A3 HazardsA3 Hazards

Lesson 6Measuring weather

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Why measure the 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 The main weather stations run by the met office carry out all these met office carry out all these measurements every hourmeasurements every hour

• Temperature, dry-bulb• Temperature, wet-bulb (used in conjunction with the dry-bulb

reading to derive the vapour pressure, relative humidity and dew-point)

• 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 temperatureMeasuring 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 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 screenA Stevenson screen

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?

What are the little gaps in the side for?

(louvers)

Met office Stevenson

screens in the UK have

doors facing north. Why?

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A wet and dry thermometerA 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 Measuring precipitation (rain only)(rain only)

• The instrument used to 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 SpeedWind Speed

• Wind speed is measured 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 DirectionWind Direction

• A wind vane is used to measure wind direction.

• It is measured using compass directions (north, south, east or west) from which the wind has blown.

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Air Air PressurPressur

ee

• One instrument used to measure air pressure is called a barometer.

• 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 measuredHow 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

AnemometerDual thermometer

and screen

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If this is to be of any use …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’ Notice this say ‘High’ over the UKover the UK

• Notice the white lines 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’ Notice this say ‘High’ over the UKover the UK

• The white line – isobars

• Iso = same

• Bars are measure of 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?So what have we found out?• When the pressure is high, the sky is

generally 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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• 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!

This is visibility – see the white area – that is fog

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Why is that?Why is that?

• Dense air pushed downwards giving high 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?Why?

We had a lot of low pressure areas called

Depressions

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• 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 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 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 Comparing anticyclones (highs) and depressions (lows)depressions (lows)

• Anticyclones

• > 1020 mbar (approx)

• Clear skies, no rain, morning mist especially in Autumn and Winter

• Light breezes only, 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 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 AssignmentNew Practical Assignment

• You may do a microhabitat project – see 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 AssignmentNew 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 Now here is one that does not work!work!

Maybe if I had done daily total and the

maximum temperature?

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Once you have a pattern …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.