Last updated on April 23, 2014 at 12:54 EDT


Rain is a type of liquid precipitation. It is defined as condensation of atmospheric water vapor that forms into drops that become heavy enough to fall, and often making it to the surface. Rain is the primary source of most of the fresh water in the world. Rain provides suitable conditions for diverse ecosystems, water for power sources and for crop irrigation. Not all rain reaches the surface. Some rain evaporates as it falls, especially through dry air. This type of rainfall is known as “virga”, a phenomenon often encountered in hot desert regions.

The hydrologic cycle of rain plays an important role for life on earth. Moisture from the oceans evaporates into the sky, condenses into droplets, precipitates from the sky over land and other areas, and eventually returns to the ocean through rivers and streams, and the cycle repeats itself over and over again. Water vapor from plants also contributes to this cycle. The “˜Bergeron process’ is a scientific explanation of how rain forms and falls.

The phenomenon of rain is not isolated planet Earth. Saturn’s largest moon Titan has numerous channels on its surface that are thought to be carved by methane rain. Venus’ atmosphere is plagued with sulfuric acid that evaporates 15 miles above the surface. The gas giants of Jupiter, Neptune, Uranus, and Saturn most likely have rain of various compositions in their upper atmospheres.

Rain on our own planet is sometimes produced by human activities. Car exhaust and other human sources of pollution form condensation that evaporates into the air and produces clouds and possible rain. Weekly commuting and commercial traffic causes pollution to build up and peak by Saturday. After five days of weekly pollution build up, the condensation that forms clouds usually produces rain. In areas that are quite populated, such as the eastern seaboard of the US, there is a 22% higher chance for rain on a Saturday than on a Monday.

Precipitation can be classified by the rate at which it falls. Very light rain is rain that falls at less than 0.01 inch per hour. The classifications continue with light rain, moderate rain, heavy rain, very heavy rain, and extreme rain with rainfall amounts of 2 inches per hour or more. Most forms of precipitation are measured using a rain gauge.

Raindrops are often depicted as being “teardrop-shaped”, but this is incorrect. Only drops of water forming from certain objects are tear-shaped at the moment of formation. Small raindrops are more spherical in shape. Large raindrops become flattened on the bottom, much like a hamburger bun. Even large ones can be shaped like parachutes. Sometimes very large (0.20 in/diameter) raindrops will become unstable and fragment. The average raindrop is about 0.05 inches in diameter. The largest raindrops ever recorded were in Brazil and the Marshall Islands in 2004 with droplets as big as 0.4 inches in diameter. Large drops are usually formed by smaller drops colliding with one another and forming one drop. This typically only happens with storms carrying a high content of moisture.

Rain has a normal pH level just under 6. This is due to atmospheric carbon dioxide that dissolves in the droplet and forms extremely small quantities of carbonic acid, which partially detaches, and thus lowering the pH. In some regions, like deserts, airborne dust particles contains enough calcium carbonate to counter the acidity levels in precipitation, making it neutral or alkaline. Rain with a pH of 5.5 or lower is considered acid rain.

Rain plays an important role in agriculture. All plants need at least some water to survive, and rain is the most effective means of watering. A regular rain pattern is normally vital to healthy plants. Too little or too much rainfall can be detrimental or even devastating to crops. A drought can be drastically devastating and kill numerous crops, while extremely wet weather can cause disease and fungus. Some plants thrive in areas with limited precipitation, such as in deserts, while tropical plants need hundreds of inches of rain per year to survive.

Protective devices have been developed to deal with the presence of rain, such as umbrellas and raincoats. Diversion devices such as gutters and storm drains have also been created. Many people just stay indoors on rainy days, especially in the tropical climates where rain is commonly accompanied by severe thunderstorms and can be extremely heavy. Rain is often harvested or used as gray water. Excessive rain, especially during periods of long dry spells that harden the soil to the point that it cannot absorb water, may cause flooding. Many people find the scent, either during or immediately after a rain, pleasing and distinctive. The source of this scent is the oils produced by plants that are released into rocks and soil and later released into the air during rainfall.

Depending on the region, precipitation derives from different areas and can vary greatly. In the United Kingdom most rain is driven into the country by the south-western trade winds along the warm gulf stream currents. The western coastal regions can receive up to 40 inches of rain per year at sea level, and up to 100 inches in the mountains.

In the United States, the city of Seattle, Washington is well known for its rainy climate. However, despite its rainy climate, the city experiences less rainfall (37 inches per year) on average than New York City (46 inches per year). Seattle does have more cloudy days (201 per year) than New York City (152). The wettest city in the 48 contiguous United States is Mobile, Alabama, with an average rainfall of 67 inches per year. Alaska’s temperate rainforests of the southeast get an average of 160 inches per year.

Although Australia is the world’s driest continent, more than 300 inches of rain falls on Mount Bellendon Ker in the northeast each year. 472 inches were recorded in the year 2000. Melbourne’s climate is similar to that of Seattle, Washington in the USA, but only gets about 25 inches per year. Sydney receives an average of 48 inches of rain per year.

Cherrapunji, a town on the southern slopes of the Himalayas in Shillong, India is one of the wettest places on Earth. The average annual rainfall is about 450 inches per year. The highest recorded rainfall was 904.9 inches in one year in 1861. Although Mawsynram, Meghalaya, India has an estimated yearly rainfall of 467.4 inches, Cherrapunji is known as the wettest, as rainfall records were never supervised properly at Mawsynram. It should be noted that Cherrapunji is also considered the wettest area on Earth as most of its rainfall occurs during the monsoon seasons.

The town of Lloró, situated in Chocó, Colombia, is considered the place with the most rainfall, averaging 523.6 inches per year. Along with Cherrapunji in Asia, the town of Tutunendo in Colombia, South America is also one of the wettest on the planet. In 1974 the town recorded 86.3 feet of rainfall, the largest annual rainfall measured in Colombia. Unlike Cherrapunji, which gets most of its rain between April and September, Tutunendo receives rain throughout the year. Tutunendo has an average of 280 days of rain per year, of which 68% of the rain falls at night. Some storms in Chocó have been known to drop as much as 20 inches of rain in one day.

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