Download Y10UA3.6 Measring weather 14_5 Dec PP

A3 Hazards
Lesson 6
Measuring weather
Why measure the weather?
• Meteorological observations are made for a
variety of reasons. The aim of all these
observations are essentially to provide data and
are a valuable source of information, which is
used in a variety of different ways, for example:
 as input to supercomputer forecasting models, which
produce local/global weather forecasts
 to help with any weather related issues, e.g. aviation
safety
 for climatological analyses, i.e. frequency of extreme
events, such as gales or heavy rain
 to answer legal enquiries, where data may have to be
presented in a law court
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The main weather stations run by the met
office carry out all these measurements
every hour
• Temperature, dry-bulb
• Temperature, wet-bulb (used in conjunction with the dry-bulb
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reading to derive the vapour pressure, relative humidity and dewpoint)
Amount of precipitation
Amount of cloud – including amount, type and height of cloud base
Present weather
Visibility
Wind speed and direction
Pressure tendency and characteristic
Atmospheric pressure
Past weather and any special phenomena occurring either at the time
of observation or since the last observation
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Measuring temperature
• A thermometer usually consists of a hollow glass bulb
attached to a narrow stem with a thread-like bore.
The bulb is filled with liquid,
 If it is silver it is mercury
 If it is red it is alcohol
• The liquid in the tube expands when the temperature
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rises and contracts when the temperature falls. The
amount of expansion and contraction is measured by a
numbered scale.
Whilst thermometers are really measuring their own
temperature, they are used to measure the
temperature of the surrounding air.
To make sure that the temperature of the surrounding
air is the same as the thermometer, it must be shaded
from sunlight and be exposed to adequate ventilation.
These conditions are provided by a Stevenson screen.
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A Stevenson screen
What are the
little gaps in
the side for?
(louvers)
Met office
Stevenson
screens in the
UK have
doors facing
north. Why?
Why do we need
one of these?
Why does it
have legs?
Why is it painted
white?
Why has it got such a large
roof?
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A wet and dry thermometer
• How does it work? The dry
bulb measure the air
temperature. The wet bulb
uses up some heat energy
evaporating the moisture and
so is cooler. The drier the air
is, the more the water can
evaporate. So that if a lot
evaporates, this shows that
air is drier and so the
humidity must be low.
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• Here is a chart that shows how the dry
temperature and the difference between
the wet and dry thermometer gives the
humidity
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Measuring precipitation
(rain only)
• The instrument used to
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measure rainfall is a rain
gauge.
Notice that the
precipitation runs into a
funnel and from there
into a bottle.
The bottle will be
calibrated to measure
the depth of rain falling
on a 127mm circle.
Notice that the storage
bottle is below ground.
Why might that be?
Why do you think
the distance above
ground of the
gauge top is also
fixed?
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Wind Speed
• Wind speed is measured
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using an anemometer.
An anemometer is made
up of cups attached to
handle with a scale on it.
The stronger the wind the
faster the cups rotate
and the higher the
reading on the scale.
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Wind Direction
• A wind vane is used to
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measure wind direction.
It is measured using
compass directions
(north, south, east or
west) from which the
wind has blown.
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• One instrument used to
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Air
measure air pressure is
called a barometer.
Pressure
But a more usual one used in
weather stations is a
barograph.
The motor rotates the drum
containing the paper chart.
The capsules are flexible
metal discs from which nearly
all the air has been extracted.
As the external air pressure
increases, the discs squash
together more, and this
rotates the arm so that the
end with the pen on draws an
upward line on the drum.
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How weather is measured
• Much of it these days is done by electronic kit
which links to a computer where the data is
stored.
• But as you will
see, the parts
that go up to
make them are
often similar to
the manual
versions that
have been used
for years!
Rain gauge
Weather vane
Anemometer
Dual thermometer
and screen
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If this is to be of any use …
• You need to know what kind of weather
may occur, so that when you see reading
they will be gin to tell you something about
the kind of weather is likely to come next,
and why the weather you see is like it is
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Notice this say ‘High’
over the UK
• Notice the white lines
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and the number 1028 on
the second ring out from
the inner High circle.
What are the white lines
called? each ring is 4
different from the ones
either side. What do you
think the inner ring is?
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Notice this say ‘High’
over the UK
• The white line – isobars
• Iso = same
• Bars are measure of
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pressure
The unit of the numbers
are millibars or mbar
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• What do you notice about
the size and direction of
the arrows around the
high?
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So what have we found out?
• When the pressure is high, the sky is generally
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clear, the winds are light and revolving around in
a clockwise direction.
With clear skies, in summer what would you
expect the temperature to be during the day?
If the skies are clear at night, what happens to
the temperature? Why?
So in winter, with short days and the sun not
very strong, what is going to happen to the
temperature?
Now generally a high brings clear skies. But
there are exceptions…..
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This is visibility – see the
white area – that is fog
• According to that, it
should be 1 deg C at 3am,
but just on midnight it is
2.7 – the fog is acting as a
blanket!
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Why is that?
• Dense air pushed downwards giving high
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pressure.
As it descends, the air molecules are squashed
together, raising the temperature a little so any
clouds evaporate - hence clear skies.
But at night, in the winter, the heat from the
ground radiates out into space and the ground
becomes cool, cooling the air above it, so that
the water vapour turns into mist at ground level.
Once the sun comes up, it warms the air up again
and the mist burns off.
But once there is a layer of mist, this acts as a
blanket and stops more heat radiating out into
space.
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Why?
We had a lot of low
pressure areas called
Depressions
• Warm air and cool air do not mix
• Between 45N and 64N cold air from the poles
meets warm air from the topics to form a polar
front
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• As warm air meets cold air, the temperature gradient is
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strengthened
This leads to instability and a wave forms that develops
into a triangular shape – warm air inside the triangle and
cold air outside it
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• The leading edge of the depression is called the warm front. Here
the warm air, which is lighter rises above the cold air in front of it.
• The cold air moving behind the warm air travels faster and pushes
its way under the warm air. This is a cold front
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• At the top of the triangle pressure falls as the warm air
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rises in a spiral movement.
Winds develop and blow around the centre of the low
pressure in an anticlockwise direction.
The boundary between the warm air and the cold air is
called a front
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• Cool to cold or
warm?
• Any cloud?
• Any precipitation?
• Wind?
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• Cool to cold or
warm?
• Any cloud?
• Any precipitation?
• Wind?
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• Cool to cold or
warm?
• Any cloud?
• Any precipitation?
• Wind?
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• Cool to cold or
warm?
• Any cloud?
• Any precipitation?
• Wind?
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Comparing anticyclones (highs) and
depressions (lows)
• Anticyclones
• > 1020 mbar (approx)
• Clear skies, no rain,
morning mist especially in
Autumn and Winter
• Light breezes only,
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blowing in clockwise
direction
Hot summer days, cold
winter days – nights much
cooler except if there is
mist.
• Depressions
• < 990 mbar (approx)
• Mainly cloudy – different
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clouds at different
phases – lots of rain –
some steady, some short
and stormy
Windy – some times gales
blowing in an anticlockwise
direction
Mild days – nights not
much cooler
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New Practical Assignment
• You may do a microhabitat project – see
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attachment sent out
Or you can try to become a weather forecaster
using clouds
However you may do a virtual one.
I have collected and summarised data form my
electronic weather station and you can graph the
data (paper graphs are supplied but you can do
them yourselves in EXCEL). Do this and comment
on each graph separately (or set of graphs – see
wind direction) and you are half way there. At
this point some of you may want to stop there!
That’s fine.
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New Practical Assignment
• But for those hoping for higher grades
than a C, I want you to go to the next
phase.
• Try and see if there are patterns between
sets of data – is there a link between say
temperature and the amount of rainfall?
• Plot rainfall along the bottom and temp up
the side and see if a pattern emerges –
see over! I plotted each 6 hour period as
one cross a-piece –
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Now here is one that does not work!
Maybe if I
had done
daily total
and the
maximum
temperature
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Once you have a pattern …
• Then describe what it is – ‘As x increases
then y …..’
• Now using the info from today’s lesson and
other things in the textbook wiki and the
lesson’s wiki –
 any changes I make will be notes on:
• Try and work out what is happening and
why.
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