<P> The next most populous county is Pima County, which had a 2000 population of 843,746 . The county seat of Pima County is Tucson, where nearly all of the population is concentrated . Combined, nearly 80% of Arizona residents live in either Maricopa County or Pima County, even though the two counties make up 16% of Arizona's total area . Because of the high population of Maricopa County and Pima County, both counties are dominant in state politics . </P> <P> About 15% of Arizona is privately owned, the remaining land consisting primarily of public forest and park land, Native American reservations, military institutions, and swaths of wilderness held by the Bureau of Land Management . Arizona is home to 21 federally recognized tribes, which are each semi-autonomous . The large majority are part of the Navajo Nation, which is the largest Native American reservation in terms of population and size . The Navajo Reservation covers all of northeastern Arizona along with portions of New Mexico and Utah, and had a population of 180,462 as of the 2000 census . </P> <P> Due to the state's large area and range of elevation, there is a variety of localized climate conditions . Overall, most of Arizona receives little precipitation, and is classified as having either an arid or semi-arid climate . The northern parts of the state and the mountainous areas tend to have cooler climates, while the southern parts of the state tend to be warm year round . </P> <P> Precipitation in Arizona is governed by elevation and the season of year . The peak periods of rainfall are during the early winter, when storm systems from the Pacific Ocean cross the state, and during the summer, then moisture - bearing winds sweep into Arizona from the southeast, which obtain moisture from the Gulf of Mexico . Summer rains tend to occur in the form of thunderstorms, which result from excessive heating of the ground and the lifting of moisture - laden air along mountain ranges . These thunderstorms can cause strong winds, brief periods of blowing dust, and infrequently cause hail . The heaviest precipitation is found in the mountain ranges of central and southeastern Arizona, while the driest conditions are found in the arid regions of southwestern Arizona . </P>

What are the key aspects that make arizona's climate
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