Transcript Rain, clouds and wind - Geography at InterHigh

Rain, clouds and
wind
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This week ..
1. How do we get rain?
2. What are the clouds called (and what
might we expect when we see them?)
3. How can we tell how windy it is
without an anemometer?
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You remember we
talked about the
water cycle?
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The water cycle
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Convectional rainfall
When it is very hot
and damp
– like in the Amazon
rainforest every day –
and just occasionally in
the UK, when the
weather is REALLY hot
there is rainfall in the
afternoon after a
sunny first part to the
day.
Why?
Sun warms the soil,
and water evaporates
The air above warms
and rises with the
water vapour
As the air rises, it
cools
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Convection rainfall
More and more drops
of water join together
into big drops which
fall back to earth as
they are too heavy to
float in the air
Often there is thunder
and lightening too
Cool air can hold less water
vapour than warm air
Some of the water vapour
turns into drops of water
and these group together
as clouds
In the Amazon, this happens
nearly every afternoon at
about 4 o'clock
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This what convection rainfall looks like
This is a
cumulonimbus cloud
See later
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Formation of Relief Rainfall
Occurs in the mountains on the west coast of Britain
Evaporation
of water from
the ocean
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Formation of Relief Rainfall
Occurs in the
mountains on the
west coast of
Britain
Onshore
moisture laden
winds
Evaporation
of water from
the ocean
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Formation of Relief Rainfall
Occurs in the mountains
on the west coast of
Britain
Onshore
moisture laden
winds
Air cools
down
Mountains on the west
coast of Britain forces
the air to rise
Evaporation
of water from
the ocean
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Formation of Relief Rainfall
Onshore
moisture
laden winds
Air cools
down
Water vapour
Condenses to form
clouds
Further cooling
leads to
precipitation
Mountains on the west
coast of Britain forces
the air to rise
Evaporation
of water from
the ocean
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Formation of Relief Rainfall
Water vapour
Condenses to
form clouds
Onshore
moisture laden
winds
Evaporation
of water from
the ocean
Air cools
down
Air moving down the
mountain, gets warmer
and so reabsorbs any
remaining water
vapour.
NO MORE RAIN
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Relief Rainfall
Relief rain is formed when
air is forced to cool as it
rises over relief (height)
features in the landscape
(hills or mountains).
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Air is forced to rise and cools by 1°C per 100m.
As the water vapour in the air condenses, it forms clouds and
rains.
The air starts to descend and begins to warm up again.
As air warms up, it can hold more water vapour - clouds
disappear and rain stops. This side is known as a
RAINSHADOW.
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Fort William
Grampian
Mountains
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Rain
shadow
map
Dundee
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Grampian
Mountains
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Fort William
Rain
shadow
map
Dundee
Pennines
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Buxton
Lincoln
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Rain
shadow
map
Grampian
Mountains
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Dundee
Fort William
Pennines
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Buxton
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Lincoln
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Eastbourne
Ashburton
Dartmoor
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Lots of clouds
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This method of cloud classification was
proposed by Luke Howard (1803) who
named the clouds based on their form:
Cirrus - curl
Stratus - layer
Cumulus - heap
Nimbus - rain
and on their height:
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Cirrus & cirrostratus
The most common form of high-level clouds are thin
and often wispy cirrus clouds.
Typically found at heights greater than 6,000 meters,
Cirrus clouds are composed of ice crystals that
originate from the freezing of supercooled water
droplets.
Cirrus generally occur in fair weather and point in the
direction of air movement at their elevation.
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Altocumulus
Altocumulus may appear as parallel bands or rounded
masses.
Typically a portion of an altocumulus cloud is shaded, a
characteristic which makes them distinguishable from
the high-level cirrocumulus.
They are often come before a cold front which usually
carries rain with it.
Or altocumulus clouds on a warm and humid summer
morning is commonly followed by thunderstorms later in
the day.
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Nimbostratus
Nimbostratus are dark, low-level clouds at
a height of 2,000 meters
Nimbo meaning rain and stratus meaning
layer gives continuous not-very-heavy rain
However, when temperatures are cold
enough, these clouds may also contain ice
particles and snow.
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Stratocumulus
Stratus is a layer and cumulus means heap
So the clouds form a layer of heaps, and can
give quite heavy showers, but they do not usally
last too long
They vary in colour from dark grey to light grey
and may appear as rounded masses, rolls, etc.,
with breaks of clear sky in between.
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Fog
Fog is the lowest cloud of all!
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Fair weather Cumulus
These are know as fair weather cumulus ( heaps)
small white like cotton wool balls
They have flat bottoms which are quite low down
and a smallish heap of white cloud above
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You may sometimes see one of these
cumulonimbus
The bottom is flat and black, often with the rain already
falling out of it in huge drops.
Quite often there is thunder and lightening
The ‘heap’ part reaches high into the sky – often to the
level of the cirrus clouds
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What am I thinking of?
There is a layer of light grey lowish cloud. I
cannot see the sun but at least it is not raining
It is a lovely warm sunny morning but there are
small blobs of cotton wool clouds in the sky.
What are they called? What might happen later
on?
There are little wispy strands high in the sky –
what are they called?
Oh dear! There is a huge mountain of a cloud
going right up into the sky. It has a VERY dark
grey base. What is it called? What might happen
soon?
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The Beaufort scale was thought
up in 1805 by sir Francis
Beaufort. It is still used today.
These three versions of it were
taken from three web sites:
‘Castle Kites’, ‘Macfarlane Wind
Turbines’ and the ‘US Search
and Rescue Task Force’.
Questions:
1. Which of these versions do you prefer and why?
2.
Why did these three websites have pictures of the Beaufort scale?
3.
What other groups of people would be interested in the Beaufort scale?
4.
According to the Beaufort scale, what is the wind speed today?
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Which wind
scale do these
picture show?
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Homework: it says
Record the cloud types and the wind over 5 days
Cloud type: I have left space for a small drawing
in Paint or you may like to take a picture and
paste it in VERY small – if so you may need to
email as it might not upload.
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