Rain is precipitation that consists of drops of water. Raindrops form in clouds when microscopic droplets of water grow or when particles of ice melt before reaching the ground. Rain falls throughout most of the world. In the tropics, almost all precipitation is rain. But in the inland areas of Antarctica, all precipitation is snow.
Rain is a part of an unending process known as the hydrologic cycle. This process begins with the evaporation of water from the surface of the earth—mostly from the ocean. The resulting water vapor—the gaseous state of water—then condenses, forming clouds of liquid droplets. Some of the droplets may freeze, forming particles of ice. The droplets and particles then undergo a variety of changes in the clouds. Eventually, however, they fall to the surface as precipitation. Much of the precipitation that falls on continents eventually flows in rivers into the ocean. The process then repeats itself.
When ocean water evaporates, the salt in the ocean water remains in the ocean. As a result, rain and frozen precipitation, such as snow, hail, and sleet, are made up of fresh water. Human beings and most other creatures that live on land depend on the fresh water produced by the hydrologic cycle.
Rain cleans the air by washing away dust and chemical pollutants. But too much rain may cause flooding that destroys property and threatens lives. Heavy rainfall can also damage crops and speed up the loss of soil.
Characteristics of rain
Raindrops vary greatly in their size and in the speed of their fall. The diameter of most raindrops ranges from about 0.02 to 0.25 inch (0.5 to 6.35 millimeters). Rates of fall for these sizes range from 61/2 feet (2 meters) per second for the smallest to about 30 feet (9 meters) per second for the largest. Drizzle is rain with drops less than about 0.02 inch in diameter and falling speeds of less than about 6 is rain with drops less than about 0.02 inch in diameter and falling speeds of less than about 61/2 feet per second.
Raindrops smaller than about 0.04 inch (1 millimeter) in diameter are round. Larger drops are somewhat flat, due to the force of the air flowing around them. The largest raindrops alternate between a flattened shape and a stretched-out shape.
Most rain comes from convective clouds, in which currents of air usually rise 10 to 100 feet (3 to 30 meters) per second. Within the currents, the air undergoes many sudden changes in speed and direction. Because of the currents, the growing raindrops take complicated paths within the clouds.
It takes about 10 to 30 minutes for rain to form. The time rain takes to fall to the ground depends on the height of the cloud in which it forms and the size of the raindrops. This time ranges from a few seconds for large drops falling from low clouds to about 15 minutes for small drops falling from high clouds. In dry areas, rain from high clouds may not even reach the ground. Instead, it evaporates completely as it falls.
The intensity of rainfall varies widely. Rainfall that is too light to measure is called a trace of rain. Rain that falls at a measurable rate of up to 0.10 inch (2.5 millimeters) per hour is a light rain. A moderate rain falls at a rate of 0.11 to 0.30 inch (2.8 to 7.6 millimeters) per hour. Rain that is even more intense is heavy rain.
Acid rain is the term for rain, snow, sleet, or other precipitation that is polluted by such acids as sulfuric acid and nitric acid. The acid in acid rain forms when chemical compounds known as nitrogen oxides and sulfur dioxide react within growing droplets and raindrops. These compounds are released by motor vehicles, factories, and certain power plants. Acid rain pollutes lakes and streams. It also damages buildings and other structures and is suspected of harming forests and soil.
Formation of rain
The “life” of a raindrop begins when molecules of water vapor in a cloud condense on a tiny particle of matter called a cloud condensation nucleus. This particle can be a speck of dust or soot. However, it is usually a very tiny droplet known as a haze droplet. This consists of a concentrated solution of sea salt or of a chemical compound such as ammonium sulfate or magnesium sulfate. The compound may have formed when gases in the air interacted chemically, or it may have been given off by a motor vehicle or a factory.
Continuing condensation enlarges the droplet, which grows as long as the air is rising. But it will grow by condensation to a diameter of only a few thousandths of an inch—less than 0.1 millimeter, too small to be a raindrop. To become a raindrop, a droplet must undergo one of two processes—(1) the coalescence process or (2) the ice-crystal process.
The coalescence process
produces much of the rain that forms over the oceans and in the tropics. This process occurs as droplets fall. The larger droplets fall faster than the smaller ones. The larger droplets thus collide with the smaller ones and coalesce (combine) with them, becoming larger yet. A large droplet that falls 1 mile (1.6 kilometers) through a cloud may coalesce with 1 million small droplets.
The ice-crystal process
accounts for much of the rainfall in the two temperate zones—(1) the area between the Tropic of Cancer and the Arctic Circle and (2) the area between the Tropic of Capricorn and the Antarctic Circle. In both zones, the temperature of the clouds is usually below the freezing point of water, 32 °F (0 °C).
The clouds consist of droplets of supercooled water, water that is colder than the freezing point, but still liquid. But some of these droplets eventually freeze because they contain, or come into contact with, microscopic particles called freezing nuclei or ice nuclei. Most freezing nuclei are dust particles or tiny specks of plant debris raised by the wind.
When a supercooled droplet freezes, it turns into an ice crystal. The crystal then grows by collecting water vapor. This growth produces the complicated shapes of snow crystals. As a crystal grows, it falls faster and collides with supercooled droplets, which immediately freeze onto it. This process produces soft ice pellets called graupel or small hailstones. When these ice particles fall into air that is warmer than 32 °F, they melt. Such particles produce almost all the raindrops in thunderstorms.
Rain in long-lasting, steady storms forms in a slightly different way. The ice crystals continue to grow by collecting water vapor. The resulting snow crystals may then collide with one another and stick together as snowflakes. Snow crystals and snowflakes become raindrops when they fall into air that is warmer than 32 °F and melt.
Scientists have been trying to develop reliable methods of cloud seeding to make more rain fall from clouds. In the technique investigated the most, aircraft drop artificial freezing nuclei into clouds. The usual seeding material has been silver iodide, whose crystals have a structure much like that of ice.
Rainfall distribution
The earth as a whole receives abundant rainfall. If rain fell evenly, all the land would receive about 40 inches (100 centimeters) a year. But rainfall is unevenly distributed over the earth’s surface. Distribution is especially uneven over the continents because of mountain ranges. Heavy rain often drenches slopes where the air rises, leaving dry the slopes where the air descends. For example, southerly winds (winds from the south) rising over the Himalaya range in Asia deposit 200 to 600 inches (510 to 1,500 centimeters) of rain annually on their southern slopes. But the northern slopes of the range average less than 10 inches (25 centimeters) of rain a year.
Rainfall is generally heavy along the equator because of the high humidity and because surface winds converge (come together) there. The convergence causes the air to rise, producing clouds and rain.
Certain regions alternate between rainy and dry seasons due to shifting winds. In regions near the tropics, winds known as monsoons blow in one direction in the winter and in the opposite direction in the summer. Monsoon winds bring extremely heavy rain to southern Asia in the summer.
On both sides of the equator, generally along the Tropic of Cancer and the Tropic of Capricorn, are regions where the air usually sinks to the surface. Rainfall is therefore light in those areas, and so deserts have formed in them. Sinking air is responsible for the Sahara and the Kalahari Desert in Africa and a band of deserts in the western and central parts of Australia.
Away from the tropics, the western coasts of large land masses have more rainfall than their central regions. For example, along the west coast of North America, moist winds from the Pacific Ocean produce as much as 150 inches (381 centimeters) of rain and snow a year. To the east lies the Great Basin, a desert area that covers parts of Oregon, California, Nevada, Idaho, Utah, and Wyoming. The eastern side of North America receives moisture largely from southerly and southwesterly winds from the warm Gulf of Mexico.
Polar regions are dry partly because their cold air cannot hold much water vapor. In addition, relatively little evaporation occurs in those regions, and the wind blows away from the poles.
Measuring rainfall
People measure rainfall with a variety of devices, including simple funnel gauges and sophisticated radar systems.
A funnel gauge
consists of a funnel connected to the top of a narrow cylindrical tube. The diameter of the mouth of the funnel is much larger than the diameter of the tube. Rain falls into the funnel and collects in the tube. Markings on the side of the tube indicate the amount of rainfall. This design makes the depth of the rain easy to measure. A small amount of water falling into the mouth of the funnel will fill the tube to a considerable—and easily readable—level.
Radar systems.
Networks of gauges can measure rainfall over large regions, such as river basins, states and provinces, and even countries. However, a network can produce an inaccurate result if the rainfall is unevenly distributed across a region.
A radar system can provide a more complete picture of rainfall. In a radar system, an antenna sends out radio waves that reflect from raindrops and return to the antenna. Electronic devices connected to the antenna measure the strength of the returning waves. The strength of the reflection indicates the amount of rainfall—the stronger the waves, the heavier the rainfall.
This method of measurement avoids the limited coverage of gauge networks. However, radar may provide inaccurate results if the raindrops are unusually large or if hail is mixed with the rain. In this case, the radar waves may appear to have been reflected from a storm that is much heavier than the actual storm. A combination of radar and rain gauges usually provides the most accurate results.
