Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
Explanation and follow on work
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Introduction and aim of the activity
This activity involved experiencing two different but essential ways of observing the weather.
Weather balloons (also known as radiosondes) provide details of temperature, humidity,
pressure plus wind direction and speed up to about 20km, ideally four times a day, spread six
hours apart from 00 UTC (GMT). The data are relayed in real-time to a ground receiver and
when the ‘ascent’ is complete the whole data set is relayed to the national forecast centre and
around the world as part of the incessant exchange of data. The global weather balloon
network plays a critical role in providing details of the vertical structure of the atmosphere to
feed into operational forecast models. The aim here is to plot a selection of the temperature
and humidity data on a graph to illustrate where, for example, the air cools most rapidly with
height and where very damp and very dry layers might occur.
Observations of the weather at the surface are important too – as input for weather forecast
models and to define, over a long period of time, the ‘climate’ of a particular site. Plotting such
observations on a map is an essential step in determining where important weather features
like fronts are located.
The aim is to experience making a range of observations carefully and accurately at an
officially recognised observatory. The data will include temperature, humidity (wet-bulb
temperature), wind speed and direction, and cloud type and cloud amount.
Description of what was done and achieved
The students witness the balloon launch and will plot the ascent data in school. They can
produce two graphs. One on which a temperature line is drawn as a function of height (or
pressure) and the other on which a relative humidity line is drawn. The relative humidity will
tie in with the cloud observations taken during the visit, while the total change in temperature
from the surface to the lowest value aloft divided by the distance between them indicates the
‘lapse rate’ of temperature. This is a measure of quite how rapidly or slowly the air
temperature changes with height.
The Department of Meteorology
The Department of Meteorology was established in 1965 and is internationally renowned for its excellent
teaching and research in atmospheric, oceanic and climate science.
Walker Institute for Climate System Research
The Walker Institute for Climate System Research brings together the University of Reading’s unique breadth and
depth of climate expertise. The Institute aims to improve understanding of our future climate and its impacts for
the benefit of society.
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
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They also will have carried out a range of typical weather observations and will have logged
them. These will include drybulb temperature, wetbulb temperature, wind speed and direction
plus cloud amount and cloud type. They will have to be as precise as possible with their
readings, and be aware that some are more precise than others. For example, the temperature
readings can quite easily be made to the nearest 0.1°C while cloud amount is gauged to the
nearest ‘okta’ or eighth of the sky covered. This amount is often difficult to judge with
scattered cloud because the cover can be assessed reasonably well overhead but becomes
difficult when looking towards the horizon.
Background to the observations
Weather balloons and surface observations
You will watch the launch of a weather balloon (also called a ‘radiosonde’) from the
University’s Atmospheric Observatory and take some observations of the weather at the
surface.
Weather balloons
Every day, around the world, meteorologists launch weather balloons from about 650 places to
give us very important data, from an instrument package attached to the balloon, on how
pressure, temperature, humidity and wind change with height. The balloon is filled with
helium gas and gets bigger and bigger as it ascends because it moves into lower and lower
pressure. At some point the balloon’s skin is so stretched it bursts and the instrument package
falls back to Earth on a parachute. So from one ‘launch’ we collect very detailed information
both going up and coming down. It normally takes about an hour to reach burst height with
observations coming every few seconds, so there is a lot of data. The top of the ascent usually is
up to 20km or so above the ground, well into the stratosphere.
We can’t do proper weather forecasts without knowing quite how the atmosphere changes
upwards. In today’s forecast centres, data from balloons are fed extremely quickly into the
computer model as a critically important form of data.
Making observations at the surface are useful of course, but the ‘upper air’ data are vital – to
map the 3-D pattern of the atmosphere. Where the cold, or the dry, or the warm, or the moist,
cloudy and windy conditions are across the world, through the depth of the atmosphere, rely
in part on the network of radiosondes.
Busy upper-air stations launch four balloons a day at 00, 06, 12 and18 UTC but some might only
do one – normally either the 00 or 12 UTC observation. This may be because they can’t afford
lots of ascents at over £100 each.
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
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Sample data from an ascent
The table below has just a few observations taken from an ascent from a place near Paris
around midnight on a winter’s day. The left column shows how high the balloon is by listing
the air pressure – as the instruments go up, the pressure drops and meteorologists use pressure
to tell how high up the observation is. They also however use the more usual height above the
surface too, so the second column lists the location in metres up in the air.
The third column list the air temperature, the fourth one the dewpoint (which although
quoted as a temperature is a measure of how humid the air is – it is the temperature to which
the air must cool to make it saturated, so a small difference between it and the actual
temperature (in column three) means it’s near saturation; the larger the difference the drier
the air). The final column lists the relative humidity.
Note that the dewpoint temperature is calculated from the other values; it’s not observed
directly during the ascent.
Once you have studied the data in the table to check by eye where the dry and damp layers are
– or where the temperature is changing most rapidly, either up or down, you can plot a graph
to make the patterns clearer.
It is quite common in the atmosphere to see a lot of detail in the humidity, with dry and moist
layers one above the other. You may have seen this sometimes when you take off and land in
an airplane. There are quite often layers of cloud separated by completely dry layers.
Plotting a graph of, for example, relative humidity will illustrate where such layers might be.
The damper ones will tie in with cloud layers you might have observed from the ground.
The graph of relative humidity below depicts the data from the table – you can see that the air
is fairly humid near the ground but rapidly becomes to a very low value around 10% near 250
dm (that is 2500m). Above that is a damp layer (around 65% at 3500m) above which there are a
further two dry and moist layers.
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
A sample ascent (for Trappes, France at 00 UTC, 15 February 2009)
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Pressure (hPa) Height (m) Temp (C) Dewpoint (C) Relative Humidity (%)
1000
250
1.4
-0.8
85
940
770
-2.5
-5.0
83
925
870
-2.7
-7.4
70
900
1050
-3.1
-13.1
46
850
1530
-6.1
-20.1
32
820
1850
-8.3
-26.3
22
780
2240
-5.9
-30.9
12
720
2830
-8.7
-29.7
16
700
3050
-9.7
-14.5
28
640
3700
-12.9
-17.5
68
610
4120
-15.1
-30.1
26
570
4660
-18.1
-23.4
63
500
5570
-25.5
-30.4
63
450
6350
-31.4
-35.2
69
400
7150
-37.5
-39.9
78
300
9080
-51.5
-250
10240
-60.3
-230
10650
-63.5
-200
11230
-57.7
-Relative humidity profile
800
700
600
Height (dm)
500
400
Series1
300
200
100
0
0
10
20
30
40
50
60
70
Relative humidity (%)
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80
90
Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
Surface observations
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There are many thousand weather observation sites around the world. Busy places like
Heathrow and Gatwick Airports make observations every hour, every day, every year. Other
places observe much less, like a mountain rescue station or coastguard station where it might
be only once or twice a day.
Official hourly reporting sites take a very wide range of measures that include
temperature, humidity, pressure, cloud type and cloud amount, height of cloud above the
ground, wind direction and speed, visibility and ‘weather’ which means is there rain, drizzle,
snow, sleet, fog etc.
Like the balloon data, observations at the surface are instantly sent to the National Weather
Service HQ to be plotted on a map that is then ‘analysed’ either automatically or by hand. This
means that special lines are drawn on them that show where the important weather features
are like highs and lows at the surface or jetstreams high up in the atmosphere.
Weather maps with lots of surface weather observations on them are ‘analysed’ – that is to say
they have lines drawn on them such as isobars (lines joining places with the same mean sea
level pressure). Normally nowadays this is done automatically although occasionally a
forecaster will do a hand ‘analysis’.
www.reading.ac.uk/meteoroloy www.walker-institute.ac.uk
Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
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Activity 1
Plotting the balloon data
The instruments attached to the balloon relay observations of pressure, temperature and
relative humidity every few seconds, in real time. Because pressure decreases as height above
the ground increases, we can tell the height at which each value is reported. So when we draw
a graph of temperature changing with height, we most often mark pressure as the vertical, Yaxis. However, you can use the height in metres or decametres (dm = 10m) as your vertical axis.
On the horizontal X-axis we plot temperature or relative humidity. Plotting values of, say,
temperature at regular changes in pressure (for example at 980 hPa, 960 hPa, 940 hPa etc)
produces a vertical ‘profile’ of temperature. We can do the same thing for relative humidity.
Looking carefully at the graphs provides information on how quickly the air is getting cooler
with height – or even where it is actually warming with height. We can also tell where very
damp layers are – and therefore where the cloud is likely to be, and where dry air is located.
Use the table to write down the values from today’s balloon ascent
Pressure (hPa)
Height (metres) Temperature (C)
Relative
humidity (%)
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
Make two graphs
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Use the table of data from the ascent to make two graphs, one for temperature and one for
relative humidity.
Temperature (to nearest tenth of a degree Celsius)
(a) Write down the surface temperature and the lowest temperature in the whole
ascent. Calculate the average change of temperature per kilometre between the surface
and the lowest value by dividing the temperature difference by the height difference.
(b) Is there a layer within which the temperature actually increases with increasing height?
If so, what height is it at and how thick is the layer?
Relative humidity (to nearest 5%)
(a) Where the relative humidity is 90% or more there are likely to be clouds present. Mark
in any layers of cloud – show how they fit with your observation of the clouds at the
Observatory.
(b) Very dry air, 50% relative humidity or less, is cloud-free. Mark any layers
with such dry air.
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
Activity 2
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Making and logging surface observations
The Department’s observer takes a large range of weather data, once a day at 0900 UTC (this
is at 10 o’clock during British Summer Time). They log (write down in an official ‘log book’)
every value, and after the end of each month send the data to the UK Met Office in Exeter.
The observations include:
Maximum temperature (over the last 24 hours)
Minimum temperature ( “ “ “
“
)
Temperature at 0900 UTC
‘Wetbulb’ temperature at 0900 UTC
Rainfall total (over last 24 hours)
Wind direction and speed at 0900 UTC
Evaporation (over last 24 hours)
Cloud type and cloud amount
You will observe temperature and wetbulb temperature (both in degrees Celsius) using the
whirling psychrometer, the wind speed (knots) by using the hand-held anemometer, the
wind direction (nearest 10 degrees clockwise from north) by looking at the wind vane 10
metres up at the Atmospheric Observatory, and cloud type and amount by using the
illustrated cloud chart (for type).
Using surface data
You will be given the daily weather data for 2008 from the University’s Atmospheric
Observatory.
With many years of daily data (usually 30) we can define the ‘climate’ of the site. We have
data for the University site starting in 1970, so observations from 1971 to 2000 can be used
to calculate the 30-year averages of temperature etc for Reading’s ‘climate’.
(a) For the month of daily data you are given, calculate
•
•
•
•
•
the average maximum temperature
the average minimum temperature
the total precipitation
the number of ground frosts (grass minimum at or below 0C)
the total hours of sunshine
(b) Then, with all twelve months completed, plot a graph for the year
(January to December) of monthly average temperature (mean of the
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Department of Meteorology and Walker Institute
Resources for Schools
How do we predict Weather and Climate?
Classroom activity: Meteorological Observations
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average maximum and minimum values) and a second graph of
monthly precipitation totals.
(c) Are there certain months or times of year when it was wetter? If so,
what weather features might be linked to wetter periods?
(d) Is it the case that the sunniest month is also the warmest. If it isn’t,
why would that be?
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