All the forms of water that fall from the air to the Earth's surface are called precipitation. Whether the precipitation is snow, rain, sleet or hail depends on the temperature of the air that the water falls through. If the air is above freezing, the precipitation will most likely be rain. If the air is below freezing, the precipitation will most likely be snow. When air temperature is only a few degrees above freezing, precipitation may fall as sleet.
Hail is most commonly formed within the cumulonimbus clouds of thunderstorms. Large updrafts of air can throw rain droplets high up into the tops of the cloud. Here, the temperature is well below freezing, and the droplets freeze. The droplets then fall and can become caught in further updrafts, adding a second coating of ice to make the hailstones larger. This cycle continues until the hailstones are too heavy to be lifted again. They then falls as hail. The stronger the updrafts in the cloud, the longer the hail develops, and the larger the hailstones are when they falls.
The amount of rain, sleet, snow or hail which falls in a specified time is expressed as the depth of water it would produce on a large, level impermeable surface. Usually it is expressed in millimetres although inches may sometimes be used. Precipitation is measured daily (24 hours) by means of a rain gauge. Todays rain gauges are simple to use with pre-calibrated scales on their sides. When measuring precipitation, certain precautions have to be taken against the effects of obstructions, wind, splashing and evaporation.