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WHY IT RAINS
To have rain (and other forms of precipitation), there must be moisture in the air (from evaporation and transpiration) and the moisture must condense. Moisture condenses when the air cools (see Meteorological Concepts). Air cools when it rises. Air rises by three means:
1) Convection: heating of the ground surface heats the air above the ground causing the air to rise, thus potentially rain. This is how most of our rain occurs in the monsoon summer, and why, during this time of year, it usually rains in the afternoon. These rains are usually short in duration (and area), but intense. Most of the moisture for the summer monsoon rains comes from the Gulf of California and the Gulf of Mexico.
2) Orographic (Mountain Range): air is blown up the side of a moutain range. This is why it rains year-round more in the Santa Catalina Mountains than it does in the valley floor.
3) Frontal: a cold front pushes warm air in front of it upward. This is how we get most of our winter/spring rains. Cold fronts push down from the north, pushing our warmer air upward, causing it to rain. These rains are usually longer in duration (and cover more area), but are less intense. Most of the moisture for the winter/spring rains come from the Pacific Ocean.