Rain falls when moist air rises and cools. Cooling air is condensed and thus produces rain as it transforms from a vapor into a liquid. Four distinct weather patterns produce rain--each creating their own kind of rain, with distinct cloud formations and varied properties. The four specific types of rain commonly are referred to as frontal, relief, convection and monsoon.
A frontal rain requires the meeting of two air masses. It includes two different transformations--one being warm air meeting cold; the other being the reverse. When the movement of warm air encounters a steady flow of cold air, it is called a warm front. Because they maintain different densities, the warm and cold air cannot mix. The heavier cold air slips underneath the warm air. As they rise together, they eventually cool, thus condensing to cause clouds and rain. A warm front requires ample time to produce precipitation as its system changes from one cloud type to another before finally generating rain. Warm fronts can occur at any time, day or night, can take place over land or sea and may last from mere hours to many days.
The pattern of a cold front is reversed--with the cold air coming along to meet the steady stream of warm air. With the cold air being the predominant mass, the air is forced up quickly causing the rapid and large creation of cumulonimbus clouds. This combination produces an intense rainfall usually associated with thunder, lightning, and sometimes hail. Just as swiftly as a cold front comes on, it can dissipate and give way to clear blue skies.
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Mountainous regions maintain the elements required for relief rains. This weather pattern occurs when steadily moving air suddenly is required to move upward to cross over an elevated obstruction. As the air rises in its effort to pass over a mountain, it begins to cool, creating moisture and clouds (in the form of either mist or rain) on the side of the mountain where the air is rising. After the air begins its descent down the other side, it cools and can no longer produce moisture. Many mountains portray this meteorological phenomenon with one side being lush and green, while the other is barren and dry.
A convection rain falls from the result of sunshine, air pressure and altitude--requiring warm ground and low pressure. Low pressure creates unstable air that rises as the sun warms the ground all day. By afternoon the moist air rises rapidly, creating large cumulonimbus clouds and sudden, sometimes torrential, downpours.
Monsoons produce seasonal rainfall. They only occur in specific regions on Earth. During the dry season, the air moves steadily onto land from the ocean, where it descends and moves back out to sea. Continuing in this way, the air never rises and cannot produce moisture. The opposite holds true when monsoon season hits. Reversing its course as the land becomes warmer than the ocean, the air rises from the land, heads out to sea, descends and returns to land, rising again. This continued cycle generates copious amounts of rain with an intensity and duration that varies with every passing year.