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Essay on Temperate Cyclones

Essay # 1. Introduction to Temperate Cyclones:

Temperate cyclones, also called as extratropical cyclones or wave cyclones or simply depressions are atmospheric disturbances having low pressure in the centre and increasing pressure outward. They are in fact low pressure centres produced in the middle lati­tudes characterized by converging and rising air, cloudi­ness and precipitation. Because of their varying shapes such as near circular, elliptical or wedge (V) they are variously called as ‘low’, ‘depressions’ or ‘troughs’.

They are formed in the regions extending between 35°- 65° latitudes in both the hemispheres due to conver­gence of two contrasting air masses e.g., warm, moist and light tropical air masses (westerly winds) and cold, and dense polar air masses. The polar fronts created due to these two opposing air masses are responsible for the origin and development of temperate cyclones. After their formation temperate cyclones move in easterly direction under the influence of westerly winds and control the weather conditions in the middle lati­tudes.

Essay # 2. Origin of Temperate Cyclones:

No commonly acceptable theory of the origin of temperate cyclones could be propounded as yet. The first pioneer serious attempt was made by Fitzroy in the year 1863 in this precarious field. He postulated that extratropical or temperate cyclones originated because of the convergence of two opposing air masses of contrasting physical properties (i.e., temperature, pressure, density and humidity).

In 1911 Shaw and Lempfert pointed out that temperate cyclones origi­nated due to inflow of winds from all directions towards the centre. The theory of cyclongenesis as propounded by Shaw and Lempfert is known as dy­namic theory. At a later date convection current theory was postulated to explain the origin of temperate cyclones but this theory was severely criticised and discarded.

Thereafter came eddy theory according to which temperate cyclones originate due to the forma­tion of eddies caused by obstructions in the advancing airmasses. Two Norwegian meteorologists, V. Bjerknes and J. Bjerknes put forth ‘polar front theory’, also knows as wave theory or Bergen theory in the year 1918 to explain the origin of extratropical cyclones. Lately, baroclinic wave theory was formulated for cyclone development on the basis of recent data from middle and upper troposphere derived through satellites and radars.

Polar front theory also called as ‘frontal theory’ or ‘wave theory’ or Bergen theory as propounded by V. Bjerknes and J. Bjerknes in 1918, is primarily based on the processes of the formation of fronts. It may be pointed out that fronts are formed due to convergence of two air masses of different physical properties coming from opposite directions.

One air mass is polar in character and is cold, denser and north-easterly in direction while the other air mass is tropical or sub­tropical in origin and is warm, moist, lighter and south­westerly in direction (in the northern hemisphere). When these two contrasting air masses converge along a line in the middle latitudes (temperate regions), they move parallel to each other and thus a stationary front is formed. No cyclone can develop from such stationary front because there is no vertical move­ment in the air, rather winds are more or less stable.

On the other hand, when two opposing air masses collide against each other and try to attack the territory of one another, unstable waves are formed which help in the origin and development of temperate cyclones. In the beginning the surface separating two air masses (more technically called as surface of disconti­nuity) is almost straight but it becomes unstable and wave-like when the warm and cold air masses attempt to penetrate in the regions of one another. Such unsta­ble wavy front is called polar front.

When south-westerly warm and moist air mass enters the territory of cold polar air mass along the polar front, it being lighter rises upward, with the result a centre of low pressure is formed. Now winds from all directions rush up towards this centre of low pressure and thus a cyclone is formed.

It may be pointed out that the cyclone forming wave developed due to conver­gence of cold and warm air masses is divided into two parts e.g., the eastern part of the wave, where eastward advancing warm tropical or subtropical air mass as­cends over a wedge of cold air mass is called warm front while the western part, where cold polar air mass pushes warm air mass upward forcibly, is called cold front.

It is to be remembered that warm air mass is aggressive along the warm front where it overrides cold air mass whereas the cold air mass becomes aggressive along the cold front because it replaces warm air by pushing it upward. The south­western and north-western sectors of the cyclone are called warm sector and cold sector respectively.

The low pressure in the eastern part of the cyclone is intensified with the arrival of warm air. This draws the winds towards the centre from nearby areas, with the result cold front advances more rapidly than warm front. Consequently, cold and warm fronts come close to each other resulting into the destruction of warm front.

The cyclone dies due to disappearance of warm front. This process of cyclone destruction is called occlusion. Sometimes, weak and feable minor cyclone is formed after the occlusion of main cyclone. Such secondary cyclone is called sub-cyclone. Secondary cyclone is generally formed when some warm air still remains in the cold front after the occlusion of main front with the result a centre of low pressure is re-established and winds blow towards this centre from all sides forming a new weak cyclone.

It may be pointed out that though the formation and development of temperate cyclones is a quick process but it passes through a series of successive stages. The period of acyclone from its inception (cyclogenesis) to its termination (frontolysis or occlusion) is called the ‘life cycle of cyclone’ which is completed through six successive stages.

(1) First stage involves the convergence of two air-masses of contrasting physical properties and direc­tions. Initially, the air masses (warm and cold) move parallel to each other and a stationary front is formed. This is called initial stage.

(2) Second stage is also called as ‘incipient stage’, during which the warm and cold air masses penetrate into the territories of each other and thus a wave-like front is formed.

(3) Third stage is the mature stage when the cyclone is fully developed and isobars become almost circular.

(4) Fourth stage-warm sector is narrowed in extent due to the advancement of cold front than warm front, as cold front comes nearer to warm front.

(5) Fifth stage starts with the occlusion of cy­clone when the advancing cold front finally overtakes the warm front and an occluded front is formed.

(6) Sixth stage—warm sector completely disap­pears, occluded front is eliminated and ultimately cyclone dies out.

Essay # 3. Types of Temperate Cyclones:

Though temperate cyclones are mainly origi­nated due to convergence of two contrasting air masses in terms of temperature, pressure, and humidity but some local cyclones also form due to other reasons related to temperature variations and consequent pressure differences.

Based on above considerations tem­perate cyclones are divided into 3 categories viz.:

(i) Dynamic cyclones,

(ii) Thermal cyclones, and

(iii) Sec­ondary cyclones.

(1) Dynamic cyclones:

Dynamic cyclones are, in fact, real temperate cyclones because they are formed due to convergence of cold polar air masses and warm and moist maritime tropical air masses. These cyclones affect the weather conditions of very large areas in middle latitudes.

Different fronts (e.g., warm front and cold front) and sectors (e.g. warm and cold sectors) are fully devel­oped in dynamic cyclones. They are called dynamic because they are dynamically produced e.g. due to convergence and invasion of two contrasting air masses into the territories of one another.

(2) Thermal Cyclones:

According to Brunt ther­mal cyclones are formed due to development of low pressure centres on the continents in summers in tem­perate regions and as such winds blow from all direc­tions towards the low pressure centres. Such thermally induced temperate cyclones are stationary at their places of origin and different fronts are not developed. Such thermally induced cyclones in the middle lati­tudes have been named by Humphreys insolation cy­clones.

According to Humphreys thermal cyclones are produced due to development of low pressure centres over warm water surfaces of seas surrounded by cold land surfaces during winter season. It may be pointed out that both types of cyclones as referred to above are, in fact, thermal cyclones because they are directly related to insolation.

The only difference is that they develop over land surfaces in summers (e.g., over Iberian Peninsula, Alaska, S. W. USA, and N. W. Australia) and over sea surfaces in winters (e.g., over Okhotsk Sea, Norwegian Sea, to the south of Iceland and Greenland etc.).

(3) Secondary Cyclones:

Secondary cyclones are those which develop due to passage of cold winds over warm sea after the occlusion of main cyclone. They are short-lived and very weak.

Essay # 4. Shape, Size and Velocity of Temperate Cyclone:

Temperate cyclones are of different shapes e.g., circular, semi-circular, elliptical, elongated or ‘V’ shaped, but all of them are characterized by low pres­sure in their centres and closed isobars. The pressure difference between the centre and periphery is about 10 to 20mb but sometimes it increases to 35mb. It means that pressure increases from the centre towards outer margin. Temperate cyclones also greatly vary in size and extent.

Average large diameter of an ideal cyclone is about 1900km (1200 miles) while short diameter measures 1000km (640 miles). It may be pointed out that no two cyclones are identical in terms of their size as their diameters range from 150km to more than 3000km.

Sometimes, temperate cyclones are so large and extensive that they cover an area of 1,000,000 square kilometres. The vertical extent of an average cyclone is about 10-12 km. The temperate cyclones move eastward under the influence of west­erly winds with average velocity of 32km per hour in summers and 48km per hour in winters.

Essay # 5. Wind System in Temperate Cyclone:

Since there is low pressure in the centre of temperate cyclone and air pressure increases outward and hence winds blow from the periphery towards the centre but these winds do not reach the centre straight rather they cut the isobars at the angle of 20° to 40° due to friction and coriolis force and thus wind direction becomes anticlockwise in. the northern hemisphere and clockwise in the southern hemisphere.

The centre-bound inward air circulation becomes of convergent pattern but the winds do not aggregate at the centre but they ascend upward and expand outward so that low pressure centre is always maintained so long as the cyclone is alive. Since temperate cyclones are formed due to convergence of two contrasting air masses (i.e. cold, dry and dense air mass and warm, moist and light air mass) and hence it is natural that there are variations in the nature and direction of winds in different parts of the cyclones.

The tropical and subtropical warm and moist air is of generally westerly direction while polar cold air is generally easterly. The convergence of these air masses forms warm front, warm sector, cold front, and cold sector. Before the arrival of warm front the wind direction is easterly but it changes to southerly and southwesterly at the time of the arrival of warm front.

The warm front and warm sector are, thus, characterized by warm southerly and southwesterly winds while the direction of winds changes to west­erly, northwesterly and northerly at the arrival of cold front and cold sector. The cold front and cold sector are characterized by cold winds. It is apparent that there is sudden change in wind direction along the warm and cold fronts. The line along which wind changes its direction is called wind shift line.

Essay # 6. Temperature in Temperate Cyclone:

Different temperatures are noted in different parts of temperate cyclones because of their origin due to convergence of two thermally contrasting air-masses. The southern part of cyclone records higher tempera­ture because of the dominance of warm air while the north-eastern, northern and north-western parts record low temperature because of the dominance of cold polar air mass.

The western part records lowest tem­perature. The temperature within the cyclones depends on the properties of air masses, general weather condi­tions and moisture content in the air. Isotherms gener­ally tend in north-northeast to southwest direction in the northern hemisphere.

Essay # 7. Source Regions and Tracks of Movement by Temperate Cyclone

The areas frequented by temperate cyclones mostly lie in the middle and high latitudes extending between 35°-65° latitudes in both the hemispheres. These cyclones move, on an average, in easterly direc­tion but since their tracks are highly variable and hence cyclonic tracks are always considered in zonal pattern rather than in linear pattern. The paths followed by these cyclones are called ‘storm tracks’.

The following are the most favourable breeding areas of temperate cyclones:

(1) Cyclones after originating in the north Pacific off the north-east and eastern coasts of Asia move in easterly and northeasterly direction towards the Gulf of Alaska and ultimately merge with Aleutian Lows from where they follow southerly direction and reach as far south as southern California. The cyclones moving inland dissipate and are occluded at the windward western slopes of the Rocky Mountains.

(2) There are four principal areas of frontogenesis in North America e.g., (a) area east of Sierra Nevada Range, (b) eastern Colorado, where temperate cyclones are called Colorado Lows, (c) area east of Canadian Rocky moun­tains, where cyclones are known as Alberta Lows, and (d) Great Lakes region,

(3) The cyclones originating in the Gulf of Mexico follow northerly trajectory to the east of the Appalachians and following the course of the Gulf Stream merge with the Icelandic Zone of frontogenesis.

(4) North-west North Atlantic off the north-east coast of North America- the cyclones origi­nating in this area move in easterly direction and enter the northwestern parts of Europe,

(5) Cyclones origi­nating in the area between Iceland and Barents Sea follow easterly trajectory and affect the weather condi­tions of north Europe.

(6) There are two main zones of frontogenesis in continental Europe e.g., (a) Baltic Sea and (b) Mediterranean Sea. Some of the cyclones originating over the Mediterranean Sea after following easterly direction reach Pakistan and north India in winter season where most of the winter precipitation is received through these storms. Majority of the cy­clones of Mediterranean origin move north-eastward and reach Commonwealth of Independent States (CIS, some Republics of former USSR).

Essay # 8. Weather Conditions Associated with Temperate Cyclones:

Different parts of temperate cyclones are asso­ciated with varying weather conditions because of different types of air masses and varying temperature conditions. The observation point of a moving temper­ate cyclone experiences different weather conditions at the time of arrival and passage of warm front, warm sector, cold front and cold sector.

(i) Arrival of Cyclone:

When the cyclone coming from the western direction draws nearer to the obser­vation point, wind velocity slows down considerably, air pressure decreases and the sun and the moon are encircled by halo which is infact the reflection of thin veneers of cirrus and cirrostratus clouds in the west. Temperature suddenly increases when the cyclone comes very close to the observation point, wind direc­tion changes from easterly to south-easterly, the cloud cover thickens and the sky becomes overcast with dark, thick and low clouds.

(ii) Warm Frontal Precipitation:

Clouds become very thick and dark with the arrival of warm front of the cyclone and heavy showers begin with nimbostratus clouds. Since the warm air rises slowly along the front, and hence the precipitation is slow, gradual but of long duration. The warm frontal precipitation largely de­pends on the amount of moisture and instability of the rising warm air. If the air is full of moisture and is unstable, there is sufficient precipitation, the sky is overcast and the sun is not visible for several hours.

(iii) Warm Sector:

The warm sector comes over the observation point after the passage of warm front and there is sudden change in the pre-existing weather conditions. The wind direction becomes southerly. The sky becomes cloudless and clear. There is sudden rise in air temperature and increase in the specific humidity of the air but air pressure decreases remark­ably. Though weather becomes clear but there may be some occasional drizzles. In all, the weather is clear and pleasant.

(iv) Cold Front:

Temperature registers marked decrease with the arrival of cold front. Cold increases considerably. The cold air pushes the warm air upward and there is change in wind direction from southerly to south-westerly and westerly. Sky is again covered with clouds which soon start precipitation.

(v) Cold Frontal Precipitation:

Sky becomes over­cast with cumulonimbus clouds which yield heavy showers. Since the warm air is forcibly lifted upward hurriedly, the cold frontal precipitation is in the form of heavy downpour with cloud thunder and lightning but the precipitation is of short duration and less widespread because the cold sector is very close.

(vi) Cold Sector:

Weather again changes remark­ably with the passage of cold front and arrival of cold sector. Sky becomes cloudless and hence clear. There is sharp fall in air temperature and considerable rise in air pressure but decrease in specific humidity. Wind direction changes from 45° to 180° and thus it becomes true westerly. After the occlusion of cyclone the weather conditions of pre-cyclone period again set in.