Tor Bergeron was the Swedish

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 meteorologist who proposed a mechanism for the formation of precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 in cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s. In the 1930s, Bergeron and W. Findeisen developed the concept that clouds contain both supercooled water and ice crystals. According to Bergeron, most precipitation is formed as a consequence of water evaporating from small supercooled droplets and accreting onto ice crystals, which then fall as snow

Snow

Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

, or melt and fall as cold rain

Rain

Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 depending on the ambient air temperature. This process is known as the Bergeron Process

Bergeron process

The Bergeron–Findeisen process , also known as the cold rain or ice crystal process, is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth. The equilibrium vapor pressure over water is greater than the saturation vapor pressure over ice, at the...

, and is believed to be the primary process by which precipitation is formed.

Bergeron was one of the principal scientists in the Bergen School of Meteorology

Bergen School of Meteorology

The "Bergen School of Meteorology" is a school of thought which is the basis for much of modern weather forecasting.Founded by the meteorologist Prof...

, which
transformed this science by introducing a new conceptual foundation for understanding
and predicting weather. While developing innovative methods of forecasting, the Bergen
scientists established the notion of weather fronts and elaborated a new model of
extratropical cyclones that accounted for their birth, growth, and decay. Bergeron is
credited with discovering the occlusion process, which marks the final stage in the life
cycle of an extratropical cyclone.