Fog, Western Disturbances and the Monsoon: Indian Aviation Meteorology for DGCA Students

The DGCA exam, and real flying in India, expect you to understand Indian weather: winter fog over the Indo-Gangetic plains, western disturbances, pre-monsoon thunderstorms and the southwest monsoon.

The short version:

• Winter (Dec–Feb): western disturbances bring rain, hill snow and low cloud. When they move away, cold, clear, calm nights over moist ground produce dense radiation fog, the biggest disruptor of flights in north India.

• Pre-monsoon (Mar–May): heat, thunderstorms, Nor'westers and dust storms.

• Southwest monsoon (Jun–Sep): widespread rain, low cloud, wet runways and cumulonimbus hidden inside layer cloud.

• Post-monsoon (Oct–Dec): withdrawal of the monsoon, northeast-monsoon rain on the southeast coast, tropical cyclones, and the return of fog to the north.

This guide explains each of these and links them to what you'll see in METARs.

India's aviation weather calendar

IMD divides the year into four seasons: winter, pre-monsoon, southwest monsoon and post-monsoon. The hazards below overlap those seasons rather than fitting neatly inside them.

For the 2025–26 winter, DGCA declared 10 December to 10 February as the official fog window. During this window, airlines must roster pilots trained for low-visibility operations and deploy CAT IIIB-compliant aircraft.

Fog: the winter visibility problem

What counts as fog? It depends who is reporting

This catches many students out. METARs and IMD's public weather bulletins use different fog definitions.

In a METAR (the ICAO/WMO code):

Code	Meaning	Visibility
FG	Fog	Below 1,000 m
BR	Mist	1,000–5,000 m
HZ	Haze (dry particles, not water droplets)	5,000 m or less
MIFG / BCFG / PRFG	Shallow fog / fog patches / partial fog	Can be reported with visibility of 1,000 m or more, because the fog is shallow or does not cover the whole aerodrome
FZFG	Freezing fog	Fog below 1,000 m with temperature below 0°C

IMD's fog intensity categories, used in public warnings and press releases, are:

IMD category	Visibility
Shallow	500–999 m
Moderate	200–499 m
Dense	50–199 m
Very dense	Below 50 m

How radiation fog forms

Radiation fog is fog made by the ground cooling at night. It needs four ingredients:

1. A clear sky, so the ground loses heat freely by long-wave radiation.

2. Light wind. A little mixing spreads the cooling through a shallow layer. Dead calm tends to give dew or very shallow fog, while stronger wind mixes in warmer air and may produce low stratus instead.

3. Moist air, so only a small amount of cooling brings the air to its dewpoint.

4. A long night, giving more hours of cooling. This is why fog peaks around midwinter.

As the ground cools, the air just above it cools too and a surface temperature inversion forms: temperature rises with height for the first part of the climb. When the air in the lowest layer reaches its dewpoint (T = Td), fog forms. It is usually thickest around dawn. After sunrise the ground warms, and the fog thins from below or lifts into low stratus before clearing.

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Three ways to make fog

Type	How it forms	Typical behaviour
Radiation fog	Ground cools on clear nights; air cools to its dewpoint	Forms after sunset, densest near dawn, usually clears with daytime heating
Advection fog	Warm, moist air moves over a colder surface	Can form by day or night and persist in stronger winds than radiation fog
Precipitation fog	Rain from warmer air above evaporates into cold air below and saturates it	Linked with frontal or disturbance rain; often comes with low cloud and drizzle

In India, radiation fog dominates. A study of 69 Indian stations from 2000 to 2020 found it was the most common fog type at most locations, accounting for 68.1% of all fog events and 70.0% of dense fog events. The same study identified December and January as the main fog months.

Why the Indo-Gangetic plains are so foggy

The plains of Punjab, Haryana, Delhi, Uttar Pradesh and Bihar combine all four radiation-fog ingredients in winter: long nights, frequent clear skies, light winds and moist low-level air. That moisture is often topped up by recent rain from a western disturbance (see below). Urban pollution adds extra particles for droplets to form on, and IMD bulletins regularly report "smog" alongside fog in these regions.

Fog and flight operations

When visibility falls far enough, airports apply low-visibility procedures (LVP). At Delhi's Indira Gandhi International Airport, a 2013 parliamentary reply stated that ILS CAT IIIB was operational on runways 28, 29 and 11, allowing landings down to 50 m RVR, with low-visibility take-offs down to 125 m RVR.

The runway layout at Delhi has changed since then, so always check the current AIP India for present-day minima. The general point holds: fog operations depend on the airport's equipment, the aircraft's certification and the crew's training. That is why, in thick fog, some flights still operate while others are delayed or diverted.

Reading a fog METAR

Here is a fictional training METAR in a typical dense-fog format:

VIXX 150030Z 00000KT 0050 R27/0075N FG VV001 08/08 Q1019 NOSIG

Calm wind, visibility 50 m, RVR for runway 27 of 75 m with no distinct change, fog, vertical visibility 100 ft (sky obscured), temperature equal to dewpoint at 8°C, QNH 1019 hPa, no significant change expected.

Western disturbances

What a western disturbance is

IMD describes a western disturbance (WD) as a cyclonic circulation or trough in the mid- and upper-tropospheric westerlies. It originates over the Mediterranean, Caspian or Black Sea region and moves eastwards across north India. In daily bulletins, IMD typically describes a WD's position as a trough in the middle-tropospheric westerlies, often giving the height of its axis.

Key features for exam purposes:

• WDs are extratropical systems carried east by the subtropical westerly jet.

• They are most active in winter, with roughly 4–7 passing through north India per month at that time of year.

• They bring most of northwest India's winter rain and the western Himalaya's snow.

The weather before, during and after a WD

Before: high and medium cloud thickens from the west, and nights become less cold under the cloud cover.

During: rain over the plains and snow over the western Himalaya, with low cloud and reduced visibility. Thunderstorms and hail are possible, especially in late winter and spring.

After: skies clear from the west, colder northwesterly air follows, and a cold wave is possible.

For pilots, the hazards include low ceilings and poor visibility in rain, snow and icing in the hills, thunderstorms in the transitional months, and then fog once the system has passed.

Why dense fog often follows a WD

After a WD has passed, the ground is wet, the sky clears and cold air settles under light winds. That is an almost ideal setup for radiation fog. Forecasters have recognised the pattern for decades: a 1963 IMD Mausam paper described widespread, persistent fog following "moist" cold waves after a WD.

The link is real but not automatic. The same 2023 study found that 46.9% of radiation-fog onsets in Delhi in December–January happened alongside an active WD. Yet only 32.3% of WDs in those months coincided with the onset of radiation fog. A WD raises the chance of fog, but it doesn't guarantee it.

Pre-monsoon (March–May): heat and thunderstorms

As the land heats up, the pre-monsoon season brings:

• Thunderstorms and squalls, including the Nor'westers (Kalbaisakhi) of eastern India, with gusty winds, hail and heavy rain.

• Dust storms and raised dust over northwest India, reducing visibility.

• High temperatures, which raise density altitude and reduce aircraft take-off and climb performance.

In METARs, watch for TS, SQ, GR, DS and CB in the cloud groups.

The southwest monsoon

IMD's normal dates

Under IMD's revised normals, the southwest monsoon:

• sets in over Kerala around 1 June, with a standard deviation of about 7 days;

• covers most of India except the extreme northwest by about the end of June;

• covers the entire country by 8 July;

• starts withdrawing from northwest India around 17 September; and

• has normally withdrawn completely by 15 October.

The actual onset date varies considerably. Over Kerala, onset was 8 June in 2019 and 2023, but 24 May in 2025, the earliest since 2009.

Flying hazards in the monsoon

• Embedded cumulonimbus. Thick layers of nimbostratus can hide CB cells inside them, along with their turbulence, hail, icing, lightning and heavy rain. Without weather radar, you may fly straight into one.

• Heavy rain cuts visibility sharply and can reduce it within minutes.

• Low cloud bases at the destination and alternates.

• Windshear and downbursts beneath convective cells.

• Wet or flooded runways, which increase stopping distance and the risk of hydroplaning.

In METARs and TAFs, watch for +RA, TSRA, CB and low BKN/OVC layers. Read SIGMETs for embedded or obscured thunderstorms (EMBD TS / OBSC TS).

Post-monsoon: withdrawal, the northeast monsoon and cyclones

As the southwest monsoon withdraws, the northeast monsoon brings rain to the southeast coast, especially Tamil Nadu, from October to December. Tropical cyclones form in the Bay of Bengal and the Arabian Sea mainly in two windows: the pre-monsoon months, around May, and the post-monsoon months, around October to December. By December, the fog season has returned to the north.

Common mistakes

Mistake	Fix
Treating IMD "shallow fog" as METAR MIFG	IMD shallow = visibility 500–999 m. MIFG = fog under 2 m deep, with reported visibility of 1,000 m or more.
Coding mist below 1,000 m	Below 1,000 m with water droplets is FG. BR is 1,000–5,000 m.
Thinking radiation fog needs dead calm	Light wind helps. Calm tends to give dew or very shallow fog, and stronger wind tends to give low stratus.
Saying a WD always causes fog	A WD makes fog more likely. It doesn't guarantee it.
Calling a WD a tropical system	It is extratropical, carried by the subtropical westerly jet.
Memorising old monsoon dates	Use IMD's revised normals: whole country by 8 July, withdrawal starting about 17 September.
Confusing visibility and RVR	Visibility is general horizontal visibility. RVR is measured along a specific runway and reported as R27/0075N.

Quick-revision cheat sheet

Practice questions

These are original questions written for this article. They are not DGCA exam questions.

1. Horizontal visibility is 3,000 m because of tiny water droplets, with high humidity. Which METAR code applies?

2. IMD reports visibility of 150 m in fog. What is IMD's intensity category?

3. List the four conditions most favourable for radiation fog.

4. Why does dense fog over the north Indian plains often follow a western disturbance?

5. According to IMD's revised normals, by what date does the southwest monsoon normally cover the whole country?

6. Decode R27/0075N and VV001.

7. A METAR shows 9999 MIFG. Does this contradict IMD's definition of "shallow fog"? Explain.

Answers

1. BR (mist): visibility of 1,000–5,000 m due to water droplets.

2. Dense fog (50–199 m).

3. Clear sky, light wind, moist air and a long night.

4. WD rain moistens the ground and the lowest layer of air. The skies then clear and cold air settles under light winds, which allows strong overnight radiative cooling to the dewpoint.

5. 8 July.

6. RVR for runway 27 is 75 m with no distinct change; vertical visibility is 100 ft, meaning the sky is obscured.

7. No. MIFG is fog less than 2 m deep, so visibility at eye level stays high. IMD's "shallow fog" is a visibility band of 500–999 m. The terms come from different systems.

FAQs

What visibility counts as fog in aviation?

In METARs, fog (FG) is reported when visibility is below 1,000 m because of water droplets. Between 1,000 and 5,000 m it is mist (BR).

Where do western disturbances come from?

IMD describes them as systems that originate over the Mediterranean, Caspian and Black Sea region and move east across north India in the mid- and upper-level westerlies.

Why does fog usually clear by late morning?

After sunrise, heating of the ground breaks the surface inversion. The fog thins from below or lifts into low stratus before clearing. Very dense fog under weak sunshine can persist much longer.

Do all flights stop in dense fog?

No. Operations depend on the airport's ILS and lighting category, the aircraft's certification and the crew's low-visibility training, so CAT IIIB-equipped and trained crews can operate when others cannot.

Is the southwest monsoon onset over Kerala the same every year?

No. The normal date is 1 June with a standard deviation of about a week, and recent years have ranged from 24 May (2025) to 8 June (2019, 2023).

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