Rain

Rain is a familiar source of water over much of the earth -- drops of liquid water falling from the sky in billions from rainclouds or thunderstorms, or as part of a short shower from a smaller patch of clouds. The sound of rain pattering on leaves, windowpanes, or a car roof is soothing to many people, and the distinctive water smell of rain is also enjoyed by many. Grass, trees, gardens, and crops all benefit from regular, moderate rainfall. The plant life of much of the earth depends on rain, and the rivers, springs, and streams where humans and animals get their water are also replenished by rain.

Rain falls when the temperature is above freezing, both at the ground and aloft, and when the air is saturated and there is a lot of instability, meaning that the air is rising and water is condensing out of it to form precipitation. The basic mechanism producing rain is the same as that for other kinds of precipitation. As moist air rises, pushed upwards by such forces as the pressure of frontal boundaries or the heating of air near the ground by the sun, it cools, and as it does so, water vapor condenses out of it into droplets, since cool air can hold less water vapor than warm air. These droplets form clouds and, if the condensing process is strong enough, rain.

Interesting facts about rain

Raindrops form by coalescence, when droplets of water condense out of the air as it cools and can no longer hold the water in its gaseous state. These droplets run into each other in the cloud as air swirls and shifts, and combine into larger drops. Eventually, the drops become heavy enough to fall, and tumble out of the cloud base as rain.

Rain is one of the key links in the overall "water cycle," which moves water from the ground into the oceans, back into the atmosphere, and into the ground again through rain and snow. Rain is fueled by evaporation, which moves water from the surface into the air -- much of this evaporation occurs over the equatorial regions, where heating produces enormous updrafts and fast evaporation, and the heat of the air means that it can contain large amounts of water vapor. Evaporation occurs everywhere over the planet, however, adding to the overall moisture content of the atmosphere and thus fueling rain.

Rain tends to fall in waves, known as rainbands. These bands are most noticeable in strong storm systems such as hurricanes or very strong cold fronts. Rainbands can also be triggered ahead of a front by the atmospheric disturbance it causes as it pushes forward. For this reason, it is sometimes difficult to tell exactly when the frontal boundary passes over a certain point without the data from radar, satellite photos, and the like.

Rain, along with temperature, is one of the two main elements that makes up climate. The type of plant growth in an area, for example, is determined by how much water is available at different times of the year, as well as the temperature in the summer and winter, since most plants need at least some moisture to grow. The difference between a hot rain forest and a hot desert is the amount of rain that falls in each; both have a warm climate, but the presence or absence of rain makes a striking difference in their respective landscapes.

Raindrops are not shaped like the classic raindrops that artists and cartoonists draw. The smallest are almost round, while larger raindrops are flattened on the bottom and eventually become shaped like an upside-down U. The intensity of rainfall varies by the size of the drops as well as the number that fall per second in each square foot. Fog, the weakest type of precipitation, drops only 0.005 inches per hour, while an intense cloudburst can drop 4 inches per hour.

Effects of rain

The most obvious effect of rain, from a human perspective, is that when enough falls in a region, agriculture is possible there. Annual rainfall must be high enough for a specific crop to survive in order to grow that crop, or water must be available from a river or aquifer -- which, of course, means that the farmer is simply indirectly tapping rain that fell elsewhere on the planet's surface. Rain is therefore essential to human civilization -- without it, crops could not be grown on a scale needed to support a large human population.

Rain can also cause harm, of course. When there is too much rain, crops can literally 'drown' in the fields, or harmful molds and fungi can grow, as was seen in New Orleans after Hurricane Katrina. Large amounts of rain in a short time can lead to flooding, which can destroy buildings, fields, animals, and kill people who are unable to escape or who make the mistake of trying to cross flowing water in a vehicle.