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Year 10 Outdoor Education
Weather
High and Low Pressure Systems
Introduction
Regions of sinking air are called highs, high pressure regions or anticyclones. Clear skies
and fair weather usually occur in these regions.
Regions of rising air are called lows, low pressure regions, depressions or cyclones.
Clouds rain and strong winds often occur in these regions.
Look at the map below (Figure 1). This shows a typical pattern of high and low pressure
regions. The curved lines are called isobars. These pass through areas with the same air
pressure.
(Figure 1)
Air pressure is measured in hectopascals. On this weather map the isobars are at 4
hectopascal intervals.
The black arrows show the wind direction. High and low pressure regions do not stay in
the same place. They move over the Earth's surface.
The line with barbs is a cold front. The barbs point in the direction the front is moving. In
the diagram the cold front is moving in an easterly direction.
What do weather maps show?
The most obvious features of the media's weather maps (Figure 2. is an example) are the
patterns of high and low pressure, and the barbed lines identifying cold fronts. In the
southern hemisphere, the earth's rotation causes air to flow clockwise around low
pressure systems and anticlockwise around high pressure systems. (The opposite applies
in the northern hemisphere.) Friction over the earth's surface causes the winds to be
deflected slightly inwards towards low pressure centres, and slightly outwards from high
pressure systems. Wind strength is inversely proportional to the distance between isobars
-- the closer the lines, the stronger the winds. This rule does not apply in the tropics
where the effect of the earth's rotation is weak. For this reason, tropical meteorologists
usually replace isobars with streamline arrows which indicate wind and direction without
directly relating to the pressure gradient. Shaded areas on weather maps show where
there has been rain in the previous 24 hours, and wind direction is shown with arrows that
have a series of barbs on their tails to indicate speed.
(Figure 2)
Clouds
Of all weather phenomena, clouds are among the most fascinating. From the silky
filaments of high altitude cirrus to the towering, threatening mass of storm-bearing
cumulonimbus, clouds are as varied as the weather itself.
Apart from their beauty and interest, clouds can provide a useful indication of weather
conditions.
Clouds have their origins in the water that covers 70 per cent of the earth's surface.
Millions of tons of water vapour are evaporated into the air daily from oceans, lakes and
rivers, and by transpiration from trees, crops and other plant life.
As this moist air rises it encounters lower pressures, expands as a result, and in doing so
becomes cooler. As the air cools it can hold less water vapour and eventually will become
saturated. It is from this point that some of the water vapour will condense into tiny water
droplets to form cloud (about one million cloud droplets are contained in one rain-drop).
Thus, whenever clouds appear they provide visual evidence of the presence of water in
the atmosphere.
There are four ways in which moist air can be lifted to form cloud. They are:
Orographic lifting occurs when air is forced upward by a barrier of mountains or hills.
Convective lifting occurs when air heated at the earth's surface rises in the form of
thermal currents or bubbles.
Widespread ascent results from the interaction of air masses, or the movement of a cold
air mass forcing warm air to rise ahead of it.
Mechanical (or frictional) turbulence occurs when the air flow is deformed into a series
of eddies as it moves over the earth's surface.
There are ten main cloud types, which are further divided into 27 sub-types according to
their height shape, colour and associated weather, Clouds are categorised as low (from
the earth's surface to 2.5 km), middle (2.5 to 6 km), or high (above 6 km). They are given
Latin names which describe their characteristics, e.g. cirrus (a hair), cumulus (a heap),
stratus (a layer) and nimbus (rain-bearing). It's an interesting fact that all clouds are
white, but when viewed from the ground some appear grey or dark grey according to
their depth and shading from higher cloud.
Typical examples of the ten main cloud types are shown
Cirrus: high level, white tufts or filaments; made up of ice crystals. (No precipitation.)
Cirrocumulus: high level, small rippled elements; ice crystals. (No precipitation.)
Cirrostratus: high level, transparent sheet or veil, halo phenomena; ice crystals. (No
precipitation.)
Altocumulus: middle level layered cloud, rippled elements, generally white with some
shading. May produce light showers.
Altostratus: middle level grey sheet, thinner layer allows sun to appear as through ground
glass. Precipitation: rain or snow.
Nimbostratus: thicker, darker and lower based sheet. Precipitation: heavier intensity rain
or snow.
Stratocumulus: low level layered cloud, series of rounded rolls, generally white.
Precipitation: drizzle.
Stratus: low level layer or mass, grey, uniform base; if ragged, referred to as
"fractostratus". Precipitation: drizzle.
Cumulus: low level, individual cells, vertical rolls or towers, flat base. Precipitation:
showers of rain or snow.
Cumulonimbus: low level, very large cauliflower-shaped towers to 16 km high, often
"anvil tops". Phenomena: thunderstorms, lightning, squalls. Precipitation: showers of rain
or snow.
Questions
1. Define the term isobar.
2. What unit is air pressure measured in?
3. In which direction does the air flow around a low pressure system?
4. In which direction does the air flow around a high pressure system?
5. Using a weather map, what indicates wind speed?
6. What does a shaded area on a weather map indicate?
7. Using the typical summer weather map complete the followinga/ In which direction would we expect the wind to be in Melbourne at the time of this
weather map being drawn?
b/How strong would we expect the wind to be?
c/What changes would we expect in weather and wind direction in the near future?
8. Using the typical winter weather map complete the followinga/ In which direction would we expect the wind to be in Melbourne at the time of this
weather map being drawn?
b/ How strong would we expect the wind to be?
c/ What changes would we expect in weather and wind direction in the near future?
d/ If you were planning a trip to the snow, could we expect likely snow showers.
Explain your answer.
9. Explain the four ways moist air can be lifted to form clouds?
10. Name the 3 main categories of clouds.
11. Outline the cloud types that will indicatea/ No rain or precipitation
b/ Rain or snow
c/ Drizzle
d/ Thunderstorms
12. What predominate cloud condition are we are experiencing today? Explain your
answer.
Weather maps
A typical summer chart
A typical winter chart