Thunderstorm- An Extreme Weather Events, their forecasting and Mitigation of the after-effects
Authors: Brijesh Yadav, Poonam Yadav and Avinash Goyal
ICAR-Indian Agricultural Research Institute, New Delhi, 110012

An extreme weather event includes unusual, severe or unseasonal weather, weather at the extremes of the historical distribution, the range that has been seen in the past.It includes cyclones, snow storm, hailstorm, drought, thunder storm, dust storm, heavy & unseasonal rainfall and flood. According to WMO, thunderstorm is defined as “One or more sudden electrical discharges, manifested by a flash of light (lightning) and a sharp or rumbling sound (thunder) occurs from a cloud of vertical development”. Thunderstorms usually have higher strong winds, heavy rain, thunder, lightning and sometimes hail, or no precipitation at all. Frequency of thunder storms decrease with increase in altitude because beyond tropopause instability of atmosphere decreases due to increase in temperature with height. Lines that joints equal occurrence of thunderstorms activity called isobronts. The main region of high thunderstorm activity in India are categories in three regions-1) Northeast India (2) Southwest peninsula (3) Northwest India .

National Weather Service defined the severe thunderstorm as a thunderstorms having at least one of the following facts:(1) A tornado (2) Damaging winds, or winds in excess of 50 knots / 58 mph (3) Hail three-quarter inch in diameter or larger. Thunderstorm day is observational day during which thunder is heard. Thunderstorm is a mesoscale phenomenon. Typical meso scale winds range from a few kilometres to about a hundred kilometres in diameter. Generally, they last longer than micro scale motions, often many minutes, hours, or in some cases as long as a day.

Conditions for thunderstorm formation –

Thunderstorm development is dependent on three basic components: moisture, instability, and some form of lifting mechanism / convergence.

1.Moisture – As air near the surface is lifted higher in the atmosphere and cooled, available water vapour condenses into small water droplets. As condensation of water vapour occurs, latent heat is released making the rising air warmer and less dense than its surroundings and form a strong updraft.

2.Stability and Instability of atmosphere – Instability is a prerequisite for severe weather. If greater the instability, the greater the potential for severe thunderstorms. If a parcel of air expands and cools, or compresses and warms, with no interchange of heat with its surroundings, this situation is called an adiabatic process. If parcel is moist and dry than it called moist and dry adiabatic lapse rate respectively. The rate at which the air temperature surrounding us will be changing if we were to climb upward into the atmosphere, we will refer to it as the environmental lapse rate. If dry adiabatic rate is less than the environmental lapse rate, the parcel will be warmer than the surrounding air and will continue to rise, constantly moving upward, away from its original position. So the atmosphere is unstable.

3.Lifting mechanisms / Low level convergence – Lift is needed to overcome convective inhibition. There are mainly four trigger mechanisms i.e.- Sea breeze and land breeze, Orographic lifting, Frontal lifting and Convergence lifting.

Thunder storms types

On the basis of instability and relative wind conditions at different layers of the atmosphere, thunderstorm classified into four categories-

1. Single cell –

This term technically applies to a single thunderstorm with one main updraft. Single-cell thunderstorms form in environments of low vertical wind shear and last only 20–30 minutes. Such storms are rarely severe and are a result of local atmospheric instability.

2.Multi-cell cluster –

This is the most common type of thunderstorm and consists as a group of cells. Although individual cells tend to last only 20-30 minutes, the cluster itself may last for several hours. Multi-cell storms can produce moderate size hail, flash floods, and weak tornadoes.

3.Multi-cell lines –

Multi-cell or “squall” line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. These storms can produce small to moderate size hail, occasional flash floods, and weak tornadoes.

4.Super cell thunderstorm –

The super cell is a highly organized thunderstorm. They have separate downdraft & updraft with single strong rotating updraft .Although super cells are rare, they pose an exceptionally high threat to life and property.

Life cycle of a thunderstorm -

On the basis of magnitude and direction of the ascending and descending air current, thunderstorm have three stages. A thunderstorm takes 30 to 60 minutes to cover complete life cycle.

1. Cumulus stage -

Characterized by an updraft throughout most of the cell,which causes the cloud to grow. With continued upward motion a large amount of water condenses and eventually falls as precipitation. The cumulus stage typically lasts 10-15 min.

2. Mature stage –

Characterized by the presence of downdrafts and updrafts, as water droplet size exceeds the capacity for the updraft to suspend the droplets, the droplets begin to fall. The mature stage lasts 15-30 minutes on average. Surface boundary that separate the advancing cooler air from the surrounding warmer air called gust front.

3. Dissipating stage –

In this stage updraft becomes weaker and downdraft is dominant. Downdraft will push down out of the thunderstorm, hit the ground and spread out called downburst. It takes 20-30 minutes

Damage due to thunderstorms

1. Tornadoes- It causes an average 70 fatalities & 1500 injuries each year. It produced wind speed more than 250 mph.

2. Lightning- It causes 80 fatalities per year. It kills more people then tornado.

3. Strong winds- Wind speed more than 100 mph. It causes damage to crop/tree, aviation etc.

4. Flooding- Thunderstorm cause flooding which affect humans, livestock.

5. Hails, downburst

Predictions & Forecasting

SPC forecasters, NWS forecasters, NSSL researchers and other groups work together to develop and evaluate the best thunderstorm forecasting tools, including computer forecast models and new forecasting techniques.

Computer forecasting models-

Meteorologists often rely on massive computer programs called numerical weather prediction models to help them decide if conditions will be right for the development of thunderstorms. Data is gathered from weather balloons launched around the globe twice each day, in addition to measurements from satellites, aircraft, ships, temperature profilers and surface weather stations.

Satellites –

Satellites are critical in short-term forecasting. Satellite images can give an early indication of a developing thunderstorm by showing where cumulus clouds are forming. Cumulus clouds grow rapidly into cumulonimbus clouds if conditions are right, and you can track their growth using satellite images.

Data are collected from different data source like Radars, satellites, numerical models etc. integrated by hydromet decision support system and they predict thunderstorm, strong winds, tornados, flooding and lightning. Thunderstorm prediction is based on atmospheric stability and instability. In operational forecasting, many indices, thermodynamic diagram are used to assess the stability of the atmosphere and predict the likelihood of severe thunderstorm development. Generally following techniques are used for stability and unstability determination-

1.Assessing stability / instability indices

The stability indices examined are the Showalter index, lifted index, SWEAT, K index, total totals, CAPE, CIN, and equilibrium level pressure. They can be determined by mathematical formulae or by plotting on a skew-T log-p diagram. The lifted index, CAPE, and CIN are calculated using virtual temperature as well as normal air temperature.

(a) Showalter Index:

SI = T500 - 850 → 500

Where T500 is temperature of actual atmosphere and T’850-500 is temperature of air parcel when air parcel is lifted dry-adiabatically from 850 hPa (mb) level to the lifted condensation level (LCL) and from LCL up to 500 hPa moist-adiabatically

(b) Lifted Index:

LI= T500 – TP500

Where T is temperature of environment and Tp is air parcel lifted adiabatically. If LI value is -2 to -6 than Thunderstorm probability will be more.

2. Thermodynamic diagrams

A thermodynamic diagram is a graph that shows the relationship between five atmospheric properties- (a) Pressure (millibars) (b)Temperature (°C) (c) Potential temperature (°C – must be converted to K) (d) Equivalent Potential Temperature (C – must be converted to K) (e) Saturation Mixing Ratio (g/kg). Since pressure rapidly decreases with altitude, thermodynamic diagrams are used most commonly to display vertical profiles of atmospheric properties. They determine some properties like - Atmospheric Stability cloud layers, height of the tropopause, cloud top temperatures, frontal zones, vertical wind shear, helicity location of inversions, precipitation type, height of the freezing level, locations of upper level fronts.

Awareness Information

1. Severe Thunderstorm Watch- It is expected in the next 6 hours or so on. Area approximately 120 to 150 miles wide and 300 to 400 miles long. The NWS Storm Prediction Centre issues such watches.

2. Severe Thunderstorm Warning- It indicate that a hazardous event is occurring in about 30 minutes to an hour. Local NWS forecast offices issues such warning.

3. Significant Weather Advisory- Strong thunderstorms with wind gusts of 40 to 55 mph and hail iess than 1.00” in diameter will be approaching

1. Asoi Lal. 1989. Forecasting of thunderstorm around Delhi and Jodhpur. Mausam, 40:267-8.

About Author / Additional Info:
PhD 1st year, Division of Agricultural Physics, Indian Agricultural Research Institute, New Delhi