Thunderstorm

A thunderstorm (TS) is caused by intense convective currents within Cumulonimbus (Cb) clouds and is officially reported when thunder is heard.
The diameter of a Cumulonimbus cloud can be as much as 10 km.
Single-cell thunderstorms usually occur during the non-winter months due to intense surface heating (air-mass thunderstorms).
Multi-cell cluster thunderstorms are commonly associated with mountains, cold fronts and low-pressure systems.
Multi-cell line thunderstorms develop along squall lines associated with frontal systems.
Supercell thunderstorms may extend into the stratosphere and are capable of producing tornadoes.

Thunderstorms develop in an unstable atmosphere where the Environmental Lapse Rate (ELR) is greater than the Saturated Adiabatic Lapse Rate (SALR).
High humidity and a triggering mechanism are essential for thunderstorm formation.
Common lifting mechanisms include:
- Surface heating (Insolation) causing local convection.
- Orographic lifting.
- Convergence of air.
- Frontal lifting.
- Radiational (Katabatic) cooling.

Heat (Air-Mass) Thunderstorms develop due to intense afternoon heating and the formation of heat lows.
Low pressure causes convergence of warm, humid air, leading to the development of Cumulonimbus clouds.
Orographic thunderstorms occur when warm, moist air is forced to rise over mountainous terrain, particularly during the afternoon.

Steady-state thunderstorms form due to prolonged convergence associated with winds or upper-air troughs.
Frontal thunderstorms usually develop along cold fronts and occasionally along warm fronts.
Strong uplift of warm, moist air along a cold front can produce thunderstorms during both day and night.

Squall lines consist of a series of thunderstorms arranged in a line.
They are typically located 100–300 km ahead of a cold front.
Squall lines may be embedded within layered clouds, making them difficult to detect visually and therefore particularly hazardous.

A Mesoscale Convective Complex (MCC) consists of multiple clusters of thunderstorms.
MCCs may cover very large areas and are capable of producing cyclones, tornadoes and flash floods.

The initial growth phase of a thunderstorm is known as the Cumulus Stage and typically lasts 15–20 minutes.
The rising air cools initially at the Dry Adiabatic Lapse Rate (DALR) until saturation occurs.
After saturation, cooling continues at the Saturated Adiabatic Lapse Rate (SALR), allowing Cumulus clouds to develop into Cumulonimbus clouds.
Only updrafts exist during this stage, while rain and ice particles grow within the cloud.
Entrainment (inflow of surrounding air into the cloud) occurs during this stage.

The Mature Stage is the most violent stage and generally lasts 20–40 minutes.
Precipitation reaches the ground during this phase.
Both strong updrafts and down-drafts are present until the entrained air becomes relatively dry.
Roll clouds and Shelf clouds are commonly observed during this stage.

Secondary thunderstorms develop when cold down-drafts create squalls and gust fronts accompanied by heavy showers.
The cold gust front undercuts warm air, forcing it upward.
The rising moist air develops additional Cumulonimbus cells.
Down-drafts from the parent storm interact with low-level winds, creating convergence that produces daughter cells.
This repeated process results in multicell thunderstorms.

Thunderstorms generally move with the prevailing winds at approximately the midpoint of the cloud’s vertical extent.
The direction and speed of movement are mainly controlled by winds near 10,000 feet or between the 700 hPa and 500 hPa pressure levels.

In ordinary thunderstorms, down-drafts during the mature stage suppress further upward growth.
Supercell thunderstorms develop under conditions of extreme instability, abundant moisture and strong vertical wind shear.
Typically, the atmosphere is relatively stable near the surface but highly unstable at higher levels.

Supercells initially develop under slight instability, which increases with continued surface heating.
Vertical wind shear results from winds blowing from different directions and at different speeds with height.
Wind shear tilts the thunderstorm, separating the updrafts from the down-drafts.
This separation allows the storm to continue growing into a long-lived supercell.

Light Thunderstorm: Characterised by faint thunder.
Moderate Thunderstorm: Loud thunder, lightning, showers and winds between 15–40 knots.
Severe Thunderstorm: Continuous thunder and lightning, heavy showers and winds exceeding 40 knots.
Over plains, thunderstorms usually occur during the afternoon.
Over valleys, thunderstorms are more common at night and during the early morning.
Over the sea, thunderstorms are generally more frequent during the night.
In mid-latitudes, thunderstorms are most common during summer, whereas in the tropics they are often associated with winter cold fronts.
The first gust associated with a thunderstorm usually produces a sudden increase in atmospheric pressure.
Thunderstorms are forecast by comparing existing atmospheric conditions with those required for development and by monitoring weather radar and satellite imagery.
Aircraft should avoid thunderstorms by a wide margin, especially at lower altitudes.
Jet streams create upper-level wind shear that tilts the updrafts and supports the development of supercell thunderstorms.

Dust and sand storms develop over desert regions under conditions of high temperature, low humidity and atmospheric instability.
Slight moisture intrusion may produce Cumulonimbus clouds that generate strong winds, lifting dust to heights of about 10,000 feet and reducing visibility to as little as 50 metres.
During the pre-monsoon season, these storms resemble thunderstorms with relatively low cloud tops and precipitation that evaporates before reaching the ground.
Norwesters approach from the northwest over Bengal and are associated with violent thunderstorms and squall lines.
Tornadoes are rotating funnel clouds formed by intense convergence and low-level wind shear.
Waterspouts are tornadoes that develop over water and may lift spray, water and debris.
Dust Devils are small rotating columns of air formed by localized surface heating, capable of lifting dust to heights of about 2 km and reaching diameters of up to 10 km.