Thunderstorm: IC Joshi Chapter 13, DGCA Notes and Practice Questions
Thunderstorm is a High-weight chapter in IC Joshi, and it draws on half the book: stability, clouds, icing, winds and met services all meet here. Know the three ingredients, the three stages, and which hazard belongs to which stage, and most DGCA thunderstorm questions answer themselves.
What DGCA asks from this chapter
Thunderstorm questions in the DGCA Meteorology paper fall into five groups. If you can answer one question from each group without hesitating, you have the chapter.
- Formation. The three conditions a thunderstorm needs, and which trigger produces which type of storm.
- Life cycle. The three stages, what defines the start of each, how long each lasts, and what the vertical air currents are doing.
- Hazards. Turbulence, icing, hail, lightning, gusts, microbursts, wind shear and altimeter errors, and when in the life cycle each is worst.
- Indian storms. Nor'westers, dust storms and the pre-monsoon thunderstorm season.
- Avoidance. Distances, radar detection and what not to do around a cumulonimbus.
The three ingredients
A thunderstorm needs all three of these at once. Remove any one and you get, at most, a large cumulus.
| Ingredient | Why it matters |
|---|---|
| A deep layer of moist air | Condensation releases latent heat, which keeps the rising air warmer than its surroundings and feeds the updraught. |
| Instability through a great depth | Usually conditional instability: an environmental lapse rate between the SALR and the DALR, so saturated air keeps rising once lifted. Covered in Chapter 9, Stability of Atmosphere. |
| A trigger to start the lift | Surface heating, a front, hills or mountains, or low-level convergence. The trigger decides the type of storm. |
Types, by trigger
- Air mass (heat or convective) thunderstorms: surface heating on hot afternoons over land. Usually isolated, typically forming in the afternoon and decaying by evening.
- Frontal thunderstorms: lift along a front, most violently a cold front. Can form at any time of day and often appear in lines.
- Orographic thunderstorms: moist unstable air forced up a mountain slope.
- Squall lines: a line or band of active thunderstorms, often ahead of a cold front. Among the most severe for aviation, because there may be no way round.
Life cycle: the three stages
Every thunderstorm cell goes through the same three stages. A large storm is often several cells at different stages side by side, which is why a storm can last for hours while each cell lives for well under one.
| Stage | Begins when | Air currents | Typical duration |
|---|---|---|---|
| Cumulus | Cumulus cloud starts growing vertically | Updraughts throughout the cloud | 10 to 15 min ⚑ |
| Mature | Precipitation first reaches the surface | Updraughts and downdraughts side by side. Strongest of the cycle. | 15 to 30 min ⚑ |
| Dissipating | Downdraughts spread through the whole cell | Downdraughts only. The supply of warm moist air is cut off. | about 30 min ⚑ |
Hazards to aircraft
| Hazard | What to know for the exam |
|---|---|
| Turbulence | Severe to extreme inside the cloud, from adjacent updraughts and downdraughts. Also found below the cloud, around it, and in clear air near the top and under the anvil. |
| Icing | Heavy clear (glaze) ice is likely in the updraughts above the freezing level, where large supercooled drops are carried aloft. See Chapter 12, Ice Accretion. |
| Hail | Most likely in the mature stage. Can fall from under the anvil, in clear air, well ahead of the storm. |
| Lightning | Can strike aircraft in or near the cloud. Effects include temporary blindness, compass errors, and damage to radio and electrical equipment. |
| Gust front and squall | Cold downdraught air spreading out ahead of the storm. A sudden wind shift and increase, a sharp drop in temperature and a rise in pressure, often before the rain arrives. |
| Microburst | An intense, small-scale downdraught. Under about 4 km across, lasting about 5 to 15 minutes ⚑. The most dangerous low-level wind shear for an aircraft on approach or departure. |
| Altimeter errors | Rapid pressure changes under and around the storm make the altimeter unreliable near the ground. |
| Heavy rain and reduced visibility | Torrential rain, possible water ingestion, and visibility near zero in the heaviest showers. |
Microburst on approach: the sequence
This is examined often because the first sign is misleading. An aircraft flying through a microburst on approach meets three things in turn:
- Headwind increase. Airspeed and performance rise, and the aircraft tends to go above the glidepath. The natural reaction is to reduce power.
- Downdraught. Strong sinking air in the centre.
- Tailwind. Airspeed and lift fall sharply, just when the aircraft is lowest and the power has been reduced.
That's why a sudden, unexplained airspeed gain near a thunderstorm on approach is a warning sign, not good news.
Thunderstorms in India
DGCA questions use Indian examples, and IC Joshi covers them in detail.
- Nor'westers (Kalbaishakhi): violent pre-monsoon thunderstorms over West Bengal, Odisha, Assam and Bangladesh, mainly in April and May. Named for the direction they arrive from: they usually move from the north-west towards the south-east. They bring squalls, hail and heavy rain, often in the afternoon or evening.
- Dust storms (Andhi): pre-monsoon storms over north-west India, including Rajasthan, Punjab and Haryana. Strong winds lift dust and sand, and visibility can drop to near zero.
- The pre-monsoon season (March to May) is the peak thunderstorm period for much of India. See Chapter 19, Climatology India.
Detection and avoidance
- Weather radar detects precipitation, not cloud or turbulence. The strongest echoes and the steepest gradients between them mark the worst areas. A hook or finger-shaped echo suggests severe weather.
- Radar attenuation: heavy rain close to the aircraft can hide a more severe cell behind it.
- Avoid severe thunderstorms by at least 20 NM ⚑, and treat an embedded or obscured storm as severe.
- Don't fly under the anvil: hail can fall from it in apparently clear air.
- Don't take off or land with a thunderstorm approaching or overhead the aerodrome.
- Reports and forecasts: TS and CB appear in METARs and TAFs, and active thunderstorm areas are warned of by SIGMET. See Chapter 21, Meteorological Services for Aviation.
Common traps
Practice questions
Three questions from the 158 on this chapter. Each one cites the book it comes from.
1. During which stage of thunderstorm development do both updrafts and downdrafts coexist?
Show answer
Answer: B. Mature stage
The mature stage represents the most violent phase of thunderstorm development, characterized by the coexistence of strong updrafts and downdrafts within the same cell. During this stage, precipitation begins to fall, creating downdrafts while updrafts continue to feed the storm with moisture and energy. This creates the most hazardous conditions for aviation, including severe turbulence, hail, lightning, and heavy precipitation. The mature stage typically lasts 15-30 minutes and represents the peak intensity of the thunderstorm's life cycle.
Source: Aviation Meteorology — IC Joshi · Chapter 13 — Thunderstorm · Page 145
2. What is the primary requirement for thunderstorm formation?
Show answer
Answer: B. Atmospheric instability
Thunderstorms require three essential ingredients for formation: atmospheric instability, moisture, and a lifting mechanism. Atmospheric instability is the most fundamental requirement as it allows air parcels to continue rising once lifted, creating the strong vertical motion necessary for cumulonimbus cloud development. Without instability, convective processes cannot sustain themselves, and thunderstorm formation becomes impossible. The instability is typically measured by environmental lapse rates that exceed the moist adiabatic lapse rate.
Source: Aviation Meteorology — IC Joshi · Chapter 13 — Thunderstorm · Page 142
3. What is the typical horizontal extent of an air mass thunderstorm?
Show answer
Answer: D. 5-10 kilometers
Air mass thunderstorms, also known as ordinary or single-cell thunderstorms, typically have a horizontal extent of 5-10 kilometers and a life cycle of 30-60 minutes. These storms develop in relatively uniform air masses with weak wind shear and are characterized by their isolated nature and predictable three-stage development pattern. Unlike organized thunderstorm systems such as supercells or squall lines, air mass thunderstorms remain relatively small and localized. Their limited horizontal extent makes them easier to circumnavigate during flight operations, though they still pose significant hazards within their immediate vicinity.
Source: Aviation Meteorology — IC Joshi · Chapter 13 — Thunderstorm · Page 147
Frequently asked questions
Which chapter of IC Joshi covers thunderstorms?
Chapter 13, Thunderstorm, of the 2019 edition of IC Joshi's Aviation Meteorology. It covers the conditions a thunderstorm needs to form, the three stages of its life cycle, the hazards to aircraft, Indian storms such as nor'westers and dust storms, and detection and avoidance.
What are the three stages of a thunderstorm?
Cumulus, mature and dissipating. The cumulus stage has updraughts throughout the cloud. The mature stage has updraughts and downdraughts side by side and is the most hazardous. In the dissipating stage downdraughts spread through the whole cell and the supply of warm moist air is cut off.
When does the mature stage of a thunderstorm begin?
When precipitation first reaches the surface. That is also when the first gust, the sharp temperature drop and the pressure jump arrive at the surface, and when turbulence, hail, icing and lightning are all at their worst.
What is a nor'wester?
A violent pre-monsoon thunderstorm, also called Kalbaishakhi, over West Bengal, Odisha, Assam and Bangladesh, mainly in April and May. Nor'westers usually move from the north-west towards the south-east and bring squalls, hail and heavy rain, often in the afternoon or evening.
Related chapters
Practise Chapter 13: Thunderstorm
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