Rain is liquid water in the form of droplets that have condensed fromatmospheric water vapor and then precipitated—that is, become heavy enough to fall under gravity. Rain is a major component of the water cycleand is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystem, as well as water for hydroelectric power plants and crop irrigation. The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such ascumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains.
On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across easternNorth America and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 millimetres (28.1 in), but over the whole Earth it is much higher at 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar. Rain is also known or suspected on other planets, where it may be composed of methane, neon, sulfuric acid or even iron rather than water.
The water cycle The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go, in and out of theatmosphere. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes ofevaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In so doing, the water goes through different phases: liquid, solid (ice), and gas (vapor). The water cycle involves the exchange of energy, which leads to temperaturechanges. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment.
When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and transports minerals to different parts of the globe. It is also involved in reshaping the geological features of the Earth, through such processes as erosion and sedimentation. Finally, the water cycle figures significantly in the maintenance of life and ecosystems. Convectional rain Convectional rain happens in places of the world that are hot and wet. Sometimes, it also takes place in tropical deserts and inlandareas during summer, when temperatures are hot. During the day, the sun makes the ground very hot. Air near the ground surface is heated by conduction. The heated air expands, becoming less dense and rises in a strong upwards air current. When the temperature of the rising air falls to the dew point, water vapor shrinks into thick clouds and forms convection rain. Depending on the temperature it may fall as sleet or snow.
Orographic rainfall Orographic rainfall is when a parcel of air containing water vapor gets to a mountain, it is then forced upwards (the mountain blocking its way). As the parcel of air is forced upwards, it expands (adiabatic expansion) and also cools. As it cools, it becomes saturated because the amount of water it can hold decreases, so the water vapor begins to condense and finally falls as rain. Cyclonic or ‘frontal’ rain Cyclonic or ‘frontal’ rain occurs where two air masses meet and form a ‘front’. The warmer, moisture laden air rises over the colder air as a ‘warm front’. As the air rises it cools, and its relative humidity increases. Clouds form as water vapour condenses, and then fall as rain. Rain gauge Rain is measured in units of length per unit time, typically in millimeters per hour, or in countries whereimperial units are more common, inches per hour.
 The “length”, or more accurately, “depth” being measured is the depth of rain water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour. One millimeter of rainfall is the equivalent of one liter of water per square meter. The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties. The inner cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to 0.25 mm (0.0098 in) resolution, while metal gauges require use of a stick designed with the appropriate 0.25 mm (0.0098 in) markings.
After the inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty. Other types of gauges include the popular wedge gauge (the cheapest rain gauge and most fragile), the tipping bucket rain gauge, and the weighing rain gauge. For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gauge if left out in the open, but its accuracy will depend on what ruler is used to measure the rain with. Any of the above rain gauges can be made at home, with enough know-how. When a precipitation measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as CoCoRAHS or GLOBE. If a network is not available in the area where one lives, the nearest local weather or met office will likely be interested in the measurement.
Advantages of rain 1. Irrigation
* Water is necessary for almost all trees and crops to grow. Rain’s primary advantage, therefore, is the regular and automatic dispersion of the water needed for agriculture and natural vegetation. Other methods of watering via mass transport and desalinization from oceans is extremely ineffective by comparison. Due to the vast volume of water deposited in the form of rain around the world, there is no viable alternative to rain for supporting plant life on a large scale. Without it, much of the Earth’s plant life would likely die, leading to food and drought crises around the globe.
Freshwater Lakes and Rivers
* Rain collects in large reservoirs such as lakes and ponds, and feeds rivers and springs. Human civilization has always tended to thrive near these bodies of water because of their clear advantages in transportation, drinking water, food sources and recreation. Without rain, these bodies would not exist, and drinking water would have to be processed through desalinization facilities. Fresh water lakes and rivers are also vital components of the ecosystem. The species within them serve as important links in food chains around the world.
* Rain’s formation of rivers and lakes allowed humans to construct hydroelectric dams to generate electricity. But this is only a direct advantage of rain in producing electricity. Freshwater (which was collected from rain) is also used in coal, nuclear and thermal electricity generation. Rain’s advantages therefore extend to the majority of the energy sector. Without it, energy costs would be far more expensive as water would have to be transported from the oceans to these plants and mines to keep them operational.
Reduced Water Demand and Costs
* Rainfall reduces the need to transport water long distances for domestic needs such as cooking, washing, watering and consumption. Instead, towns and villages can use wells to collect rain that has been absorbed into the ground and stored in the region’s water table. This reduces the demand on large water reservoirs such as rivers, lakes and oceans and saves people and communities money.
* In the event of excessive rain following a dry period, mudslides can occur, with devastating effects. Not only do they erode valuable soil, they also can destroy homes and can be life-threatening. Usually, they occur on hills, and even if there aren’t homes on the hillside, there is the danger of potentially fatal carnage to roads, businesses and houses at the bottom of the hill.
* Flooding destroys homes, businesses, and, in worst case scenarios, lives. Sometimes the flooding is cumulative, as when its source is several days of steady rain. However, flash floods are perhaps even more dangerous because they can sneak up on people who are either unprepared for them or who have underestimated their danger. There is also an negative economic effect, as cities and towns are forced to rebuild.