air. This in turn means that there are fewer collisions of molecules that we register as temperature. On average atmospheric temperature decreases 3.6 F for every 1000’ of elevation gain – this is known as the average environmental lapse rate. However, this rate does fluctuate day to day and with the seasons. In summer the environmental lapse rate is usually higher than average and in winter it is usually a little lower than average. This means that in summer the mountain peaks of southeastern Arizona may be 25 F to 35 F cooler than the surrounding desert valleys while in winter they may be only 10 F to 15 F cooler. Compare the maps below of average January and July temperatures. Arizona temperatures are controlled not only by elevation, but also to a lesser degree by latitude. The latitude of a location marks its angular distance from the earth’s equator. Places farther from the equator receive incoming solar radiation at a more oblique angle and therefore receive less energy from the sun per unit area. The latitudinal control on atmospheric temperature is stronger in winter than in summer. In winter places on the Colorado Plateau in northeastern Arizona are almost 10 F colder than places at the same elevation but farther south in the mountains of the Basin and Range. The extremely high temperatures recorded in Arizona’s desert regions in summer are due not only to low elevation, but also to the dry clear skies characteristic of the area. Dallas, Texas and Atlanta, Georgia lie at roughly the same elevation and the same latitude as Phoenix. Why then does Phoenix have summer temperatures averaging 10 F to 15 F hotter than these other cities? Why do some places in Arizona reach temperatures over 120 F? It is due to the fact that the humid cloudy skies of these other cities, while making it feel stickier, actually reflect incoming solar radiation and keep the temperature cooler. The southwest U.S. in general has some of the most cloudless skies on earth. Why? It seems that our planet has five cells of what are called sub-tropical high pressure (STH). These STHs are all centered at roughly 30 latitude, and are located just off the west coasts of continents. High-pressure air is sinking air and as air sinks it compresses and heats up – just like compressing air into a car tire. Heating the air increases its water vapor capacity and therefore dries it out. Winter temperatures Average temperature values in January range from 20 to 25 F in the high regions of the San Francisco Peaks, White Mountains, and Kaibab Plateau (all of which are on the Colorado Plateau), while average temperatures in January approach 55 F in the region of the lower Gila and Colorado Rivers (which are found in the Basin and Range). Winters are coldest on the high elevation plateaus of the Colorado Plateau province, particularly above 7000'. Temperatures below freezing are common in this region from November through April. Temperatures below -30 F have been recorded at three places on the Plateau all fairly distant from one another: -32 F at Chinle in the far northeastern corner of the state; 33 F at Fort Valley near Flagstaff; and -40 F at Hawley Lake in the White Mountains. Temperatures as low as -50 F must certainly have occurred near the summits of the San Francisco Peaks and in the highest portions of the White Mountains. Winter temperatures are far warmer in the low-lying basins of the Basin and Range province, especially in southwestern Arizona. The warmest winter temperatures are found in the low-lying valleys of the lower Colorado River, the lower Gila River, and along the Santa Cruz River where average winter temperatures reach 50 F. Average minimum winter temperatures over practically the entire desert floor of southwestern Arizona do not go below freezing. Some places are warm enough at night to have average winter minima of up to 45 F. It is these places which support Arizona's citrus industry. Surprisingly, the warmest spots at night in winter are not at the absolute lowest elevations, as you would think. On winter nights, heavy cold air from surrounding mountain ranges sinks/slides downhill to occupy the lowest layers of the valleys in between the mountains. These cold air layers are relatively thin (500' to 1000'). Therefore, on winter nights the warmest spots in the desert are located on the bajadas and pediments of desert mountain ranges around the edges of valleys and up off the desert basin floor. For example, winter nighttime temperatures are warmer in White Tank Regional Park 15 miles west on Olive than here in Glendale due to Glendale's lower elevation. Summer temperatures Average temperatures increase from 30 F to 40 F from January to July. The high sections of the Colorado Plateau now enjoy average temperatures of 70 F, with higher sections such as the SF Peaks, the Kaibab Plateau, and the White Mountains having average temperatures of only 60 F. These very high areas may still have winter minima below freezing, but most of the Colorado Plateau has minima above 40 F. The rest of the state is quite warm in the summer except for the isolated high peaks of the southeastern and northwestern Basin and Range region. Practically all areas below 5000' have summer maxima in excess of 90 F. In fact, practically all of the southwest section of the state report daily maxima in summer of over 100 F. Temperatures over 120 F have been recorded at all towns on the Colorado River south of Lake Mead, and all towns on the Gila and Salt Rivers from Phoenix downstream. These river valleys have the lowest elevations and therefore the hottest temperatures of all. The highest temperature on record for Arizona is 128 F registered at Lake Havasu City in June of 1990. The southeastern portion of the state contains higher elevation basins within the Basin and Range province. Therefore the summer temperatures in these desert basins are cooler than in southwest Arizona. The warmest summer temperatures in Arizona usually occurs in late June and early July. This period directly precedes the summer monsoon season which brings with it cloud cover and rain. Once the monsoon season begins, increasing cloud cover reflects solar radiation and temperatures begin to decrease slightly. The intense solar radiation of late June heats the desert surface to such a degree that hot rising air creates a thermal low-pressure system. It is this thermal low-pressure system that brings on the moistureladen winds of the monsoon season. Precipitation Precipitation amounts Arizona tends to be very dry in general due to its west coast location and 30 North latitude – which put it under the influence of sub-tropical high pressure. As we discussed, these locations on all continents tend to be fairly dry due to subsiding(sinking) air from the upper atmosphere. Subsiding air is compressing, heating, and drying out and resulting in intense solar radiation. Roughly half of Arizona receives less than 10 inches of rain a year (see map at left). This includes most of southwestern Arizona and most of the northeast as well. The most arid region, receiving less than 5 inches of rain a year, are the low elevation areas in the west and southwest portions of the state along the valleys of the Gila and Colorado rivers. Yuma receives less than 3 inches of rain per year. Certain regions in the central portion of the state, however, receive significant amounts of precipitation. The Kaibab Plateau, the San Francisco Peaks, the Mogollon Rim and the White Mountains, all of which lie on or near the southern/western edges of the Colorado Plateau physiographic province, receive more than 20 inches of precipitation per year with some very local areas receiving up to 30 inches per year. Other rainier regions of the state include the higher mountain ranges of the Transition Zone province (Mazatzals; Bradshaws; Sierra Anchas) and of the Basin and Range (Catalinas; Rincons; Santa Ritas; Huachucas; Chiricahuas, Pinalenos). (see map) All these regions lie at high elevations. High elevations – being cooler – have an easier time reaching saturation. Just as atmospheric temperature in Arizona is linked to elevation, elevation exerts a strong influence on precipitation as well. Two ingredients are necessary to create clouds and precipitation – a moisture source to put humidity into the air and a lifting mechanism to cool the air toward saturation. For much of the year Arizona is under the influence of sub-tropical high-pressure which keeps Arizona dry. However, twice a year low pressure (rising air) makes its way into the state and brings Pacific moisture with it. These two rainier times of year will be discussed below. Precipitation seasons The graph (below) and table (below) show the seasonality of Arizona's precipitation. From these it should be apparent that Arizona's precipitation tends to arrive in two different seasons. There is a summer rainy season (July and August are the rainiest months for all portions of the state) and a winter rainy season (lasting from December through the middle of March). The table demonstrates that eastern Arizona benefits most from the summer rains, while the western portion of the state gets more total rain in the winter. The arid low-lying deserts of the southwestern portion of the state are not the driest portion of the state in all months however. During the winter, the cold high plateaus in the northeastern portion of the state are even drier due to surrounding mountain ranges blocking arriving moisture. Winter up here in this region is very cold and very dry. Arizona's dry seasons are the spring and the fall. Drought conditions are greatest in May and June. The rainy parts of the state may go weeks without rain while the low-lying southwest deserts receive rain during May and June only once every three years on average. The second dry season is the fall. This period of drought is not as severe as the May-June drought. The fall drought seems more pronounced in October in southern Arizona and more pronounced in November in northern Arizona. Causes of precipitation Four major factors favor precipitation: moisture/humidity in the air; mountainous terrain; convergence of air flow from the south; and intense thermal heating. Moisture/humidity in the air - this factor seems fairly obvious. We can not get rain unless there is water in the air to form rain. Because Arizona is dominated by high pressure, by subsiding air from the upper atmosphere, our air is usually very dry. Special conditions must arise to bring moisture to our state. These conditions will be discussed in the next section. Mountainous terrain forces air to rise and cool. Because cool air can not hold as much moisture in the form of invisible gaseous water vapor as warm air can, cool air becomes saturated by water vapor more easily. Saturated conditions are what leads to clouds (water in the visible liquid state). Convergence of air from the south crowds the air and forces it to rise and cool. Intense thermal heating of the air by hot desert ground surfaces forces air to rise and cool (see above) Arizona's two rainy seasons Keeping the above causes of precipitation in mind it is now possible to explain Arizona's two seasons of increased precipitation. July and August (monsoon season) In July and August the ground surface of the desert becomes so hot that it superheats the air above and causes it to rise. This creates what is called a thermal low-pressure system centered in the Yuma area. A low-pressure cell is an area of rising air. This lowpressure cell pulls in air from the surrounding region as it rises. One region providing air to this rising low-pressure cell is the tropical Pacific Ocean west of Mexico. This oceanic air is very humid and provides a good moisture source for Arizona. When this humid air arrives in Arizona it is forced to rise due to intense heating and due to large mountain ranges in the southeast corner of our state and in central Arizona. This rising air cools, the invisible gaseous water vapor condenses to clouds (liquid water) and it rains. The portions of the state receiving the most rain are the mountains of the southeast corner (they get the first crack at the moist air) and then the mountains to the north in the transition zone and on the southern edge of the Colorado Plateau. Because this moisture arrives from the south, the southern slopes of many mountain ranges often receive the highest amounts of rain. Phoenix, because of its low elevation and lack of nearby large mountains, receives very few of these summer rainstorms. Summer rainstorms occur most often in the late afternoon, as this is the time of the most intense heating. December through March (winter rains) In winter our moisture source is the North Pacific Ocean. At this time of year we can not rely on intense heat for air to rise. However, all year long warm moist air from the subtropical latitudes collides with cold dry air from the polar latitudes. These collisions result in intense large storms known as mid-latitude cyclones. The warm air rises over the top of the cold air forming centers of low pressure. These low-pressure cells (mid-latitude cyclones) are born over the North Pacific Ocean and move from west to east across the United States most of the year. However, they usually stay far to the north of Arizona. However, occasionally these storms will veer southward if atmospheric conditions change. These storms usually are moving fairly quickly and linger no longer than two or three days. However, these storms do last longer than the short thunderstorms associated with our summer rains. Several storms days or weeks apart may track over Arizona before the normal atmospheric conditions return and the subtropical cyclones once again pass to the north. It is these winter storms that provide the higher elevations with snow.