thumb|San Carlos during the wet season Precipitation is seasonal and mostly occurs in the higher elevations. In hot and arid or semi-arid lands, evaporation vastly exceeds precipitation.Yetman, p. 9 Mexico's most arid area, the Altar Desert is located in this state. The east of the state is dominated by the Sierra Madre Occidental, which has less extreme temperatures and relatively more rainfall due to altitude. Most moisture comes in from the Pacific Ocean and the tropics, which is pushed against the Sierra Madre. This cools the air masses, leading to rain and occasionally snow in the higher elevations. While most of the rain falls in the mountainous areas, much of this water finds its way back to the western coastal plains in the form of rivers and streams that empty into the Gulf of California and fill underground aquifers. Most of the year's precipitation falls during the rainy season, which is locally called "las aguas" (the waters). These last from July to mid-September, when monsoon winds bring moist air from southerly tropical waters. Most of this is from the Pacific Ocean west of Central America but can also come from Gulf of Mexico as well. This moister flow results in nearly daily afternoon thunderstorms. After the las aguas, there may be additional moisture brought in by hurricanes, which generally move west along the Pacific coast of Mexico and occasionally come inland, especially in southern Sonora. However, these storms tend to drop large quantities of rain in a short time, causing flooding and destruction.
thumb|San Carlos during the wet season Precipitation is seasonal and mostly occurs in the higher elevations In hot and arid or semi-arid lands evaporation vastly exceeds precipitationYetman p 9 Mexico's most arid area the Altar Desert is located in this state The east of the state is dominated by the Sierra Madre Occidental which has less extreme temperatures and relatively more rainfall due to altitude Most moisture comes in from the Pacific Ocean and the tropics which is pushed against the Sierra Madre This cools the air masses leading to rain and occasionally snow in the higher elevations While most of the rain falls in the mountainous areas much of this water finds its way back to the western coastal plains in the form of rivers and streams that empty into the Gulf of California and fill underground aquifers Most of the year's precipitation falls during the rainy season which is locally called "las aguas" (the waters) These last from July to mid-September when monsoon winds bring moist air from southerly tropical waters Most of this is from the Pacific Ocean west of Central America but can also come from Gulf of Mexico as well This moister flow results in nearly daily afternoon thunderstorms After the las aguas there may be additional moisture brought in by hurricanes which generally move west along the Pacific coast of Mexico and occasionally come inland especially in southern Sonora However these storms tend to drop large quantities of rain in a short time causing flooding and destruction