Transcript Weather! - fhouses-school

You may have heard
this before
somewhere…
Most of the world has climate
But in Britain they have ….
Weather!
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This is because …
We are on the edge of several air masses
All of which are having arguments with
each other
And who wins changes all the time.
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The High’s and Low’s bring us different
weather!
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We are going to watch a video (I hope!)
It is on http://ysgolrhyngrwyd.wikispaces.com/Unit+2+Year+9
I want you to notice the weather.
We are then going to talk about what
causes it.
So you have seen it(??)
What was the weather like in the video?
This due to a high pressure system called
an anticyclone
We are going to see what an anticyclone is
like
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Weather on Friday at Midday
Does anyone know what these
lines are called?
What do they measure?
Why do you think the word
HIGH is written in the middle
over the UK?
I have been watching out for a
map that looks like this but we
don’t seem to have had one!
From the pictures you saw
,what do you think the weather
was like over the UK that day?
Notice that compared with the
low pressure, the lines are
widely spaced
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Here are the isobars again. What does the other map show?
Where the high is, what is the wind like?
Which direction is the wind flowing around the high?
Look at the spacing of the isobars around the high and the
low. What do you notice?
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What do you notice about the cloud?
The cloud to the NW of the UK is a thick layer of cloud. Do
you know what that is called?
The cloud over southern France and South Spain is little
lumps. Do you know what that is called?
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So an anticyclone has …
High pressure
Hardly any wind at the centre
Very little cloud
Very light winds around the outside going
clockwise on this occasion
In fact in the northern hemisphere ALL
anticyclones have winds rotating in a
clockwise direction. How do you think they
rotate in the southern hemisphere?
The rotation is due to the fact that earth
is spinning known as the Coriolis Effect
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So why does high pressure usually give
clear skies?
The air has high pressure, so it sinks towards
the ground.
As the air descends, the molecules become
compressed, the pressure increases and it
warms.
When air is warming, any moisture in the
atmosphere is evaporated so no clouds can form.
The sky is clear.
Anticyclones can be very large, typically at least
3,000 km wide.
Once they become established, they can give
several days (or even longer) of settled
weather.
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But weather forecasters will get
caught out by high-pressure conditions.
Usually they will produce clear skies, but occasionally
it's a little different.
The sinking air warms up, but there is a possibility that a
temperature inversion could occur, where there is
warmer air above cooler air
(usually it is the other way round in the atmosphere).
The inversion happens because the clear skies mean the
Earth's surface loses a lot of heat - there is no
insulating layer of cloud.
A cool surface means cool air temperatures near the
surface.
This cool air is dense and hard to shift, so the warmer
descending air sits on top.
If this air is moist, the water vapour can condense into
a dreary layer of cloud which hangs around for days,
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since the winds are too light to disperse it.
This has happened in the recent past in
parts of the UK
There was an inversion layer that kept the cloud
hanging about
For others the problem was early morning mist
that took a long time to clear
As there is no cloud, once the sun has gone
down, heat is radiated out from the Earth into
space.
Falling temperatures allow any water vapour in
the air to condense into low lying mist.
It takes the sun a while in the morning to ‘burn
it off’ – in other words warm up the air enough
to evaporate the water droplets into water
vapour
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Another issue with Anticylones
Because the air is pressing down and there is
little wind.
In cities, such as London, and other places too,
the air gets a hazy look, both in Summer and in
winter.
This is because any pollution produced is stuck
at a low level and builds up over time.
In the past when everyone burnt coal on their
fires, winter fogs could last for days. These
were called smogs ( smoke + fog) and lead to
many deaths from pneumonia, pleurisy and
bronchitis.
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Choked streets: For years London was
synonymous with thick fogs - indeed, the
city's pea-soupers were immortalised in the
works of Charles Dickens. But the smokeladen fog that shrouded London from 5-9
December 1952 was the worst single
incident of air pollution in the UK.
Conductors walked in front of buses holding
torches or flares in order to see street
signs - and so drivers could see them.
Coal deliveries: Although soon coal deliveries
were to become a thing of the past, change
came too late to save the thousands who fell
prey to the trapped toxins.
The smog of 1952 was thought to have killed
at least 4,000 people - particularly the
elderly and infirm - and the death rate
remained above average up until Christmas
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Next week we are
going to look at an
altogether more
complex weather
system
As part of that, you need to
be familiar with various cloud
types – so a small deviation!
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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
Middle layer
clouds
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 clouds.
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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Alto stratus
Alto is middle level
Stratus is layered
Not much rain
should come with
this one
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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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