Rain is a form of precipitation in which water falls back to earth as a liquid. Precipitation that occurs in a solid form is snow or hail. There is always some water in the air, but the amount of water the air can hold depends mostly on temperature. Very warm air can contain more water than cooler air. Sudden drops in temperature can force water to fall, or precipitate, out of the air. In fact, this happens every morning when the cooler night air loses some of its moisture as dew.







The Water Cycle

Water moves on the earth in a cycle. The water that falls as rain comes mainly from oceans, but also to a lesser extent from lakes and rivers. Heat from the sun causes water to evaporate, that is, to change phase and become a vapor. The water vapor rises and, as it does, reaches a level in the atmosphere where the temperature is cool enough for it to change back into a liquid. The change of phase from vapor to liquid is called condensation -- the exact same process that causes steam from a hot shower to fog up a cold mirror.

    Water condenses in the air in tiny droplets about 1/100 of a millimeter in diameter, so small they remain aloft as clouds. Usually for rain drops to form there must also be small solid particles suspended in the air. When they are too heavy to remain in the sky, they fall back to earth as rain. The water then runs into lakes, rivers and ultimately the ocean, completing the water cycle.

Measuring Rainfall

Rainfall is typically measured using a rain gauge. Rain gauges are usually about 50 cm tall and are placed on the ground just high enough to avoid splashes. Rain water that is caught in a funnel on top, runs down into a measuring cylinder below – where it can be recorded.




It is expressed as the depth of water that collects on a flat surface, and is routinely measured with an accuracy up to 0.1 mm or 0.01 in. Rain gauges are usually placed at a uniform height above the ground, which may vary depending on the country. 


There are two types of gauge:

1. Storage Rain gauge: These are used to make daily or monthly measurements. 

2. Recording Rain gauges: Can measure the intensity of rainfall using a 'tipping bucket' which will only tip when a certain volume of water is in it. An electrical switch can be used to record the tips.

Rain gauges should be placed in an open area where there are no obstacles, such as building or trees, to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings.