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Weather and climate
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6- precipitation
Question 34
G31 Precipitation
►Air contains water in the form of vapour. Vapour consists of drops of water or
tiny pieces of ice.
● These drops of water and tiny pieces of ice fall onto the Earth as
precipitation. (ex. snow, hail, rain, mist)
G40 The causes of precipitation
►Precipitation is connected with rising air. (The higher you go, the colder it
gets) When air rises, it cools. Cool air can hold less water, so it rains.
● When air goes down, it gets warmer. Warm air can hold more water so
clouds disappear, the sun shines.
GG 41 Precipitation in mountains
►Windward side= the side of the mountain that faces the wind
Relief rainfall= air hits the windward side of the mountain, rises and cools
down. There will be condensation and then it rains.
►Leeward side= the side of the mountain that faces away from the wind
Rain shadow= at the leeward side, the air goes down and gets warmer. There
will be vaporization. There is very little precipitation.
GG 42 Wet tropics, dry deserts
►Convectional rainfall The sun shines vertically at the equator (=tropics).
The heat makes the air rise and the air cools down. When it cools down, it can
hold less vapour. There is condensation and it starts to rain.
● Desert Cold air goes down at 20º-40ºN/S. When air goes down the air is
getting warmer. Warm air can hold more water. That's why there are no clouds
and it doesn’t rain.
Question 35
a
b- In the tropical rainforest it rains a lot (more than 2000mm/year).
In the savannah area there is a wet period of 8-9 months (1000-2000mm/year).
In the steppe area there is a short wet period of 3-4 months (250-500mm/year)
In the desert area it is almost always dry (less than 250mm/year)
c- Because the angle of incidence. At the equator the sun is all year round high
in the sky. Further away from the equator the angle of incidence will become
smaller. So it's less warm and there is less rainfall. Near the desert there is a
low pressure area, so air goes down. This air warms up and can hold more
water vapour (so less precipitation).
d- In the steppe there is not enough rainfall for trees to grow. So there are only
thorny shrubs and grasses.
Question 36
a- Warm air can contain much moisture. The moisture is present in the air as
invisible water vapour. When air on the windward side of a mountain is
forced to rise, it cools down. Cold air can contain less water vapour, causing
condensation. Water vapour turns into visible water droplets, which flow on
the rising air. When water droplets cling together, they become a cloud.
When the water droplets are too heavy for the air to carry them, they fall down
as precipitation.
On the leeward side of the mountain, the air starts to descent. This air
becomes warmer and can contain more water vapour. Possible present
water droplets in the descending air will evaporate. On the side of the
mountain where the air is descending it is clear weather.
b- 53th map 47G and 36 54th map 41A and 23
Zuid-Limburg – Veluwezoom (Arnhem/Nijmegen) - Hondsrug
c-
Bergen
Norway
Mariehamn Finland Amsterdam
Annual
precipitation
> 2000mm
400-600mm
600-800mm
Temperature in
July
10º-15ºC
15º-17,5ºC
17,5º-20ºC
(15º-17,5ºC)
d- Bergen is at the windward side of the Scandinavian Highlands, Mariehamn is
in the rain shadow.
e- Mariehamn, because the temperatures are better and there is less
precipitation.
Question 37
a- There is also precipitation caused by fronts: places where warm and
cool/cold air come together. These frontal rains are often seen by colliding
air masses. On the front line we see clouds in Europe. Warm air from the
tropics transfers to the north and clashes against cold air. Where warm and
cold air collide, there’s the front. The cold air is heavier and forces the warm
air to rise. But when warm air rises it cools down, causing a wide front . It
starts to rain and it will keep raining until the front has passed.
b- High pressure: air comes down from above. It gets warmer, there are no
more clouds and the sky is clear. There is no rainfall.
Low pressure: air goes up and cools down. It brings clouds and rain.
Question 38
Snow is a form of solid precipitation. It is formed when water vapour
changes directly to ice without first becoming a liquid,
high in the atmosphere at a temperature of less than 0°C and
then falls to the ground.
Hail is a form of solid precipitation. Hail is caused when
raindrops are lifted up into the atmosphere during a
thunderstorm and then super cooled by
temperatures below freezing, turning them into
ice balls.
Dew is a liquid form of water. It is water in the form of droplets that
appears on thin, exposed objects (ex. plants, grasses) in (mostly)
the morning due to condensation. As the exposed surface cools
by radiating its heat, atmospheric moisture condenses, resulting
in the formation of water droplets.
Ripe is the deposit of ice crystals on objects exposed to the free air,
such as grass blades, tree branches or leaves. It is formed by
direct condensation of water vapour to ice at temperatures
below freezing point. It is also called hoarfrost.
Fog can be considered a cloud at ground level. It is made up of
condensed water droplets which are the result of the air being
cooled to the point where it can no longer hold all of the water
vapour it contains. Fog is mostly formed by infrared cooling. This
happens due to the change of seasons. During the summer the
ground absorbs solar radiation. As air passes over
it is made warm and moist. When the seasons change this
mass of warm moist air collides with the cooler that is now
prevalent. This causes the water vapour in the air mass to
condense quickly and fog is formed.