Precipitation occurs when moist air rises to cooler altitudes, condensing the water out of the air into droplets. Once these droplets become heavy enough, often by coalescing around motes of dust or other particles, they fall out of the cloud as precipitation. Without significant updrafts bringing more moisture to the cloud layer, the condensed water may remain light enough to stay aloft, which is why not every cloud brings rain.
Rain clouds may form in a variety of meteorological conditions. Commonly, they occur across frontal boundaries, where a mass of warm air is forced upward by a cold front, pushing a large amount of moisture into the upper atmosphere. This is the cause of long, soaking rains due to the sheer amount of condensation produced by the updrafts. Air masses may also be pushed upward by topographic features like mountains, producing rain on the lee side of these geologic features.
Another type of rain is produced when the sun's heating creates a convective current in the atmosphere. This energy heats the air near the ground, causing it and its moisture to rise in a relatively small area. This pattern tends to create small, isolated showers that can be extremely vigorous but may not move quickly, a common weather pattern in the summer months.