There are 5 synoptic features, which can be identified on weather charts.
· An Anticyclone or High
· A Ridge of High Pressure
· A Col
· A Depression or Low
· A Trough of Low Pressure
Depressions in tropical latitudes are also called by a range of local names such as hurricane, cyclone and typhoon. Traditionally seafarers use the term “tropical revolving storms”.
An anticyclone or high is characterised by its centre of high pressure around which are roughly concentric isobars and by its clockwise (N.H.) circulation of winds. Anticyclones are three-dimensional features and their structures vary with height.
Apart from their pressure and wind patterns, highs and lows show other important differences. In general, highs cover much larger areas, move slower and are more persistent than lows. Their pressure gradients are less steep, especially near their centers, so the winds are characteristically light and variable near a centre. A new high forms almost always either as an extension of an existing high or as a centre near the original, eventually replacing it.
Summer: Fine, quiet weather, sunshine and high temperatures. At night, clear skies and ground mist or fog tend to form over the land. In coastal waters, there is probably frequent persistent fog.
Winter: a) By day, fine clear and very cold. By night, frost and fog over the
Land.
b) Sky is persistently covered by low cloud sheet (Sc). This is the “Anticyclonic Gloom” typical of November and December, and the clouds are Sc due to the inversion brought about by the subsiding air heating adiabatically.
Anti-Cyclone Centered Over UK
An elongated extension of high pressure anticyclone system is called a RIDGE.
A system of isobars in which pressure is higher on the inside that the outside therefore has anticyclonic properties. Ridges are often located in the outer regions of anticyclones.
Fast moving ridges occur between individual members of a depression family, moving along the polar front. They bring intervals of fair weather between the periods of rain or showers associated with fronts of a depression.
Weather conditions in ridge resemble those in anticyclones. The ridge brings fine weather on the east side, but for a short period only. Frequently the pre-cursor of a depression, so weather deteriorates on the west side.
Ridge of High Pressure Over UK
The Col is an area of intermediate pressure and slack isobars between two highs and two lows diagonally disposed.
The weather in a col is basically any type which is compatible with calms, airs or a light breeze. This can include fog, mist, haze, dew, hoar frost, and with a northerly origin Pm air mass, showery Cu cloud extending to Cb with thunderstorms and hail.
Winds are light and variable and it is the meeting place of two sets of wind from different directions, having different temperatures and humilities.
Winter: Mixing fogs and mists.
Summer: Clouds and thunderstorms
4. Depression
A system of closed isobars with lower pressure on the inside is a depression or low. There are many types of depressions, but the most important depressions are those which develop on the Polar Front called Frontal Depressions. The Polar Front is the boundary between warm tropical air and cold polar air. The Polar Front bends and forms a warm front and a cold front.
Intense Depression
Formation of a Frontal Depression
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Master29NS/H.EI/CB.11/01/07
Polar Front
Frontal wave forms if Fronts converge
If the wave becomes unstable it will grow in amplitude and develop a strong circulation with low pressure at the apex. Central pressure falls as circulation develops.
FRONTS
General properties of fronts
A front is the boundary between two air masses. These air masses have different temperatures and therefore have different densities. They may be thought of a 'discontinuities'. If the earth did not rotate the air masses would quickly move so that the warm air lay above the cold air. However on the rotating earth, Coriolis force produces a front, which is in equilibrium when angled to the surface.
In middle latitudes the slope of fronts may be between 1:50 to 1:200.
Temperature height diagram for a front
Isobars at a front
Isobars trough (are 'kinked') towards high pressure. This means that the wind direction changes at a front. It veers in the Northern Hemisphere.
Types of fronts
1. Stationary front
No horizontal movement of the warm and cold air.
2. Warm front: A warm front is a front along which warm air replaces cold air.
Symbol
An Observer ahead of the front would normally observe the following sequence as the warm front approaches.
· Ci, Cs, As, Ns – all stratiform due to the gentle lifting action.
Most of the precipitation will be steady in nature:-
· Drizzle, rain, sleet, snow or freezing rain depending on ambient temperatures.
Where the precipitation is drizzle/rain there is usually evaporation as it falls though the cooler air and this may cause saturation and the formation of stratus clouds and often fog.
As the front approaches the pressure gradient usually increases causing increase in wind speed. There is a characteristic kink in the isobars at the front (pointing towards high pressure). This causes the wind to veer at the passage of the front (northern hemisphere)
Pressure. As the front approaches there is more and more warm less dense air aloft and so the pressure falls.
2. Warm Sector
Between the fronts of the depression we are in the warm moist Tm air. The area is recognizable on the weather charts by nearly straight often parallel isobars.
At sea we usually have;
· Cloudy conditions with low Stratus or Stratocumulus clouds with occasional drizzle, visibility is moderate or poor and fog may occur.
The nearer we are to the centre of the low pressure the thicker the cloud becomes and the heavier the precipitation.
Cold front: A front along which cold air replaces warm air.
A wedge of cold air is pushing its way under the warm air ahead of it thus forcing it to rise, sometimes rapidly. The steepness of the cold front and therefore the type and severity of the weather depends on the temperature gradient across the front.
With weak fronts the weather may well be like a warm front in reverse, very active fronts result in line squalls with extremely violent gusty winds and thunderstorms.
The “Typical” cold front is somewhere between the two.
At the passage of the front there will be a well marked veer in the wind. Temperature will suddenly fall. Heavy rain, sometimes hail and perhaps thunder will occur from cumulus and cumulus nimbus cloud.
At the rear of the front the heavy rain usually clears quickly and is followed with isolated showers becoming less frequent. The clouds will be A, Ac and then Cu and possibly Cb with blue sky’s increasing. Visibility will rapidly improve.
3. Occluded Front
The cold front moves faster than the warm front, gradually overtaking it.
The resulting boundary is called an OCCLUDED FRONT and gradually extends down from the centre of the depression as the cold front catches the warm front.
An occlusion is classified as WARM or COLD depending on whether the air behind the cold front is warmer or colder than the air ahead of the warm front.
On the weather charts;
If the warm front is an extension of the occluded front is a warm occlusion.
If the cold front is an extension of the occluded front it is a cold occlusion.
The source of energy for the development of a frontal depression comes mainly from the supply of air in the warm sector.
Depressions with a wide warm sector will continue to deepen and grow even during the early stages of occlusion. As the depression occludes, and the supply of warm air is reduced, the development stops.
Movement slows at this stage and, over a period of a few days, the depression FILLS. The direction and speed of movement is dependant on the general flow of air over a wide area.
Warm Occlusion
Where the Polar air behind the depression is warmer, and therefore less dense, than that ahead of it. It is like an extension of the warm front with the original cold front rising up the slope of the original warm front, pushing the warm sector ahead of it.
The weather is a combination of both fronts with the main bands of cloud and precipitation ahead of the surface occluded front.
Cold Occlusion
Where the Polar air behind the depression is colder, and therefore more dense, than that ahead of it.
It is like an extension of the cold front with the original warm front rising up the slope of the front, lifting the warm sector.
The weather is a combination of both fronts with the main bands of cloud and precipitation behind the surface occluded front. They tend to be less extensive, cumuliform and therefore showery in nature.
dariusz.lipinski