Area Climatology

Winter runs through January and February, and December to February over northwest India. A large belt of high pressure stretches from the Sahara to Siberia, centred near 45°N and 105°E. India lies on its southern edge, so pressure is high over the north and low over the Indian Ocean.

Pressure and wind

Isobars from 1012 to 1018 hPa at 2 hPa spacing cover the country, with the 1018 hPa isobar over Pakistan and Jammu and Kashmir. A shallow trough runs from Kerala to Gujarat along the west coast. Northwesterly winds prevail over north India and northeasterly winds over the peninsula. Up to about 10,000 ft the winds stay northwesterly over the north and northeasterly over the south, with westerlies above. The subtropical jet stream sits over north India near 27°N at about 12 km, blowing westerly at 80 to 100 kt and up to 200 kt.

Systems and weather

North India is worked by the subtropical jet and by 5 to 7 western disturbances a month. A western disturbance is an occluded front of extra-tropical origin that travels east on the jet. As one passes, the sequence is: winds back from northwesterly to southwesterly or southeasterly, temperature rises, pressure falls, cirrus appears and thickens to altostratus and nimbostratus, and drizzle or rain follows. Then the cold-front phase brings cumulus and cumulonimbus, rising pressure and thunder. Behind it, clear cold dry northwesterly air sets in, with widespread radiation fog and a cold wave. A cold wave is a minimum temperature 4.0°C or more below the normal minimum; a cold day is a maximum of 16°C or less in the plains.

Aviation hazards

1. Poor visibility is the main hazard Fog, mist and haze form morning and evening, worst after a western disturbance over northwest India, Uttar Pradesh, Bihar, north Bengal and Assam. Polluted fields such as Palam hold fog into the afternoon.
2. Advection fog Forms over the coasts and near large water bodies, usually clearing between 1000 and 1100 IST.
3. Clear air turbulence Near the subtropical jet over northeast India, with mountain waves over northern high ground.
4. Icing The freezing level descends, so ice can be picked up at lower levels than in summer.

March, April and May are a transition. The winter pattern breaks down: temperature rises steadily, pressure falls, and the gradient goes slack. Only two isobars, 1006 and 1008 hPa, lie across the country, roughly parallel to the coast.

Pressure and temperature

A trough runs down the peninsula near 78°E, and by May it shifts to 79°E along the Chennai coast. A weak low lies over upper Sindh in Pakistan, a trough over east Uttar Pradesh and Bihar, a low over Myanmar, and a closed high over the east-central Bay. Daytime maxima reach 40 to 42°C over most of the country and are highest in May. The diurnal range reaches about 18°C over Gujarat, Saurashtra and Madhya Pradesh. A heat wave is declared at 4°C or more above the normal maximum, or at a maximum 5°C or more above normal.

Systems

The upper westerlies strengthen and the subtropical jet weakens and moves north. Norwesters, known as Kalbaishakhi, strike east and northeast India in the afternoon and evening, triggered by western disturbances. Dust storms cross Rajasthan and north India. Dust-raising winds run roughly 0900 to 1700, at 30 to 50 kt, and can drop the visibility to a few hundred metres. A few tropical cyclones strike the Bengal coast and then move northwest.

Why it is the worst season to fly

1. Violent thunderstorms With hail, strong gusts and wind shear.
2. Dust storms and line squalls Common over Rajasthan and the north.
3. Widespread dust haze to 7 to 8 km So visibility is poor even in the cruise.
4. Norwesters over the east Build quickly to cumulonimbus and are a serious hazard to aircrew.

June to September is the single most important feature of the Indian climate. The season is spread over four months, but the local length runs from under 75 days over west Rajasthan to more than 120 days over the southwest.

Onset and withdrawal

Normal onset dates are South Andaman Sea 20 May, Kerala 1 June, Mumbai 15 June, New Delhi 1 July, and the whole country by 8 July. Withdrawal begins from Rajasthan by 1 September and northwest India by 15 September, clears most of the country by 15 October, and is then replaced by a northerly continental flow, the northeast monsoon. The current advances in two branches, the Arabian Sea branch and the Bay of Bengal branch.

The monsoon trough

A low near 994 hPa lies over Baluchistan. The monsoon trough runs from Sriganganagar through Allahabad and Kolkata to the Head Bay, and it coincides with the ITCZ. It slopes south with height: near 700 hPa it lies roughly along 22°N, near 500 hPa around 10 to 15°N, and it is not traceable above that. Its position governs the rain. In its normal position the rain is well spread over the country. When the trough shifts north to the Himalayan foothills, a break monsoon sets in: the plains go dry while heavy rain and floods hit the sub-Himalayan belt, Bihar and the northeast. Pressure rises over the country and strong westerly surface winds run over east Uttar Pradesh and Bihar.

Systems and strength

Monsoon depressions form in the north Bay of Bengal, 2 to 3 a month, and track northwest or west, bringing well-distributed rain. A mid-tropospheric or subtropical cyclone between 500 and 700 hPa near 18 to 28°N and 60 to 80°E, with a surface trough below it, gives very heavy rain over western India and the northeast Arabian Sea, as in the 2005 Mumbai rains. Monsoon strength is graded from the sea wind and the land rainfall: weak up to 12 kt and under half the normal, moderate 13 to 22 kt, strong 23 to 32 kt, vigorous over 32 kt and over four times the normal. El Nino, the warm phase of ENSO, weakens the southeast trades and the southwest monsoon, and severe Indian droughts occur in El Nino years. La Nina gives an active monsoon.

Flying the monsoon

1. Low cloud 150 to 200 m agl with strong surface wind during strong-monsoon spells and passing depressions.
2. Turbulence Smooth in the stratiform layers, turbulent in embedded cumulus and cumulonimbus.
3. Visibility Very good except in precipitation. Layers have clear gaps, so a cloud-free level is usually available, except near a depression.
4. Tropical easterly jet Easterly along about 13°N at 15 to 16 km, 70 to 80 kt and up to 150 kt.

October and November, running into December, are the retreating southwest monsoon, as the northeasterly regime takes over the subcontinent.

Pressure and wind

A single isobar, 1010 hPa, rings the whole country. The monsoon trough shifts into the Bay of Bengal along 13°N, and the pressure field is flat, with a weak low of 1008 hPa off the Chennai coast and another over Baluchistan. The surface wind swings from southwesterly to northeasterly, the northeast monsoon. It crosses the country diagonally from northeast to southwest and picks up moisture over the Bay before it reaches Cape Comorin. So Tamil Nadu, coastal Andhra Pradesh, Rayalaseema and Puducherry get their main rains. For Tamil Nadu this is the principal rainy season, about 48% of the annual total and nearly 60% in the coastal districts. Rain falls mostly between 2100 and 0300 IST in spells of 3 to 4 days. Winds are light and variable in the flat field, and the sea breeze is strong and is felt up to 160 km inland. Temperatures are nearly uniform at 18°C with a diurnal range of 6 to 7°C.

Upper winds

Westerlies re-establish at all levels and the subtropical jet reappears over Jammu and Kashmir. Easterlies shrink to the far south and weaken to about 20 kt.

Tropical cyclones

1. Maximum cyclonic storms occur in October over the Bay of Bengal and in November over the Arabian Sea. Severe cyclonic storms are more frequent in November than October.
2. Usual track They form near the Andaman Sea and move towards Tamil Nadu, Andhra Pradesh and Odisha, normally striking the Tamil Nadu coast.
3. Northern recurve Some move north and strike West Bengal or Bangladesh.
4. Re-emergence A few cross the peninsula, re-emerge over the Arabian Sea, revive, recurve and strike the Maharashtra and Gujarat coasts.

Aviation hazards are poor visibility near industrial areas at dawn and dusk, fog or mist after a western disturbance, and low cloud, strong wind, heavy rain and thunder during a cyclonic storm over south India, with clear air turbulence from the subtropical jet in the north. This is the best period for flying except over the south.

The seasonal picture reduces to two cruise-wind regimes over India, one for the eight dry months and one for the four monsoon months.

The two regimes

1. October to May, eight months. Above 3 km the wind is westerly everywhere over India and strengthens with height. The subtropical jet, westerly at 80 to 150 kt, lies over the Delhi to Kolkata route near 12 km. Below 3 km it is northwesterly to westerly over the north and northeasterly over the peninsula.
2. June to September, four months. Above 6 km the wind is easterly and strengthens with height. The tropical easterly jet, easterly at 60 to 100 kt, lies over Chennai near 15 km. Below 6 km it is westerly to southwesterly over the south and southeasterly over the north.

A disciplined brief

Start with the seasonal expectation, then replace it with current evidence.

1. Season and monsoon set Which of the four seasons, and which way the low-level wind is set.
2. Current observations METARs and the satellite sequence confirm or challenge the pattern.
3. Vertical structure The significant weather and upper-wind charts give the jet, icing, cloud tops and turbulence.
4. Decision record State the route, level, alternate and fuel consequence.

The IMD forecast

Since 2007 the southwest monsoon rainfall forecast is issued in two stages, a first forecast in April and an update in June, with model errors around 4 to 5%. Seasonal rainfall forecasts are also issued for four broad regions of the country.