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UNDER PRESSURE: PART 1.
How heavy is the weight of air above your head? At first,
this seems a ridiculous question. However, gases do
actually have a weight and since the atmosphere consists
of a mixture of gases (mainly Nitrogen and Oxygen), air has
a weight. This weight is also described as pressure. In fact,
the atmosphere is exerting about 5 tonnes of pressure on
your head. A cubic metre of air typically weighs about 1.2
kilograms (kg) at sea-level. Thus, the weight of air in a car
or a large tea chest is about 1 kg, which is approximately
the weight of a large bag of sugar. Fortunately, we do not
experience this weight or pressure because our internal
pressure acts as a counter-balance. However, we do
feel rapid changes in pressure. For instance, if you dive
into a swimming pool, the sudden increase in pressure
is expressed particularly by compression on the head,
especially the eardrums. This is because water is about
1000 times denser than air. Furthermore, we also sense
changes in pressure during air travel. Aircraft are normally
pressurized to the equivalent pressure at about 2000 metres. Therefore, on ascent and
descent we can experience changes in pressure, this is usually noticeable by a slight discomfort in your eardrums, with
the drums tending to ‘pop’.
Large animals have difficulty in adjusting rapidly to pressure changes. For
instance, there have been reports of cows exploding when hit by tornadoes
(extremely violent whirlwinds). The inner core or ‘vortex’ of tornadoes is the
lowest pressure system on Earth. When these extremely low-pressure systems
hit cattle, they cannot rapidly adjust their internal pressure and they can
therefore explode. For the same reason, people in tornado-vulnerable areas
(such as ‘tornado alley’ in the central USA) are advised to open windows in
advance of possible tornadoes, as this diminishes the chances of the house
exploding when hit by a tornado (i.e. very
low pressure outside and relatively high
pressure inside).
Evangelista Torricelli was one of the first
scientists to experiment with pressure. In
1644, Torricelli inverted a tube of mercury
into a vessel containing mercury (a liquid
metal). In the upper part of the tube a
vacuum was created when the mercury fell. This meant Torricelli was able to show
that air pressure was capable of supporting a column of mercury. When the air
pressure rose, the height of mercury increased. When the air pressure decreased,
the height of the mercury decreased. To illustrate air pressure to yourself, simply
place a drinking glass under the water level in a bowl. Raise the inverted full glass
above the surface of the water, keeping the rim below the water level. Air pressure
keeps the water in the glass, even though it is above water level.
Are you intrigued to know more about pressure? If so, then read on.
Notes written by:
Professor Mike Fullen and Dr Ken Addison,
School of Applied Sciences, The University of Wolverhampton, Wolverhampton WV1 1SB.
Tel. (01902) 322410, fax. (01902) 322468, e-mail [email protected]
UNDER PRESSURE: PART 2.
We all understand the everyday life idea of being ‘under pressure’. However, it is difficult to comprehend that the
atmosphere above us exerts pressure. Especially as the air above your head weighs several tonnes. In fact, the typical
pressure is 10 tonnes per square metre. Therefore, we can define pressure as weight per unit area.
Early experiments on pressure were carried out by Evangelistica Torricelli (a student of Galileo). In Florence (Italy) in
1644, Torricelli demonstrated that the Earth’s atmosphere exerts pressure, using a simple experiment. Torricelli inverted a
tube full of the liquid metal mercury (also known as ‘quicksilver’) into a vessel. In the upper part of the tube a vacuum was
created when the mercury level fell. Torricelli showed air pressure was able to support a column of mercury.
The instrument used to measure pressure is the barometer; the term derives from the Greek ‘baros’ meaning ‘weight’.
The barometer uses the principles discovered by Torricelli; that is, air pressure can support a height of mercury (chemical
symbol of mercury: Hg). As pressure increases, the height of the mercury rises. Likewise, as pressure decreases the
height goes down. Initially, pressure was expressed in terms of height of mercury, but more recently, pressure has been
expressed in millibars (mb). Thus:
1000 mb = height of mercury of 29.53 inches = 750.06 mm.
Knowing this, on average, a cubic metre (m) of air (i.e. 1 m x 1 m x 1 m = 1 m3) weighs 1.2 kilograms (kg) (assuming a
pressure of 1000 mb and a temperature of 20 degrees centigrade (oC)).
Measuring pressure is extremely important, especially the relative change in pressure, which is referred to as the
barometric tendency. Generally, rising pressure is associated with calm stable weather, while falling pressure indicates the
likelihood of wet and stormy conditions. Aircraft cannot directly measure their altitude, thus aircraft instruments measure
pressure, which is then converted to altitude. Hence, all airports have high quality meteorological stations. Therefore, on
approach, the pilot is informed of the local air pressure and then fine-tunes the altimeter accordingly. This allows accurate
readings of altitude as the aircraft approaches the airport. Accurate measurement of pressure is therefore literally a matter
of life and death!
Many barometers, especially older ones, use the Torricelli principle and these were particularly sought after as ornaments
in Victorian households. They played an essential role in weather prediction and were of particular importance to the
safety of shipping. Hence, barometers can be seen in most ports, usually on the wall of the harbour master’s office.
Barometric tendency is also reported on the BBC Shipping Forecasts.
In the nineteenth century, aneroid barometers were invented (Figure 1).
The aneroid is one or more sealed metal capsules, which have had most
air pumped out of them and so reacts to pressure. In fact, the term aneroid
means ‘without liquid’ and aneroid capsules were invented by the French
engineer Lucien Vidie, in 1843. As air pressure increases, the aneroid capsule
assembly contracts, and as air pressure decreases, the assembly expands.
These movements are transmitted to a moving arm. Therefore, by placing
a pointer over the arm, then we can observe if pressure rises or falls over
a period of time. This tells us the barometric tendency and whether calm
or stormy conditions are likely. Sometimes these pointers have a pen-nib
attached and an adjacent revolving drum with a paper chart, which allows
continuous recording of air pressure. This is known as a barograph (Figure
2). Aneroid barometers have the advantage of being more robust and
transportable than mercury barometers.
We have briefly mentioned that particular weather conditions are associated
with different pressure systems. High pressure indicates descending air.
Figure 1. An aneroid barometer.
Normally, this means very calm, dry, clear, hot and sunny conditions in
summer. This is usually associated with the movement (or advection)
of tropical continental high-pressure air from Southern Europe and/or
North Africa over the British Isles. Likewise, in winter, this means calm,
clear and cold conditions, usually associated with the advection of
polar continental air from Asia, particularly from Siberia. These highpressure systems are known as anticyclones.
In contrast, rising air is associated with low pressure. Low-pressure
systems are coupled with stormy, unsettled and usually rainy weather.
These systems are known as cyclones. There are several types of
cyclone. In Britain, we usually experience depressions. These are
low-pressure systems with air revolving around the low-pressure
centre in an anticlockwise direction. In the tropics, cyclones can be
extremely violent and damaging. These tropical cyclones are known as
Figure 2. A barograph.
hurricanes in the North Atlantic and Caribbean and typhoons in the Pacific.
Air pressure varies both in space (that is vertically and horizontally) and through time and decreases with altitude. Since
air pressure is the overlying weight of air, it is logical that, as we ascend, pressure must decrease. For instance, as
we ascend a mountain, some mass of air must be beneath us and so less weight of air is above us (so pressure must
decrease). If you were on the top of Mount Everest, about 70% of the mass of the atmosphere would be below you. About
50% of the weight of the air is in the lower 5 km of the atmosphere. In the lower atmosphere, pressure deceases very
rapidly with altitude. The reason for this is that the Earth’s gravity pulls the atmosphere towards the Earth’s surface. In a
way, the atmosphere ‘hugs’ the Earth and so pressure decreases markedly with altitude.
Generally, there is a 10-fold decrease in air pressure for every 16 km increase in altitude (Table 1).
Table 1. Typical air pressure values (mb) at different altitudes (km)
Altitude (km)
0
16
32
48
Pressure (mb) 1000 100
10
1
Due to the rapid decrease in pressure with altitude, many climbers need additional oxygen for high altitude climbing in
mountain ranges, such as the Himalayas. Consequently, often people feel unwell (nauseous and dizzy) at high altitude,
especially if the ascent has been rapid. This is known as altitude sickness. As we already know, aircraft pilots use these
altitude-pressure relationships to calculate their altitude. Barometer readings are usually adjusted to sea-level and the
Meteorological Office issues values to alter pressure from a given altitude to sea-level. As an approximate generalization,
pressure decreases by about 1 mb per 10 metres increase in altitude. For instance, a barometer at 100 metres (m)
altitude would need 12.1 mb to be added to adjust it to sea-level and 24.4 mb at 200 m. This is assuming the temperature
is 10oC, as the density of mercury varies with temperature.
Most of the weight or mass of the atmosphere is in the lowest zone of the atmosphere, known as the troposphere.
The troposphere extends from 7 km deep at the poles to 14 km deep at the equator, averaging at about 10 km depth.
The reason of the geographical variation is the spinning of the Earth causes the troposphere to ‘bulge’ outwards at the
equator, due to the effect of centrifugal force (i.e. the force that throws you outward on a revolving roundabout on a
fairground). This is the zone in the atmosphere in which we have what we would typically call ‘weather’, that is day-today variations in meteorological parameters, such as rainfall, temperature, wind and humidity. The troposphere contains
virtually all the atmosphere’s water. This exists as water vapour (water as a gas), liquid water (cloud droplets and rain)
and ice (tiny ice crystals in clouds and larger hailstones). Liquid water and ice form the main constituents of clouds. In
fact, the thermal properties of planet Earth are unique in the Solar System. Being the ‘third rock from the Sun’, water
exists in solid, liquid and gaseous phases. Closer to the Sun, on the much hotter Venus, all water is water vapour; further
away from the Sun, on the much colder Mars, all water is ice.
The troposphere contains about 90% of the mass of the atmosphere. Therefore, only about 10% of the mass of the
atmosphere is above the troposphere in the zones known with increasing altitude as the stratosphere, mesosphere and
thermosphere.
Pressure also varies through time. The highest recorded pressure in Britain was 1055 mb, in 1902, in Aberdeen, and the
lowest recorded pressure was 925 mb, in 1884, in Perthshire. However, pressure does not normally vary that much in
the British Isles. For instance, a stormy low-pressure depression would typically have a pressure of about 980 mb, while
calm anticyclonic conditions would have a pressure of about 1020 mb. The highest pressures are typically associated
with the centre of the Siberian anticyclones, rising up to about 1070 mb. The lowest pressures are associated with tropical
cyclones, with a pressure of 882 mb measured in the centre or eye of Hurricane Gilbert in 1988. The most extreme low
pressures are associated with the vortex of tornadoes, but since they destroy weather stations little is known about their
pressure conditions, but they may be as low as 700 mb. The global average pressure is 1013 mb.
Pressure varies across the surface of the Earth. By having a network of barometers, both on land in meteorological
stations and at sea on ships, it is possible to have a ‘snapshot’ of pressure conditions. From these data, meteorological
services, such as the Meteorological Office, can construct maps of atmospheric pressure. These use lines of equal
pressure or ‘isobars’, from the Greek ‘isos’ meaning equal and ‘baros’ meaning weight. Isobars are very much analogous
to contours, which are lines of equal altitude. A universal law is that wind always blows from high (= descending air) to
low (= rising air) pressure. Adjustments need to be made both for the effects of the spin of the Earth and for surface
roughness on wind direction. Therefore, using a pressure map, it is also possible to predict wind direction.
Pressure maps also allow us to calculate wind velocity. The nearer isobars are packed together, the greater the pressure
gradient. The greater the pressure gradient, the higher the wind velocity. Again, this is analogous to contours. If contours
are packed closely together on a map, then the slope or gradient is steep. A ball rolling from the high to low ground
would roll down a steep slope and so gain velocity. Conversely, if isobars are far apart, then the pressure gradient is
gentle and wind velocities are low. The analogous slope would be gentle and a ball rolling across it would have a low
velocity. For instance, in the calm conditions associated with an anticyclone, there may be few isobars. Therefore,
the Meteorological Office usually issues a general pressure value (height of mercury and mb) to allow observers to
fine-tune their barometers. A comprehensive network of recording meteorological stations, both at home and abroad,
enable the Meteorological Office to continuously, and usually accurately, predict wind velocity and its direction.
Mathematical equations can predict wind velocities from the pressure gradient and the powerful computers of the World’s
meteorological services are continually doing this. Figure 3 shows a representative pressure map for western and central
Europe; that is at the sub-continental (or synoptic) scale.
Amateur meteorologists can also develop skills in weather
observations. For instance, if you, or someone you know,
have a barometer, why not note the daily pressure and
barometric tendency? Before doing so, remember to
give your barometer a gentle tap, as inertia can prevent
full movement of your pressure pointer. How do your
pressure measurements compare with the actual weather
and changes in the weather? Next, try comparing these
with weather charts published in newspapers and shown
on TV. However, remember, it is the relative changes in
pressure, which are particularly informative, rather than
the absolute pressure. With a little practice, it is possible to
make your own weather forecasts.
Figure 3. Pressure map associated with the passage of a depression over the British Isles.
Further reading
Roger G. Barry and Richard J. Chorley (1987). Atmosphere, Weather and Climate (5th Edition). Routledge, London, 460
pp. (ISBN 0-415-04585-1).
[Comprehensive overview of the main atmospheric and climatic systems].
Ron Lobeck 1991. The Barometer: Your home weather forecaster. Geerings of Ashford Ltd., Ashford, 72 pp. (ISBN 1873953-01-1).
[Useful summary of the history and principles of barometers. To obtain a copy contact: Barometer World, Quicksilver
Barn, Merton, Okehampton, Devon EX20 3DS; Tel. (01805) 603344].
Robin Mcllveen 1986. Basic Meteorology: A Physical Outline. Van Nostrand Reinhold (UK), Wokingham, 457 pp. (ISBN
0-442-31769-7).
[Detailed overview of meteorological processes, including atmospheric pressure].
Meteorological Office 1982. Observer’s Handbook (4th Edition). Chapter 7 Atmospheric Pressure, p. 98-109, HMSO,
London. (ISBN 0-11-400329-7).
[This handbook gives precise information on the measurement and recording of meteorological parameters, including air
pressure].
Notes written by:
Professor Mike Fullen and Dr Ken Addison,
School of Applied Sciences, The University of Wolverhampton, Wolverhampton WV1 1SB.
Tel. (01902) 322410, fax. (01902) 322468, e-mail [email protected]