Hail
Information on Hail and ice pellets during storms. Learn how hailstone is formed.
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The amount of water vapor which the air space can hold before becoming saturated depends mainly on the temperature. At higher temperatures more water molecules are energetic enough to evaporate. So the saturation point is reached when there is more water vapor in the air space.
Water in its liquid state is composed of millions of molecules. Some are energetic enough to escape into the air. The number able to do so depends on the temperature.
In time as many return to the liquid as are evaporated from it. When this happens the space above the liquid is said to be saturated.
Evaporation
Evaporation is the process that changes a liquid into a gas. It can occur at any temperature, and is due to the movement of molecules in the liquid. For example, water left in a saucer will gradually evaporate. The water molecules escape into the air as a gas that is water vapor. The warmer the air, the more moisture it can hold in the form of vapor. Every second, about 15 million tons of moisture is evaporated from the Earth's surface by the heat of the Sun. Water evaporated from the ocean often travels hundreds of miles before falling as rain.
When air contains the maximum water vapor possible for its temperature, it is said to be saturated. Below this temperature, called the dew point for a particular mass of air, the water vapor begins to condense into water droplets.
The amount of water vapor in the air is called its humidity. Relative humidity is the actual amount of water vapor in the air compared with the amount the air would hold if saturated. Relative humidity is usually expressed as a percentage, and in many regions is of major importance in weather reports. Humidity is very high in the tropics, particularly in coastal regions.
