Chapter 4(b)

Atmospheric Motion & Wind

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.

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.

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

The General Circulation of the Atmosphere

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.

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.

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.

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.

Jet Streams

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.

We will look at four major items on the synoptic scale:

air masses,

fronts,

cyclones, and

anticyclones.

Synoptic scale motion

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)

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

Air Masses

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.

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.

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.

Cold Fronts

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.

Warm Front

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.

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.

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.

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.

Cyclones

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.

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.

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:

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.

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.

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.

Thermal Breezes

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.

Katabatic winds

Though technically meaning any wind that ‘flows’ down-slope, 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.

Meso scale mechanical circulations

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.

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.

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.

Micro-scale motion