Theories:
The Bergeron-Findeison process (Ice crystals)
 Developed by two Norwegian scientists, Bergeron and Findeison, in the 1930s.
 Assumes that ice crystals and water droplets coexist in clouds.
 Condensation occurs relatively easily in comparison with freezing in clouds, as
there are more condensation nuclei than freezing nuclei.
 Spontaneous freezing occurs when small water droplets are super-cooled to -40ºC.
 The freezing nuclei that are needed to encourage ice crystals to form consist of
fine soil particles and dust from volcanic eruptions.
 Following formation, large ice
crystals can splinter in turbulent
Cloud seeding
air currents in clouds to form
p. 108 in Atmospheric Processes and
small splinters that can act as
Human Influence
additional nuclei.
This is the artificial process of rain
making and is largely based on the
 Moving ice crystals in large
Bergeron-Findeison theory.
clouds, such as cumulonimbus,
This is because the basis of the activity
are believed to grow by further
lies in the importance of freezing nuclei.
condensation and freezing.
 Water vapour can condense very
rapidly and turn straight from a gas to ice – often called sublimation.
The coalescence theory
 In the tropics, strong thermals help the development of precipitation, as large
clouds can contain several turbulent currents. Eventually, droplets reach a size
where gravity pulls them to the ground.
 It is possible for both theories to occur at the same time. The Bergeron-Findeison
would take place in the upper parts of clouds, and the coalescence process at
lower altitudes.
Types of rainfall:
Convectional rainfall:
 Results from strong upward-moving, buoyant air currents.
 In the British Isles, convectional rainfall is particularly common in the summer
when there are consistently high temperatures, resulting in short-lived showers
and thunderstorms.
 Most notably occur in the south-west of England where storms have developed at
sea and tracked eastwards towards the land.
Orographic (relief) rainfall:
 Associated with upland areas that encourage warm, moist air to rise, leading to
conditional instability.
Case study: Tenerife in the Canary Islands
p. 110-111 in Atmospheric Processes and Human Influence
 Trade winds from north over Atlantic bring 450 mm of rainfall each year,
falling on the north side of the island.
 When air currents reach heights of 1500-1800 m, condensation leads to a
layer of cloud.
 By the time air masses have crossed the uplands, most moisture has been
released.
 The leeward side of the mountains are dry and referred to as the rain
shadow.
Frontal (cyclonic) rainfall
 Particularly important in the mid-latitudes, about 40-60º north and south.
 Low pressure systems, or depressions, moving from west to east, influence the
climate of these zones.
 When warm air is forced to rise over cold, cloud development and rainfall can
occur.
Test yourself
1. What does the Bergeron-Findeison theory (Ice crystal theory) assume?
2. What is the term which describes water vapour condensing very rapidly and
turning straight from a gas to ice?
3. What is the name of the theory which is the artificial process of rain making and
is largely based on the Bergeron-Findeison theory?
4. Why is this theory strongly based on the Bergeron-Findeison theory?
5. What is explained by the coalescence theory?
6. Why is it possible for the Bergeron-Findeison theory to occur at the same time as
the coalescence theory?
7. Which form of precipitation results from strong upward-moving, buoyant air
currents?
8. Which form of precipitation is associated with upland areas that encourage warm,
moist air to rise, leading to conditional instability?
9. Which other form of precipitation is particularly important in the mid-latitudes,
about 40-60º north and south?
10. Where in the British Isles does convectional rainfall most notably occur?
Answers
1.
2.
3.
4.
5.
Assumes that ice crystals and water droplets coexist in clouds.
Sublimation.
Cloud seeding.
Because the basis of the activity lies in the importance of freezing nuclei.
In the tropics, strong thermals help the development of precipitation, as large
clouds can contain several turbulent currents. Eventually, droplets reach a size
where gravity pulls them to the ground.
6. Because Bergeron-Findeison would take place in the upper parts of clouds, and
the coalescence process at lower altitudes.
7. Convectional rainfall.
8. Orographic (relief) rainfall.
9. Frontal (cyclonic) rainfall.
10. South-west of England.