The ice-crystal theory to explain precipitation was propounded by Tor Bergeron, an eminent meteorologist from Norway, in 1933. It is also called the Bergeron Process after the name of its discoverer.
It is based on two special meteorological properties of water. First, the water droplets in a cloud do not freeze at 0°C. In the atmosphere super-cooled water has been observed down to below -40°C. When water remains in liquid state below 0°C it is referred to as super-cooled.
The super-cooled water tends to freeze, if it is disturbed. Being of aircrafts which fly through a cloud consisting of super-cooled droplets offer a typical example of this phenomenon. Besides, super-cooled droplets also freeze when they come into contact with a tiny solid nucleus, about 1 micro-meter in diameter.
image source: noaa.gov/features/02_monitoring/images/snowflake_c.jpg
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This is called a freezing nucleus. Most of the nuclei become active at- 20° to -25°C. However, freezing nuclei are sparse in the atmosphere. Thus, when the ascending air currents rise well above the freezing level, some of the water droplets will be changed into ice and through sublimation water vapour will enter into solid state.
According to Taylor, if a single ice-crystal is introduced into a cloud of super-cooled water droplets, the entire cloud rapidly changes over to an all-ice cloud. This abrupt change from water to an ice cloud is caused by different vapour pressures existing over super-cooled water droplets and ice crystals at the same temperature.
Now, we have to consider the second special property of water. Over ice, the saturation vapour pressure is lower than what it is over water. In other words, when air is saturated (100 per cent relative humidity) with respect to water, it is supersaturated (relative humidity greater than 100 per cent) with respect to ice.
From it is clear that at – 15°C, when the relative humidity is 100 per cent with respect to Water, it is about 115 percent with respect to ice. In this case, vapour diffuses rapidly from air to ice crystals so that the ice crystals begin to grow at the expense of water droplets.
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The growth of ice crystals is rapid enough to generate crystals large enough to fall. While falling from the cloud, the ice crystals grow by intercepting cloud droplets that freeze upon them. Sometimes these falling ice crystals are broken up into fragments which again become freezing nuclei for other water droplets.
A chain reaction takes place. These ice crystals by accretion grow further in size to become snowflakes before leaving the cloud.
Snowflakes generally melt before reaching the ground and fall as rain. Because of the glaciations of the upper part of the cloud, its cauliflower like top becomes anvil-shaped, so typical of a cumulonimbus.