Synoptic Meteorology and

ClimatologyATC Chapter 7 & 8

Aim

To learn how to decipher a meteorological chart including all weather systems and to learn about climatology in Australia

Objectives1. Explain what an air mass is2. State and explain the different types of fronts3. State the different pressure systems4. Decipher a synoptic chart5.Explain the seasonal weather of Australia6. Describe the life cycle of a Tropical Cyclone

1. Air massesAir masses

If air remains over a surface for long enough, it takes on the characteristics of that surfaceOnce it has taken on the characteristics, it is called an air mass

Ie if a cold body of air moves over a warm surface, it will gradually become warmer

1. Air massesAir masses

Air masses can be classified as either polar or tropicalThe air within the polar cell is called polar air, while all other air is tropicalAir that remains over land for a period, gradually loses its moisture and is classified as continentalAir that remains over sea gradually increases its moisture and is classified as maritimeTherefore, air masses can be classified as:• Tropical maritime• Tropical continental• Polar maritime• Polar continental

2. FrontsFronts

When an air mass moves over a different region, it often encounters air of different temperaturesBecause the air has different densities, the warmer air tends to rise and the colder air tends to sinkThe boundary of these air masses is called a frontThe different types of fronts are:• Cold fronts• Warm fronts• Occluded fronts• Quasi stationary fronts

2. FrontsCold Fronts

A cold front occurs when a cold air mass moves into a region previously occupied by warm airThe boundary of these air masses is called a cold frontThe cold front on a synoptic chart is shown as a line with solid barbs pointing in the direction of travel

2. FrontsCold Fronts

As the cold air advances, it lifts the warm air aboveThe exact cloud formed depends on the stability and humidity of the warm air and the speed at which the cold air is advancingGenerally as the warm air is forced to rise, it promotes instability and cumuliform clouds are likelyThe passage of a cold front can be characterised by:• The temperature falling• The pressure rising• The wind backing

2. FrontsWarm Fronts

A warm front is characterised by cold air retreating from advancing warm airThe position of a warm front is shown on a synoptic chart as a line with solid semicircles pointing in the direction of travel, ie into the cold air

2. FrontsWarm Fronts

As the warm front approaches, it is characterised by:• The temperature rises• Wind backs• Pressure falls

Cloud associated with a warm front are usually cirrus first, followed by a gradual lowering of the base to form middle level altostratus. Nimbostratus can then form to produce rain until the front passes

2. FrontsOccluded Fronts

Because cold air is heavier, it tends to surge forward much more rapidly than a warm airAs days go by, the cold front moves faster over the ground than the warm frontEventually the cold front overtakes the warm front producing an occluded frontAn occluded front is shown on a synoptic chart as a solid line with a mixture of barbs and semi circles

2. FrontsOccluded Fronts

The cloud associated with an occluded front depends on what clouds were associated with the cold and warm frontsIt is not unusual for stratiform clouds from the warm front to conceal cumuliform clouds such as cumulonimbus from the cold front making flight very dangerous

2. FrontsQuasi stationary Fronts

A quasi stationary front is a front that tends not to moveThese types of fronts tend to align themselves with the isobars and have generally benign weatherThe quasi stationary front is shown on a synoptic chart as a solid line with barbs and semicircles on either side of the line

3. Pressure systemsPressure systems

To understand how to read synoptic charts, we must first be able to understand the pressure systems, how they are indicated, and what weather is associated with themOn a synoptic chart, the pressure systems which we see are:• High pressures or anticyclones• Low pressures or depressions

3. Pressure systemsAnticyclones

When isobars close around a region of high pressure, the feature is called a high pressure system or anticycloneIn the southern hemisphere, the rotation of winds around a high pressure are anticlockwiseIn the surface layer, the result of friction causes the wind to blow slightly outwards and across the isobarsThe flow associated with a high pressure system is:• An outflow of air in the lower layers• A subsidence of air over the wide area from above• Convergence of air in the upper layers

3. Pressure systemsWeather associated with anticyclones

As air is subsiding, the air in the top layers will be warming quicker than the air in the lower layers due to compressionThis can cause a subsidence inversion and trap pollutants such as smoke or haze beneath itThis can affect flight visibility

3. Pressure systemsRidges of high pressure

A ridge is an elongated area of high pressureA ridge is generally associated with fine weather conditions

3. Pressure systemsDepressions

When isobars form around a region of low pressure it is called a low pressure system or depressionIn the southern hemisphere, the winds about a low tend to blow clockwiseDue to surface friction, the winds tend to blow inwards and across the isobars

3. Pressure systemsDepressions

Compared with a high pressure, a low pressure has a stronger pressure gradient, that is a greater change of pressure with distanceBecause of the inflow of air towards the centre of a low pressure system, the wind tends to get stronger at the centre• This is called convergence and causes the air to also rise

at the centre due to the lower pressure• This in turn causes a divergence in the upper layers

3. Pressure systems Weather associated with a Depression

In a depression, the rising air in towards the centre will also be coolingIf the air reaches dew point temperature, cloud will formInstability in the rising air can cause a large vertical development of cumuliform clouds accompanied by showersBecause of the vertical motion of air, good visibility can result due to the vertical motion taking away all the particles suspended in the airThis can also cause turbulence

3. Pressure systemsTroughs of low pressure

An elongated area of low pressure is known as a troughConvergence will occur and the air will riseIf the air is unstable, large cumuliform clouds can occur

3. Pressure systemsCol

A col is an area between 2 areas of low pressure and 2 areas of high pressure, or 2 anticyclones and 2 depressions.It is an area of almost constant pressureA col usually has light and variable winds

4. Synoptic ChartsSynoptic Charts

When presented with a synoptic chart you must be able to identify:• Pressure systems• Fronts• Troughs and ridges• Surface wind and pressure at a particular location

4. Synoptic ChartsSynoptic Charts

When analysing a chart we need to remember that:• A front ALWAYS originates from a low• The circulation around a low is clockwise• Pressure gradient runs down into a low (up into a high)• The gradient wind cannot blow in a reverse direction

4. Synoptic ChartsSynoptic Charts - Example

A

B

CD

E

F

Example, given the chart below, assuming an isobar spacing of 4hPa determine:1. The pressure systems marked: A, B, C2. The name of the features: BD, BE, CF3. The surface wind and pressure in Hobart

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:1. The pressure systems marked: A, B, C

A

B

CD

E

F

- Fronts ALWAYS start from a low, therefore B is a Low- Once we have identified one pressure system we can

draw arrows to indicate wind direction – clockwise around a low

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:1. The pressure systems marked: A, B, C

A

B

CD

E

F

- The wind is blowing in an anti-clockwise direction around A and C therefore they are Highs

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:2. The name of the features: BD, BE, CF

A

B

CD

E

F

- Looking at the symbols on the chart, BD is a cold front, BE is a warm front

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:2. The name of the features: BD, BE, CF

A

B

CD

E

F

- CF is an elongated section originating from a high, therefore it is a Ridge

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:3. The surface wind and pressure in Hobart

A

B

CD

E

F

- Wind: Gradient wind is a South Westerly, the surface wind will be Westerly due to veering from surface friction

4. Synoptic ChartsSynoptic Charts - Example

Example, given the chart below, determine:3. The surface wind and pressure in Hobart

A

B

CD

E

F

- Pressure: Start from the known pressure then write what the pressure is at each isobar, remember pressure decreases as we move further from the high. Hobart = 1018

1022

1018

5. ClimatologyWeather in Australia

Australia is a vast country, and with it carries a vast range of weatherThis weather is created on a large scale and depends on the seasons and airstreams Generally speaking we can divide the weather in Australia into two categories:• Tropical – Northern Australia• Sub-Tropical – Southern Australia

Sub-Tropical MeteorologyThe weather in Southern Australia is classified as Sub-TropicalThe weather in this region is mainly influenced by the sub-tropical ridgeThis causes us to have the four seasons:• Summer• Autumn• Winter• Spring

5. Climatology

Sub-Tropical Meteorology - SummerSummer comprises the months December to FebruaryIn summer the highs which form the subtropical ridge move to the southWinds are generally south easterlyIn Western Australia the wind is generally very hot and dry

5. Climatology

Sub-Tropical Meteorology - AutumnAutumn comprises the months March to MayAutumn generally brings long periods of fine weather and is often considered the best time to go flyingAlthough passage of a cold front can bring cold conditions and the possibility of frontal thunderstormsFog can be expected during the mornings as overnight temperatures can get low

5. Climatology

Sub-Tropical Meteorology - WinterWinter consists of the months June to AugustDuring the winter months the sub-tropical ridge moves to the north causing the mid-latitude depressions and the associated fronts to lie just to the south of the continentWinds are generally westerlyCloud is often widespread stratocumulusPassing fronts bring periods of rain

5. Climatology

Sub-Tropical Meteorology - SpringSpring consists of the months September to NovemberSpring weather is often quite variableConditions can be generally fine one minute then can be un-flyable the nextThis is due to fast moving cold fronts and troughs

5. Climatology

Tropical MeteorologyThe weather in the North of Australia is classified as TropicalThere are 2 major wind streams in the tropics and they are largely responsible for the weather in these regions

The wind streams are the trade winds and the monsoons

5. Climatology

Tropical Meteorology - Trade WindsThe regions between the subtropical high pressure belts and the equatorial trough are occupied by the trade windsIn the southern hemisphere, Coriolis deflection produces the south east tradewinds, and in the northern hemisphere, the northeast trade windsThe winds do not always blow exactly in these directions, changes due to pressure gradients and the Coriolis force cause the wind direction and speed to change from place to place

5. Climatology

Tropical Meteorology - Trade WindsThe Trade Winds are responsible for the “Dry Season” in Northern AustraliaThe characteristics of the trade winds are:• Winds are South-Easterly (in the southern hemisphere)• It blows at a consistent velocity for very long periods of time

(months)• Becomes weaker at lower latitudes – due to less Coriolis force• Generally stable• Relative humidity is high at low levels• Often produces showers near the coast but is dry inland

5. Climatology

Tropical Meteorology - The monsoonsDue to specific heats, continents tend to warm up quickly in summer and cool rapidly in winter, whereas oceans tend to maintain a more even contrast throughout the yearWhen large continental and ocean areas are involved, atmospheric circulations may develop on a large scaleA monsoon is the persistence of maritime airflow over a hot continentMoist maritime air crosses the coast to the hot land and becomes more and more unstable as it is heated from below

5. Climatology

Tropical Meteorology - The MonsoonsThe Monsoons are responsible for the “Wet Season” in Northern AustraliaThe characteristics of the Monsoons are:• Winds are North-Westerly (in the southern hemisphere)• Generally unstable• It is moist near the coast but dry inland• Produces areas of widespread rainfall up to 300nm inland

5. Climatology

Tropical Meteorology - The equatorial trough The equatorial trough is the name given to the area between the trade winds of each hemisphereGenerally, the weather consists of light winds and fine weather, if there is a large level of convergence, instability can exist with large cloud formations The equatorial trough follows the passage of the sun throughout summer to winter

In summer, it is lower down and covers parts of northern AustraliaIn winter, it is further north

5. Climatology

The Intertropical Convergence Zone (ITCZ)Convergence occurs on a large scale when the tradewinds of each hemisphere meet in a narrow zoneThis zone is called the Intertropical Convergence Zone (ITCZ)It can occur when the north east trade winds and the southeast tradewinds approach each other between two near-equatorial troughsThe ITCZ can produce extreme weather conditions over a wide areaWeather can include heavy rain, frequent thunderstorms and violent wind squalls

5. Climatology

6. Tropical CyclonesTropical Cyclones

Tropical Cyclones are strong, deep depressions that form over the oceanTropical cyclones derive their energy from warm waters, usually above 28 degrees CelsiusTropical cyclones form usually between 5-15 degrees latitude This is due to the coriolis force required to develop the balanced circulation flowTropical cyclones can decay if they move into colder waters (further away from equator), or if they move inlandThe stages of a tropical cyclone are:• Formative stage• Immature stage• Mature stage• Decaying stage

Tropical Cyclones - lifecycleFormative stage – A tropical cyclone usually develops in a pre existing low pressure toughAtmospheric wind shear is low and there is an abundance of moisture for vigorous convective cloud growthThe system becomes more circular, the pressure in the centre decreases, still above 1000hPa, and wind increases

6. Tropical Cyclones

Tropical Cyclones - lifecycleImmature stage – The pressure falls below 1000hPa and winds increase to 64ktsThe convective cloud forms into spirals and the eye developsPressure continues to fall, and winds continue to increase in strength

6. Tropical Cyclones

Tropical Cyclones - lifecycleMature Stage – At this stage the cyclone is at its maximum size and strength and the centre pressure is steady, at around 950hPaThe hurricane force winds and bad weather extend to around 150nm from the centre with the strongest winds being in the left forward quadrant

6. Tropical Cyclones

Tropical Cyclones - lifecycleDecaying Stage – When the cyclone has its energy supply disrupted, by either reaching colder water or land, the decaying stage beginsThe system may continue over land as a rain bearing depression or move into higher latitudes and interact with frontal systems

6. Tropical Cyclones

Tropical Cyclones – The eyeThe eye of the tropical cyclone is an indication of a well developed stormThe conditions are benign with a constant pressure and light windsThere is generally clear skies with no precipitation

6. Tropical Cyclones

Questions?