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In this essay we will discuss about:- 1. Definition of Thunderstorms 2. Structure of Thunderstorms 3. Conditions 4. Thunderstorms and Weather 5. Classification.
Essay # Definition of Thunderstorms:
Thunderstorms are local storms characterized by swift upward movement of air and heavy rainfall with cloud thunder and lightning. According to A.N. Strahler “a thunderstorm is an intense local storm associated with large, dense cumulonimbus clouds in which there are very strong updraft of air.”
Because of heavy downpour associated with thunderstorms they are also called ‘cloud bursts’ but the rainfall is of very short duration. Thunderstorms differ from cyclones in that the latter are almost circular in shape wherein winds blow from outside towards the centre while the former is characterized by strong updraft of air. They are considered to be special case of convective mechanism.
Essay # Structure of Thunderstorms:
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A thunderstorm consists of several convective cells which are characterized by strong updraft of air. Each cell passes through a phase of life cycle consisting of youth, mature and old stages. It may be pointed out that different cells in a single thunderstorm may be in varying stages. The first stage (youth) is called cumulus stage when warm air rises strongly upward and helps in the formation of clouds.
The second stage or mature stage is characterized by both upward and downward movement of winds and occurrence of rainfall. The third stage or senile stage or dissipating stage is characterized by downward movement of winds which spread over the ground surface and stop vertical movement of winds. Clouds spread in the sky in umbrella shape and become altostratus and cirrostratus resulting into dissipation of thunderstorm.
Essay # Conditions for Thunderstorm Development:
Atmospheric instability, updraft of potentially unstable air, abundant supply of warm and moist air, thick clouds etc. are the factors which favour the development of thunderstorms. The upward movement of warm and moist air is prerequisite condition for the origin of thunderstorms. Surface heating through intense insolation causes convective mechanism resulting into updraft of air and atmospheric instability.
This is why thunderstorms originate mainly during summer season, warm days of a season, and warm hours of a day. It appears that warm, moist and rising unstable air is the most important factor in the development of thunderstorms.
This becomes possible when normal lapse rate of temperature is greater than adiabatic rate of temperature change. Besides convective mechanism, warm and moist winds also rise and become unstable due to orographic obstacles. The greater the instability of warm and moist air, the greater the intensity and duration of thunderstorms.
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There must be greater thickness of clouds between cloud base (i.e., level at which condensation and cloud formation begin) and icing level (i.e., the level at which water droplets change into ice particles). The higher the icing level above cloud base, the greater the thickness of clouds and thus the greater the intensity of convection.
Since the icing level is at very low height in the middle latitudes, thunderstorms do not develop there. On the other hand, thunderstorms are common features in the weather of low latitudes because of the higher height of icing level and greater thickness of cloud cover.
Essay # Thunderstorms and Weather:
(i) Rain Fall:
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Rain fall in thunderstorms, unlike tropical cyclones, is in the form of heavy downpour with greatest intensity of all other forms of precipitation but is of short duration because of two factors viz.:
(i) the air rises abruptly with great force due to which there is quick condensation and cloud formation, and
(ii) there is abundant absolute humidity due to high rate of evaporation consequent upon very high temperature during summer season.
The rainfall of thunderstorm is closely related to its numerous cells. There is maximum rainfall in the centre and minimum at the periphery of each convective cell. Fully developed cell yields rainfall for about an hour whereas weak cell dies out within few minutes.
(ii) Hailstorms:
When condensation occurs below freezing point, ice particles are formed which range from the size of a pea to large ball. Hail is not associated with every thunderstorm. Not only this, hail is confined to only certain cells of a thunderstorm. Hails fall down on the ground surface when the rising convection currents become weak and feeble. The sudden fail of hails inflicts great damage to human health and wealth, birds and animals and standing agricultural crops.
(iii) Lightning:
Electrical discharge centres are developed in a mature thunderstorm. The centres of positive and negative electrical charges develop in the upper and lower portions of the clouds respectively with discharge values ranging between 20 to 30 coulombs. Lightning is produced when the electrical potential gradient between the electrical positive and negative charges becomes very steep.
According to another view lightning is produced due to splitting of large water drops. Each water drop has positive and negative electrical charges which remain in neutral state when they are evenly balanced. This balance is disturbed due to splitting of these drops resulting into difference in positive and negative charges.
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(iv) Thundering:
Sound is produced due to sudden and rapid expansion of air columns caused by intense heat (10,000°C) resulting from lightning strokes. This deafening noise produced by vibrating pressure wave due to rapid expansion of air column as mentioned above is called cloud thunder.
(v) Squall:
The downward movement and divergence of cold air at the ground surface is called squall. The velocity of squalls is equal to and some times greater than hurricane velocity and hence they inflict great damage to human structures and vegetation. Squall is produced after the thunderstorm becomes mature and heavy precipitation occurs.
Essay # Classification of Thunderstorms:
Thunderstorms are generally classified on the basis of their mode of origin and lifting factors and mechanisms.
(1) Air mass thunderstorms:
(i) Heat thunderstorms.
(ii) Orographic thunderstorms.
(iii) Advectional thunderstorms.
(2) Frontal thunderstorms:
(i) Warm front thunderstorms.
(ii) Cold front thunderstorms.
(1) Air mass thunderstorms:
(i) Thermal or Local Thunderstorms:
Thermal or local thunderstorms originate due to intense heating of ground surface through insolation and resultant rising thermal convection currents. They are called local because they influence very limited area. In fact, heat thunderstorms are real thunderstorms which originate in the afternoon during summer season and die out by the evening. Heat thunderstorms are more common in the belt of doldrum because large amount of air moisture, high temperature and convergence of winds provide ideal conditions for their origin and development. Heat thunderstorms become stronger and more vigorous if the surface through which they travel is warmer, otherwise they soon are weakened if the surface is less warm.
This is why they die out when they pass through water bodies (lakes, rivers, reservoirs etc.) because of no supply of heat from below. Heat thunderstorms also originate in the inner parts of the continents during summer season in middle latitudes. Heat thunderstorms are difficult to be predicted because of their highly uncertain and variable behaviour.
(ii) Orographic Thunderstorms:
When warm, moist and unstable air strikes a mountain barrier, it is forced to rise hurriedly along the hillslope. The latent heat of condensation (release of heat after condensation) accelerates the rate of upward movement of the air. This results in the formation of most active and strong thunderstorm which yields copious heavy rains.
This is called cloud burst rain. The southwest Indian monsoon winds after striking the hillslopes produce strong thunderstorms which yield more than 12,000mm of annual rainfall at Cherrapunji. Orographic thunderstorms are more extensive, widespread, and active than heat thunderstorms and thus their forecast is easy.
(iii) Advectional Thunderstorms:
Advectional thunderstorms are produced due to substantial increase in normal lapse rate of temperature and consequent upward movement of unstable air when a cold air underlies a warm air. Such thunderstorms develop during dark nights when the sky is overcast because the upper layers of the clouds are cooled due to loss of heat through radiation, with the result cool and dense air settles downward and pushes underlying warm and light air upward resulting into convective mechanism in the air.
(2) Frontal thunderstorms:
(i) Warm Frontal Thunderstorms:
Warm frontal thunderstorms are produced when sea breezes are more humid and unstable. They are not significant because they are very weak storms. Cumulus clouds do not form because the air ascends slowly.
(ii) Cold Front Thunderstorms:
Cold front thunderstorms develop along the cold front of temperate cyclones when cold and dense air pushes warm and moist air upward with great force. Since they are associated with temperate cyclones, they are easily predicted. Cold front thunderstorms may develop at any time of a day or in any season of a year because their origin is not related to the heating of the ground surface.
Tornado:
Tornadoes are funnel shaped storms which are smallest but most violent and disastrous of all the storms. In fact, they are violently rotating column of air having upper portion of funnel shape of cumulonimbus cloud which is attached to the ground by very narrow column of air.
The diameter of the funnel increases from 90m in the lower portion to 460m in the upper portion. It is of very dark colour because of dominance of dust, sands, debris and condensed moisture. The centre of tornado is characterized by extremely low pressure; say 100mb less than the outside environment. Because of such steep pressure gradient winds rush up with great force towards the centre having furious velocity of 600 to 800km per hour.
Thus, the swiftly inward moving air is caught into a vortex of the storm and is rapidly lifted upward and cools adiabatically and forms thunderstorm. This is why tornadoes are always associated with violent thunderstorms. Tornadoes may develop singly or in groups. When tornadoes move in group (called as tornado family), they cause irreparable heavy loss to human health and wealth.
Though exact mechanism of the origin and development of tornadoes is not fully understood but most of the meteorologists believe that they are formed due to violent convection of conditionally and convectively unstable column of ascending air. The origin of tornadoes has also been related to fronts. The up-thrusting of warm and moist tropical and subtropical air mass by cold polar air mass along the cold front presents ideal condition for tornado development.
Sometimes, intense local heating of the ground surface causes strong convection which induces ideal condition for the development of tornadoes. According to Californian scientist Vernon J. Rossaw tornadoes develop because of attraction of two cloud masses. Though tornadoes may develop at any time but they are more common during spring and summer season.
Tornadoes are more common in the southern and eastern USA. The approach of tornadoes is heralded by dark and thick clouds in the sky resulting into complete darkness and minimum visibility and low air pressure. The wind blows with hyper velocity which causes cracks in the buildings. The corks of bottles suddenly open up automatically due to sudden change (lowering) of air pressure.
Tornadoes, though smallest in area of all the hazardous atmospheric storms, are very deadly to human lives and property. On an average, the annual toll caused by tornadoes in the USA includes damage to property worth 100,000,000 US dollars and 150 human deaths. The deadliest parts of tornadoes are the tornado missiles (consisting of uprooted trees, their branches, roofs of buildings etc. which are carried away by the dynamic force of winds) which inflict great damage to buildings, other human structures, and human lives.
A tornado, for example, at Lubbock (Texas, USA) in 1970 moved a long cylindrical fertilizer tank (3, 35 m x 12.5 m in size with average weight of 11 tons) for a distance of 1.21 km from its original place. It may be pointed out that a tornado becomes disaster only when its funnel of dark clouds moves by touching the ground through narrow column of swiftly moving winds.
The tornado outbreak (occurrence of tornadoes in groups involving large numbers) of 60 tornadoes on February 19, 1884 struck the states of Virginia, North Carolina, South Carolina, Georgia, Alabama, Mississippi, Tennessee, and Kentucky between 10 A.M. and 12 midnight wherein the most terrific devastation was caused by violent wind storms ever experienced in the USA before this date.
Total damages caused by these tornadoes included loss of property worth 3 to 4 million US dollars, death of 800 persons, injuries to 2,500 people, destruction of 10,000 buildings, homeless and destitute people numbering 10,000 to 15,000.
