Transcript Chapter 4(b)

Chapter 4(b)
Atmospheric Motion & Wind
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Global Scale Motion

From an early age you would have noticed yearly
weather patterns;
 Weather
tends to follow some general patterns.
 These
flow patterns are enormous, covering tens of
thousands of square kilometers and affecting large parts of
the world.
 When
describing such a scale, it is termed a global scale.
 There
are many patterns on the global scale which could be
discussed but we shall focus on those patterns that most
influence weather over Australia.
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The General Circulation of the Atmosphere

General global airflow
 When
examining the general flow of air around the globe, we
need to examine both horizontal and vertical air flow.
 In
Figure 4.6 we can clearly see the effects both of these
types of wind movement.
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The General Circulation of the Atmosphere
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The General Circulation of the Atmosphere

So what are all these circulations? As you keep Figure 4.8 in
mind, we shall examine the influences that create all of the
circulations we see;

Causes of horizontal motion (wind);

The Earth is an oblique spheroid, a three dimensional squashed ball

The Sun shines light at the Earth, which is distributed unevenly due to
curvature

The Earth warms more at the equator than at the poles creating a
temperature gradient

Heat rises at the equator, cools, and falls to the North and South of the
equator
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General airflow over Australia

In Australia, the wind generally flows from west to east
and carries lows and highs from the west coast to the
east coast over the course of several days.

Notice that westerlies flow eastward.
 Instead
of describing the direction the wind points, we use
the direction from where the wind comes.

This westerly motion is created by several factors
including the Coriolis force and the jet streams as
seen in the figure above.
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Jet Streams

Jet streams are high speed air currents found at the
top of the troposphere, often with wind speeds in
excess of ~200 km/hr.

These air streams are ribbons of high speed winds
which have lengths of thousands of miles, but are only
a few hundred kilometers wide and only a few
kilometers thick.
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Jet Streams

There are four primary jet streams on Earth;
 the
north and south subtropical jet streams and
 the
north and south polar jet streams.

The subtropical jet streams form around 30°N&S
latitude, where a general high pressure area exists.

The sinking air over this area marks the boundary
between moist, hot air to the south and cooler air to
the north.
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Jet Streams

Jet streams are important because they control the
overall flow of air over the southern part of Australia.

Bends and turns in the jet streams show us where
weather systems are and where they are going.

It is the low and high pressure systems that are guided
by jet streams and strengthened by jet streaks that
cause our everyday weather.
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Synoptic scale motion

The synoptic scale is the level just below the global
scale.

It is what we call the "weather map" scale because the
highs, lows, and fronts that you see on weather maps
fall into this scale.

The synoptic scale encompasses regions thousands
of kilometers in area with circulations that last from
days to weeks.
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Synoptic scale motion

We will look at four major items on the synoptic scale:
 air
masses,
 fronts,
 cyclones,
and
 anticyclones.
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Air Masses

An air mass is a very large body of air which has fairly
uniform temperature and moisture properties
throughout its horizontal extent, which may cover
thousands of kilometres.
 Air
masses originate in source regions, where the air is
relatively stagnant and the air can acquire the properties of
the source region.
 For
example, the Australian north interior is a source region
for the warm air that gives us our warm summers.
 An
air mass that forms in the Antarctic region is what brings
the middle to southern areas the cool changes (not to be
confused with fronts)
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Air Masses


There is a simple scheme of four classifications that are
combined to describe an air mass's properties.

A "c" stands for "continental", meaning that the air mass developed over
land.

An "m" stand for "maritime", meaning the air mass formed over water.

A "P" stands for "polar", meaning the air formed over the cold polar
regions.

A "T" means the air mass formed over a warm "tropical" region.
Air masses move from their source regions to Australia and
bring us their weather. The transition zone between two unlike
air masses is called a frontal zone
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Fronts

Fronts are zones where two air masses with differing
densities meet. The difference in density is due to
either temperature differences, moisture differences,
or both.

There are four types of fronts:
 cold
fronts,
 warm
fronts,
 occluded
fronts, and
 stationary
fronts.
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Fronts

The boundary line that separates two air masses is a
'front', from the military term describing the line
between armies.

Norwegian meteorologists developed the concept
during the First World War.

These frontal boundaries are always areas of rising air
which produce weather changes.
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Cold Fronts

A cold front is the transition zone where a colder, drier air
mass is replacing a warm, moist air mass.

Across the frontal zone, there is a sharp temperature decrease, a sharp
humidity decrease, and a shift in wind direction.

Often, there are clouds and precipitation.

Cold fronts often bring very severe weather.

This is due to the fact that warm, moist air is less dense than cold, dry
air.

The warm, moist air is forced upward as the cold front moves in, causing
thunderstorms to form.

Notice that the leading edge of the cold front has a high angle, which can
rapidly force warm, moist air aloft.
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Cold Fronts
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Warm Fronts

A warm front is the boundary zone between retreating
cold air and the advancing warm air.
 Again,
the warm air overrides the cooler air, forming clouds
and precipitation.
 However,
the weather is characterized by rain or showers,
not storms.
 Because
the slope of the warm front is not as steep as the
cold front, showers form more gradually, preventing latent
energy from reaching high altitudes.
 There
is a temperature and humidity increase with the
passage of a warm front.
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Warm Front
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Occluded Fronts

Often, a cold front will travel faster than a warm front and will
eventually catch up and overtake the warm front.

The boundary created is called an occluded front or an
occlusion.

Here, the layer of warm air is lifted above both layers of cold air.

Preceding the occluded front would be showers typical of a
warm front.

But as the front gets closer, more stormy-like conditions may
prevail.
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Stationary Fronts

A stationary front is a boundary between two air
masses that are not moving.

The weather along the front will vary from clear to
showery, depending on the air masses involved.
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Cyclones

A cyclone is any low pressure system that spins
clockwise in the Southern Hemisphere.

Cyclones range in size from tiny whirlwinds to
thunderstorms to hurricanes.

Air spirals clockwise at the surface into a cyclone.

The air is then forced upward by convergence.

This rising air is what leads to the clouds and storms
associated with the cyclone.
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Cyclones

Tropical cyclones are those which form in tropical
areas.
 These
start off as tropical depressions, then grow into
tropical storms, and may eventually strengthen into
cyclones.
 Unlike
mid-latitude cyclones, tropical cyclones are not
associated with fronts.
 They
are self-contained and intensify due to moisture and
wind convergence.
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Anticyclone

The opposite of a cyclone is an anticyclone.
 It
is a high pressure system with winds that blow out from the
center of the high in an anti-clockwise direction in the
southern hemisphere.
 High
pressure systems usually bring dry, clear weather.
 In
anticyclones, the air converges high in the atmosphere
and then sinks toward the ground.
 In
the lower atmosphere, the air spreads out, or diverges.
 The
sinking air contains little moisture, and therefore brings
dry weather.
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Meso and Micro scale phenomena

Meso-scale phenomena have an approximate size
between 1km and 100 km and a time span between a
few minutes to a day.
 This
scale includes weather such as sea breezes and
mountain-valley breezes.
 Air
flow in this scale is influenced by differential heating or
cooling over an area and by differences in terrain which can
accentuate this process.
 We
will look at these two factors and at a few examples of
mesoscale flow.
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Thermal Breezes

The sea and land breeze circulations are good
examples of thermal circulations.

The sea-breeze forms during the day, the land breeze
at night. Here's why:
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Thermal Breezes

Sea Breeze
 The
sea breeze forms due to the differential heating of the
air over a region where a large body of water and land meet.
 The
air above the land will heat and expand more rapidly
than the air over the water.
 A relative
high pressure area will form aloft over the land,
and a relative low pressure area will form over the water.
 The
pressure gradient will force air movement toward the
low pressure area, and a thermally driven circulation will
form.
 The
breeze that forms along the surface blowing in from the
water (called an onshore breeze) usually brings in cool air.
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Thermal Breezes

Land Breeze

Because the land also cools more rapidly than the water, a similar
situation occurs late at night, but in the opposite direction.

Here, the cooled air over the land contracts, forming a low pressure aloft
over land and a relative high pressure aloft over the water.

The resulting circulation is known as a land breeze, because the wind
now blows offshore.

The land breeze usually has a smaller magnitude than the sea breeze
because the temperature difference at night between land and water is
not as large.

Similar phenomena occur over smaller bodies of water (such as lakes),
but on a much smaller magnitude.

If a thermal circulation develops over a lake, it is called a lake breeze.
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Thermal Breezes

Mountain / valley breezes
 A similar
pattern occurs along mountain slopes.
 The
valley air warms rapidly and rises along the mountain
slope to form the valley breeze.
 At
night, the surface air quickly cools and slides down the
mountain slope, forming the mountain breeze.
 Although
this is due to the terrain of the mountains, it is,
technically speaking, thermally driven.
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Meso scale mechanical circulations

The other main influence on mesoscale circulations is
the topography of a region.

Because the influence is mechanical rather than
thermal, these are referred to as mechanically forced
circulations.
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Meso scale mechanical circulations

Katabatic winds
 Though
technically meaning any wind that ‘flows’ downslope, a katabatic wind usually represents wind with a higher
magnitude than that of a mountain breeze.
 Typically,
it is used to describe winds that move rapidly off a
plateau into an adjoining valley.
 The
air moves across the plateau and then warms
adiabatically as it moves downward.
 When
the air on the plateau is significantly denser than the
air in the valley gravity will ‘pull’ the air down-slope.
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Micro-scale motion

The micro-scale is the smallest scale in which we
classify weather phenomena.

Micro-scale swirls of air have widths of a few meters
and last less than a few minutes.

Like mesoscale flow, micro-scale flow is forced by
heating and obstructions to flow.

These cause tiny motions in the air to form.
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Micro-scale motion

Thermal turbulence
 Differential
heating of a surface will cause some air
molecules to become excited, while others have less energy.
 The
interactions between excited molecules and slower
ones cause tiny eddies to form.
 These
little whirlwinds add to turbulence in the friction layer
and to instability.
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Micro-scale motion

Mechanical turbulence
 Mechanical
turbulence is determined by both the speed of
the prevailing wind and the roughness of the surface over
which the air flows.
 As
wind moves through trees or over rough surfaces, the air
is broken up into eddies that make the wind flow irregular.
 We
feel these irregularities at the surface as abrupt changes
in wind speed and direction which results in gusts.
 These
eddies can either combine to form larger eddies, or
cancel each other out and lessen the effect.
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