# Synoptic Meteorology and Climatology

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Synoptic Meteorology and Climatology
ATC Chapter 7 & 8

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

Objectives Explain what an air mass is
State and explain the different types of fronts State the different pressure systems Decipher a synoptic chart Explain the seasonal weather of Australia Describe the life cycle of a Tropical Cyclone

1. Air masses Air masses If air remains over a surface for long enough, it takes on the characteristics of that surface Once 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 masses Air masses Air masses can be classified as either polar or tropical The air within the polar cell is called polar air, while all other air is tropical Air that remains over land for a period, gradually loses its moisture and is classified as continental Air that remains over sea gradually increases its moisture and is classified as maritime Therefore, air masses can be classified as: Tropical maritime Tropical continental Polar maritime Polar continental

2. Fronts Fronts When an air mass moves over a different region, it often encounters air of different temperatures Because the air has different densities, the warmer air tends to rise and the colder air tends to sink The boundary of these air masses is called a front The different types of fronts are: Cold fronts Warm fronts Occluded fronts Quasi stationary fronts

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

2. Fronts Cold Fronts As the cold air advances, it lifts the warm air above The exact cloud formed depends on the stability and humidity of the warm air and the speed at which the cold air is advancing Generally as the warm air is forced to rise, it promotes instability and cumuliform clouds are likely The passage of a cold front can be characterised by: The temperature falling The pressure rising The wind backing

2. Fronts Warm Fronts A warm front is characterised by cold air retreating from advancing warm air The 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. Fronts Warm 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. Fronts Occluded Fronts
Because cold air is heavier, it tends to surge forward much more rapidly than a warm air As days go by, the cold front moves faster over the ground than the warm front Eventually the cold front overtakes the warm front producing an occluded front An occluded front is shown on a synoptic chart as a solid line with a mixture of barbs and semi circles

2. Fronts Occluded Fronts
The cloud associated with an occluded front depends on what clouds were associated with the cold and warm fronts It 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. Fronts Quasi stationary Fronts
A quasi stationary front is a front that tends not to move These types of fronts tend to align themselves with the isobars and have generally benign weather The 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 systems Pressure 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 them On a synoptic chart, the pressure systems which we see are: High pressures or anticyclones Low pressures or depressions

3. Pressure systems Anticyclones
When isobars close around a region of high pressure, the feature is called a high pressure system or anticyclone In the southern hemisphere, the rotation of winds around a high pressure are anticlockwise In the surface layer, the result of friction causes the wind to blow slightly outwards and across the isobars The 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 systems Weather 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 compression This can cause a subsidence inversion and trap pollutants such as smoke or haze beneath it This can affect flight visibility

3. Pressure systems Ridges of high pressure
A ridge is an elongated area of high pressure A ridge is generally associated with fine weather conditions

3. Pressure systems Depressions
When isobars form around a region of low pressure it is called a low pressure system or depression In the southern hemisphere, the winds about a low tend to blow clockwise Due to surface friction, the winds tend to blow inwards and across the isobars

3. Pressure systems Depressions
Compared with a high pressure, a low pressure has a stronger pressure gradient, that is a greater change of pressure with distance Because 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 cooling If the air reaches dew point temperature, cloud will form Instability in the rising air can cause a large vertical development of cumuliform clouds accompanied by showers Because of the vertical motion of air, good visibility can result due to the vertical motion taking away all the particles suspended in the air This can also cause turbulence

3. Pressure systems Troughs of low pressure
An elongated area of low pressure is known as a trough Convergence will occur and the air will rise If the air is unstable, large cumuliform clouds can occur

3. Pressure systems Col 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 pressure A col usually has light and variable winds

4. Synoptic Charts Synoptic 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 Charts Synoptic 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 Charts Synoptic Charts - Example
Example, given the chart below, assuming an isobar spacing of 4hPa determine: The pressure systems marked: A, B, C The name of the features: BD, BE, CF The surface wind and pressure in Hobart F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The pressure systems marked: A, B, C 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 F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The pressure systems marked: A, B, C The wind is blowing in an anti-clockwise direction around A and C therefore they are Highs F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The name of the features: BD, BE, CF Looking at the symbols on the chart, BD is a cold front, BE is a warm front F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The name of the features: BD, BE, CF CF is an elongated section originating from a high, therefore it is a Ridge F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The surface wind and pressure in Hobart Wind: Gradient wind is a South Westerly, the surface wind will be Westerly due to veering from surface friction F C A D B E

4. Synoptic Charts Synoptic Charts - Example
Example, given the chart below, determine: The surface wind and pressure in Hobart 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 F C A D 1022 B 1018 E

5. Climatology Weather in Australia
Australia is a vast country, and with it carries a vast range of weather This 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

5. Climatology Sub-Tropical Meteorology
The weather in Southern Australia is classified as Sub-Tropical The weather in this region is mainly influenced by the sub-tropical ridge This causes us to have the four seasons: Summer Autumn Winter Spring

5. Climatology Sub-Tropical Meteorology - Summer
Summer comprises the months December to February In summer the highs which form the subtropical ridge move to the south Winds are generally south easterly In Western Australia the wind is generally very hot and dry

5. Climatology Sub-Tropical Meteorology - Autumn
Autumn comprises the months March to May Autumn generally brings long periods of fine weather and is often considered the best time to go flying Although passage of a cold front can bring cold conditions and the possibility of frontal thunderstorms Fog can be expected during the mornings as overnight temperatures can get low

5. Climatology Sub-Tropical Meteorology - Winter
Winter consists of the months June to August During 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 continent Winds are generally westerly Cloud is often widespread stratocumulus Passing fronts bring periods of rain

5. Climatology Sub-Tropical Meteorology - Spring
Spring consists of the months September to November Spring weather is often quite variable Conditions can be generally fine one minute then can be un-flyable the next This is due to fast moving cold fronts and troughs

5. Climatology Tropical Meteorology
The weather in the North of Australia is classified as Tropical There 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 Winds
The regions between the subtropical high pressure belts and the equatorial trough are occupied by the trade winds In the southern hemisphere, Coriolis deflection produces the south east tradewinds, and in the northern hemisphere, the northeast trade winds The 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 Winds
The Trade Winds are responsible for the “Dry Season” in Northern Australia The 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 monsoons
Due 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 year When large continental and ocean areas are involved, atmospheric circulations may develop on a large scale A monsoon is the persistence of maritime airflow over a hot continent Moist 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 Monsoons
The Monsoons are responsible for the “Wet Season” in Northern Australia The 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 hemisphere Generally, 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 Australia In 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 zone This 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 troughs The ITCZ can produce extreme weather conditions over a wide area Weather can include heavy rain, frequent thunderstorms and violent wind squalls

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

6. Tropical Cyclones Tropical Cyclones - lifecycle
Formative stage – A tropical cyclone usually develops in a pre existing low pressure tough Atmospheric wind shear is low and there is an abundance of moisture for vigorous convective cloud growth The system becomes more circular, the pressure in the centre decreases, still above 1000hPa, and wind increases

6. Tropical Cyclones Tropical Cyclones - lifecycle
Immature stage – The pressure falls below 1000hPa and winds increase to 64kts The convective cloud forms into spirals and the eye develops Pressure continues to fall, and winds continue to increase in strength

6. Tropical Cyclones Tropical Cyclones - lifecycle
Mature Stage – At this stage the cyclone is at its maximum size and strength and the centre pressure is steady, at around 950hPa The 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 - lifecycle
Decaying Stage – When the cyclone has its energy supply disrupted, by either reaching colder water or land, the decaying stage begins The 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 eye
The eye of the tropical cyclone is an indication of a well developed storm The conditions are benign with a constant pressure and light winds There is generally clear skies with no precipitation

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