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In meteorology, the condensation and separation of theatmospheric water vapour as rain, snow, hail, fog, is called precipitation. We distinguish falling precipitation such as rain or snow, deposited precipitation, such as dew or fog, and accumulated precipitation in the form of hail or snow.
When largeclouds rise, they soon reach altitudes where temperatures are below freezing. These clouds are made up of ice crystals, water vapour, and droplets of water which, in spite of the below-freezing temperature have not yet crystallised into ice. Tiny small crystals coalesce into snowflakes, which fall to the ground. Snowflakes fall in the form of dry snow to the warm ground. If the snowflakes have to pass through a warmer layer of air, they melt and change into rain.
Drops of water, which in our geographical latitudes fall to the earth, were in reality originally snowflakes. The droplets or ice crystals forming in the clouds are too light to fall to the ground. Only when millions of these droplets collide and coalesce into one drop, they are heavy enough to form precipitation.
Some flat clouds create only smaller drops of water, which fall to the ground as drizzle. When drizzle falls on frozen ground, it immediately solidifies as ice. The ground is then covered by a thin layer of ice. This poses a great danger for drivers, because the colourless and transparent ice is not visible on the dark surface of the road.
In the clouds that do not reach the altitude with freezing temperatures, air currents whirl millions of tiny droplets and join them together. These drops of water then fall to the ground as rain. An oppositeelectrical charge accelerates this coalescence.
Snowflakes show a characteristicsymmetry reflecting the coalescence of water molecules in ice. However, snowflakes deviate from this basic formula. The reason is, that the tips of crystals grow at different rate of speed. There are never two identical snowflakes. Individual ice crystals are so fine that when they fall, they touch and deform. These crystals reach the ground as deformed compounds. Tiny droplets of water and some ice crystals form in the higher layers of clouds. Moisture from the evaporating droplets combines with the ice, forming larger ice crystals. These later form snowflakes. Snowflakes fall as dry snow on cold ground, or as wet snow on warm ground.
The nucleus of a grain of hail is either an ice crystal or a grain covered by a thin layer of ice. Many additional ice layers must form before the nucleus becomes a grain. Strongrising currents cause the nuclei to be in constant swirling motion, which allows the cool water droplets to adhere. The number of ice layers (up to 25) indicates, how many times the nucleus was swirled around in the cloud. The thickest ice layer forms during the descent, because the air humidity is higher in the lower layers of the air. When the air current cannot support the grain of hail any longer, it falls to the ground. Hail may cause considerable damage.
Fog is a surface cloud layer. Tiny droplets of water are suspended in theair. This considerably decreases visibility and poses danger not only to air travel, but sea and road travel as well. Fog may even freeze (icy fog). If it combines with exhaust gases and dust particles, it may become a toxic smog.
Similarly to fog, dew and frost are included in the deposited precipitation. When the earth's surface cools down below thedew point, dew forms at temperatures over the freezing point, while frost will form at temperature below the freezing point. The dew point is the temperature at which the air is saturated with water vapour.
At one hundred percent air humidity, the dew point corresponds to the temperature of the moment. Dew is the transition stage from water vapour to water, while frost represents the transition stage of water vapour to ice. Dew and frost represent approximately 3 percent of the total amount of liquid precipitation.
In regions withlow amounts of precipitation (rain), dew may be the source of moisture for the scarce local vegetation, since the precipitation amounts differ greatly from one region of the earth to another.
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