23 Thunderstorms and Tornadoes

Dr. Jitender Saroha

Contents

Introduction

Learning Objectives

Thunderstorms – Definition and Classification

Life Cycle of Thunderstorms

Cumulus or Youth Stage

Mature Stage

Dissipating Stage

Severe thunderstorms and Supercell Thunderstorms

Weather Conditionsduring Thunderstorms

Distribution of Thunderstorms

Tornadoes

Formation of Tornadoes

Distribution of Tornadoes

Waterspouts

Summary and Conclusions

MCQ

Answers to MCQs

References

WebLinks

Introduction

All of you must have seen lightning from dense, dark and extensive clouds followed by thunder and rain. These clouds are cumulonimbus clouds and this phenomenon of lightning, thunder and rain is called thunderstorm. Thunderstorms are associated with heavy rain, thunder and lightning and occasional hail. Thunderstorm is caused by convectional activity associated with intense atmospheric instability. About 44,000 thunderstorms occur every day in the world, and predominantly in tropical regions. Tornadoes or twisters are without exception associated with thunderstorms. Tornadoes are most probable in the portion of thunderstorm adjacent to large hail activity. In this module, focus is on thunderstorms and tornadoes – their genesis and development, distribution and associated weather conditions.

Learning Objectives

After studying this module, you will be able to:

define thunderstorm and tornadoes,
explain the classification of thunderstorms,
describe the origin and characteristics of thunderstorms and tornadoes ,
explain weather conditions associated with thunderstorms and tornadoes, and describe the distribution of thunderstorms and tornadoes.

Thunderstorms: Definition and Classification

Thunderstorm represents an intense local storm associated with extensive cumulonimbus clouds, heavy precipitation along with thunder and lightning. Thunderstorms are different from tropical cyclones. Tropical cyclones have circulatory motion towards low pressure centre, but in case of thunderstorms strong convectional mechanismispredominant. This mechanism is basically a function of intense atmospheric instability which is directly related to large supply of warm and moist air and lifting mechanisms. A thunderstorm may be the result of a single cumulonimbus cloud and its influence may be local. It may be associated with a cluster of cumulonimbus clouds and prevail over a large area.

The thunderstorms are classified on the basis of areas of instability and associated lifting mechanisms in the following manner. At macro level thunderstorms are classified into two types

– (i) Air-mass thunderstorms and (ii) Frontal thunderstorms. The air-mass thunderstorms are further classified into – (a) local heat thunderstorms; (b) orographic thunderstorms and (c) advective thunderstorms. The air-mass thunderstorms, most frequently, occur in maritime tropical (mT) air-masses.Whenthese warm and moist air masses are heated from belowand lifted or lifted due to orographic barrier or go up along a front, thunderstorms occur.The frontal thunderstorms are classified into (a) warm front thunderstorms; (b) cold front thunderstorms and (c) pre and post frontal thunderstorms (Figure 1).

Figure 1: Classification of Thunderstorms

All these lifting mechanisms have been explained in detail in modules (18 and 19). Local heat thunderstorms are result of conditional instability produced by intense surface heating and convectionalaloft of warm and moist air is reflected as thunderstorms. The lifting due to orographic barrier also results into thunderstorm formation and conditional or convective instability is a pre-requisite. Advective thunderstorms are result of enhanced instability due to advection of warm and moist air at lower level and cold air at high levels. Frontal thunderstorms are result of convergence and lifting along different types of fronts produced by the convergence of two contrasting air masses.The cold front thunderstorms develop along the advancing margin of cold air mass where it forces the warm and moist air of warmer air mass to rise in the frontal zone. This trigger effect may result into atmospheric instability and moist air may become convectively unstable. Cold front thunderstorms are generally very intense and can develop at any time in a day unlike daytime condition of heating based convection. But generally, they are also most frequent in summer and during day time. Likewise the wedging of warm and moist air over cold air on warm front results into formation of thunderstorms. Usually, thunderstorms along warm front are less severe as compared with cold front.

Life Cycle of Thunderstorms

Generally, a thunderstorm has about 3 to 8 convectional cells and each cell passes through its life cycle which consists of three stages – (a) cumulus or youth stage, (b) mature stage and finally (c) dissipation or senile stage.

(a) Cumulus or Youth Stage:In the initial stage i.e. cumulus stage updrafts throughout the cell dominates and becomes stronger as the cumulus clouds develop and grow in size. The cells generally attain the height of 8 km or more. Above condensation level water droplets dominate and above freezing level ice crystals and supercooled droplets dominate (Figure 3).This initial cumulus stage is significant because it moves water vapour from surface level to aloft. This stage is dominated by updrafts. Release of latent heat of condensation enhances ascend of successive warm and moist air parcels to greater heights and therefore, the vertical growth of clouds occurs.

(b) Mature Stage: This stage is characterised by strong updrafts as well as downdrafts. The Bergeron ice crystal process and collision-coalesce process (as discussed in module 19) initiate heavy precipitation. The falling ice-crystals, supercooled droplets, snowflakes and raindrops through frictional drag create downdrafts alongside updraft. The unsaturated air from outside the clouds is drawn into the downdrafts; it causes adiabatic evaporation of some rain droplets. This causes cooling of the descending air and it becomes denser and heavier. Thus downdraft is further enhanced. As this downdraft approachesthe base, it spreads out horizontally as the strong, cool and gusty winds. Therefore, in this stage in upper part of the cumulonimbus clouds updraft dominates and in lower part downdrafts. The severity of thunderstorm is at its peak and is reflected in weather conditions such as heavy rainfall, thunder and lightning at peak level. The top of cumulonimbus clouds acquires anvil head shape.Generally, cirrus clouds made up of ice crystals dominate in top spreading part.

(c) Dissipating Stage: The disappearance of ascending currents indicates onset of dissipation This stage is dominated by downdrafts and light rainfall. The descending air is warmed adiabatically and cloud formation and precipitation stop after some time. The downdrafts produce cooling gusty winds and ultimately the convectional cell collapses.The cooling effect of falling precipitation and the intrusion of cooler air in upper part cease the thunderstorm activity.

Figure 3: Stages of Thunderstorm

Severe Thunderstorms and Supercell Thunderstorms

A thunderstorm is categorised as severe when it has winds above 93 kmph or it produces hailstones or it generates a tornado. These thunderstorms are associated with heavy precipitation, flash flooding, strong gusty winds, large hail, frequent lightning and thunder and probably tornadoes. Supercell thunderstorms are associated with extremely dangerous weather conditions. They represent a small fraction of thunderstorms but predominate in loss of life and property. Less than fifty per cent of all supercell thunderstorms generate tornadoes, yet almost all extreme type tornadoes are spawned by supercells. In a supercell thunderstorm, the vertical extent of clouds may go upto heights of 20 km and diameter of clouds in the range of 20 and 50 km. The unique feature of this type of thunderstorm is that the updraft has rotational component. In the vertical wind column cyclonically rotating part is known as mesocyclone (Figure 4).

Figure 4: Supercell Thunderstorm

Weather Conditions during Thunderstorms

A storm is identified as thunderstorm only when thunder is heard. As thunder is a consequence of lightning, it means lightning and thunder are the chief characteristic features of thunderstorms.Thunderstorms represent an extreme condition of atmosphericinstability. They are caused by intense convection of warm and moist air. Thunderstorms are associated with cumulonimbus clouds.They are associated with short duration but heavy showers, in case of the cycles of updraft and downdrafts in clouds occasionalhailstorms also occur.Precipitation is torrential in nature and is accompanied by thunder and lightning.In the mature stage of thunderstorm the clustering of positive charges in upper part and negative charges in lower part of cumulonimbus clouds takes place. Development of a secondary cluster of positive charges occasionally occurs near the base of clouds. In case, the electrical gradient between these positive and negative charges clusters becomes intense, discharge (lightning) may occur. Thus, thunderstorms produce lightening due to development of steep electrical gradient between the positively charged and negatively charged electric fields. The lightning discharge may occur within the cloud or between two clouds or between cloud and the earth. About 80 per cent of lightning occurs as sheet lightning i.e. between oppositely charged parts within cloud or between cloud zones. The second type is cloud-to-ground lightning and as the name indicates it is result of electric discharge between cloud and ground. Though it occurs only about 20 per cent times but is most threatening and damaging.Lightning is accompanied by a loud sound as thunder. The electric discharge of lightning within seconds increases the temperature upto about 33,0000C. Air is heated suddenly and expands explosively producing sound waves which are heard as thunder. Therefore, thunder is result of the vibrations produced along the channel of a lightning discharge by rapidly expanding gases (Figure 5).

Figure 5:Lightning in Thunderstorm

Distribution of Thunderstorms

There are spatio-temporal variations in the distribution of thunderstorms. About 44,000 thunderstorms occur on daily basis over the earth surface. The broad generalisation about the distribution of thunderstorms is this that their frequency decreases from equator to pole. The maximum frequency is in the doldrums region. The cumulative effect of heat, moisture, convergence, convection and presence of numerous weak disturbances create ideal conditions for their genesis and highest frequency.

The warmer landmasses in southern hemisphere doldrums have the highest number of thunderstorm-days in a year in the world. In the equatorial low pressure belt on an average 100 to 180 days are recorded as thunderstorm days in a year (Figure 6). The frequency of thunderstorms in mid latitudes in summer season during afternoon time is moderate to high. On the other hand, there are no thunderstorms in polar areas, mainly 700 N to North Pole and 600 S to South Pole. Like these cold deserts, in the subtropical hot deserts the frequency of thunderstorms is low. Majority subtropical high pressure areas have a frequency of less than 20 days as thunderstorm days in a year.In India the mean number of thunderstorm-days is about 25 days.

Figure 6: Distribution of Thunderstorms

Source:https://media1.britannica.com/eb-media/52/23852-004-5F81DDB0.jpg

Another significant observation about distribution of thunderstorms is that they frequently occur in groups consisting of many individual storms clustered together. This happens due to presence of favourable conditions over a broad area, for instance, at fronts of two distinct air masses. When these thunderstorms occur as clusters in elongated bands they represent the squall lines (Figure 7). This linear band of cumulonimbus clouds may extend for 500 km or more, with individual cells in different stages of thunderstorm formation. In case these storms are organised into roughly circular clusters, they are known as mesoscale convective complexes (Figure 8).These complexes prevail over large areas for 12 hours or more.

Figure 7: Squall Line

Source:http://climate.ncsu.edu/secc_edu/images/StormClass3.gif

Figure 8: Satellite Image of Mesoscale Convective Complex

Tornadoes

Thunderstorms, tornadoes and waterspouts are mainly localised weather phenomena. In these storms tornadoes are the most violet and destructive. Tornado represents a small but violently rotating funnel shaped column of air extending downward from cumulonimbus cloud to surface (Figure 9). Tornadoes are also known as twisters or cyclones. The diameter of tornadoes varies in the range of 100 to 600 metres. The pressure at the centre of funnel is very low as compared to its surroundings, usually a difference of 10 mb or more. Therefore, wind velocities are very high; on certain occasions they have been estimated to be as highas400 km per hour. Otherwise, in most of the tornadoes wind velocity is below145 kmph.Tornadoes are short lived and move on erratic path at an average speed of about 45 km per hour.

Figure 9: The Appearance of Tornado

Source:https://s-i.huffpost.com/gen/2176456/images/o-TORNADO-facebook.jpg

In cycling circulation of a tornado, moist air is being suckedinside at surface from all directions;it ascends in circulatory motion or spiral fashion until it eventually joins the parent thunderstorm deep in the vertical cumulonimbus cloud at top. Dust and debris lying in its pathare also picked up in this circulation.This local and short duration storm links surface with cloud.Generally tornadoes consist of a single vortex, but occasionally powerful tornadoes have multiple or many small intense whirls called suction vortices around the central larger and well established main tornado vortex, this represents multiple vortex tornadoes (Figure 10).

Figure 10: Multiple Vortex Tornadoes

Source:https://i.pinimg.com/736x/fd/7f/a8/fd7fa888e2e3f508503ea2999120a79d–water-tornados.jpg

Formation of Tornadoes

Tornadoes, without exception, are associated with intense thunderstorms. They are result of intense atmospheric instability. Although there is difference of opinion about their formation but scholars agree on the point that they are result of violent convection in conditionally and convectively unstable air column. Tornadoes associated with fronts of contrasting air masses are easy to explain. Tornadoes generally develop in association with severe thunderstorms and supercell thunderstorms. It is noteworthy that only less than 1 per cent of all thunderstorms generate tornadoes. The most probable area of origin of tornadoes is the segment of thunderstorm adjacent to large hail. These thunderstorms are associated with high velocity winds, heavy rainfall and often hail. Extreme weather conditions such as along cold fronts, squall line, mesoscale convective complex, hurricanes and severe and supercell thunderstorms can generate tornadoes.

An essential precondition associated with tornado formation is emergence of mesocyclone in severe thunderstorm. A mesocyclone is basically a vertical rotating column of air in the altitudinal range of 3 to 10 km, across that forms the updraft of a severe thunderstorm or supercell thunderstorm. This mesocyclone genesis takes place in presence of vertical wind shear. Stronger winds in upper part and weaker winds on surface side generate rolling motion along the lateral or horizontal axis. After fulfilment of this essential condition, the ascending air currents may lift this initial horizontal rotating column in vertical position. In case this happens the initial rotation within the cloud core increases the probability of tornado formation.It is noteworthy that only about half of all mesocyclones develop into tornadoes.Initially the mesocyclone is wider, slower and shorter but with the passage of time it isextended vertically and shrinks horizontally and results into accelerated winds in an inward vortex. The shrinking column of rotating air extends downward and a portion of cloud extends below the cloud base as a very dark and slowly circulating wall cloud. Further, a narrow and fast rotating vortex comes out from the base of the wall cloud to develop as a funnel cloud. When this funnel cloud extends upto surface, it is called a tornado (Figure 11).

Figure 11: Formation of Tornado

Source:http://web.gccaz.edu/~lnewman/gph111/topic_units/thunder_hurr/07_34.jpg

Distribution of Tornadoes

Tornadoes may develop in any part of the world except, the polar region and the severely cold northern regions of continents during winter season. In the USA, tornadoes are most frequent in the world. Texas, Oklahoma, Kansas, Iowa, Arkansas, Missouri, Mississippi and Alabama states of USA are most prone to tornadoes (Figure 12).In USA, tornadoes are most frequent in middle-west and a second maximum is along the Mississippi valley in southeast states.They are most frequent during the spring season when about 40 per cent of all tornadoes occur. During May and June the centre of maximum frequency shifts from south to north due to northward penetration of warm and moist (mT) air masses northward making frontal zone with contrasting air masses.Tornadoes are less frequent in other parts of the world but are experienced in Central and North Western part of Europe, Japan, eastern India and parts of Australia, South Africa, Uruguay and adjoining Argentina (Figure 13).

Figure 12: Distribution of Tornadoes in USA

Source:https://media1.britannica.com/eb-media/51/23851-004-1AB57189.jpg

Figure 13:Distribution of Tornadoesin the World

Source:http://web.gccaz.edu/~lnewman/gph111/topic_units/thunder_hurr/07_36.jpg

Waterspouts

The phenomenon similar to tornadoes over the oceans is known as waterspout. Waterspouts have usually the identical characteristics except that they are usually smaller in size or diameter (Figure 14). The funnel of waterspouts is composed of condensed moisture in low-pressure vortex. Waterspouts are common in coastal parts of USA, China and Japan.

Figure 14:View of Waterspouts

Summary and Conclusions

A storm is identified as thunderstorm only when thunder is heard. As thunder is a consequence of lightning, it means lightning and thunder are the chief characteristic features of thunderstorms.Thunderstorm represents an intense local storm associated with extensive cumulonimbus clouds, heavy precipitation along with thunder and lightning. In thunderstorms, strong convectional mechanism predominates. This mechanism is basically a function of intense atmospheric instability which is directly related to large supply of warm and moist air and lifting mechanisms. The life cycle of a thunderstorm passes through three stages – cumulus stage, mature stage and dissipation stage.Thunderstorms are classified as air-mass thunderstorms, frontal thunderstorms, severe thunderstorms and supercell thunderstorms.

The broad generalisation about the distribution of thunderstorms indicates decreasing frequencyfrom equator to pole. The maximum frequency is in the doldrums region. On the other hand, there are no thunderstorms in polar areas, mainly 700 N to North Pole and 600 S to South Pole. Like these cold deserts, in the subtropical hot deserts, the frequency of thunderstorms is low. Majority subtropical high pressure areas have a frequency of less than 20 days as thunderstorm days in a year.

Thunderstorms, tornadoes and waterspouts are mainly localised weather phenomena. In these storms tornadoes are the most violet and destructive. Tornado represents a small but violently rotating funnel shaped column of air extending downward from cumulonimbus cloud to surface. Tornadoes are also known as twisters or cyclones. The diameter of tornadoes varies in the range of 100 to 600 metres. The wall cloud and funnel cloud are structural features of tornadoes.Tornadoes, without fail, are associated with intense thunderstorms. They are result of intense atmospheric instability. The precondition for the formation of tornadoes is presence of mesocyclone. Tornadoes may develop in any part of the world except, the polar region and the severely cold northern regions of continents during winter season. In the whole world, tornadoes are most frequent in theUSA. Tornadoes are absent in polar areas and cold northern continents parts.The phenomenon similar to tornadoes over the oceans is known as waterspout. Waterspouts have usually the identical characteristics except that they are usually smaller in size or diameter.

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References

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