Route climatology over India is a sequence
A long route across India crosses more than one climate zone in a sequence. Route climatology tells the student where that sequence changes. Read it by month, route sector, height and time of day, then check it against the route forecast and the current observations.
Build the route picture
Split a long Indian route at the coastlines, the Western Ghats, the monsoon trough and the Himalayan foothills. For each sector ask which pressure system, wind regime and terrain effect rules in the month of the flight. Do not treat January and July as labels only: they are opposite low-level winds, different trough and jet positions, and different sea-surface conditions.
The two cruise-wind regimes
- October to May, eight months. Above 3 km the wind is westerly over the whole country 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.
- 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.
Forecast products fill the gaps
A tabular route forecast, the ARFOR, gives forecast weather section by section along the route. A route cross-section adds height, so cloud, freezing level and significant weather line up with terrain and flight level. These support planning and do not replace the aerodrome TAF.
The Delhi to Kolkata route
The Delhi to Kolkata sector runs along the Gangetic plain, close to the axis of the winter subtropical jet and the summer monsoon trough. It is the classic Indian jet-stream route.
October to May
Above 3 km the wind is westerly and strengthens with height. The subtropical jet lies along this route near 12 km, westerly at 80 to 150 kt. Westbound flights meet a strong headwind and eastbound flights a strong tailwind. Winter also brings western disturbances, morning radiation fog at Delhi, Lucknow and the Bihar fields, and clear air turbulence near the jet.
June to September
The monsoon trough often lies along or near this route. With the trough in its normal position the whole sector has well-spread rain, embedded cumulus and cumulonimbus in otherwise stratiform cloud, and low cloud with strong surface wind when a monsoon depression tracks across from the Bay. In a break the plains sector goes dry and hazy while the heavy rain shifts north to the foothills.
Reading it
- Jet level and core Use the upper-wind chart for flight level and groundspeed.
- Fog at the terminals Winter mornings at Delhi and the Bihar fields.
- Trough position Normal, south of normal or break decides the rain.
- Monsoon depressions Treat a northwest-moving Bay low as a low-cloud and surface-wind event.
The west coast and the Western Ghats
Routes along the west coast, Mumbai to Goa to Mangalore to Kochi, and any route crossing the Western Ghats, are dominated by orographic rain during the southwest monsoon.
Southwest monsoon
The moist southwesterly monsoon current strikes the Ghats almost at right angles. The windward slope and the coastal strip get heavy rain, low cloud and poor visibility in precipitation, while just to the lee, over the Deccan, rainfall drops sharply. Cloud base and visibility on the coast can be marginal for hours. The Arabian Sea branch feeds this rain, and a mid-tropospheric cyclone off the coast can make it extreme, as in the 2005 Mumbai rains.
The other seasons
Winter and pre-monsoon are largely dry on the west coast, with land and sea breezes and haze. In the post-monsoon the rain belt is on the east coast, not here.
Reading it
- Wind angle to the Ghats Near right angles gives the heaviest orographic rain.
- Windward against lee A short crossing can go from heavy rain to almost dry.
- Coastal cloud base Can stay low for hours in the monsoon.
- Offshore systems Check for a mid-tropospheric cyclone or an offshore trough.
The east coast and the Bay of Bengal
Routes down the east coast, Kolkata to Visakhapatnam to Chennai, and trans-Bay routes to the Andamans or Southeast Asia, are governed by the Bay of Bengal, the more active of the two seas for tropical cyclones.
Post-monsoon, October and November
The northeast monsoon brings the main rains to Tamil Nadu, coastal Andhra Pradesh and Puducherry. The Bay produces the maximum number of cyclonic storms in October, and severe storms are more frequent in November. They usually form near the Andaman Sea and track towards the Tamil Nadu, Andhra and Odisha coasts. Some recurve north to West Bengal and Bangladesh, and a few cross the peninsula and revive over the Arabian Sea.
Southwest monsoon
Monsoon depressions form in the north Bay, 2 to 3 a month, and move northwest across the coast, bringing low cloud, strong surface wind and well-spread rain inland.
Reading it
- Cyclone bulletins October and November dispatch into Chennai, Visakhapatnam or the Odisha coast needs a live check.
- Monsoon depression track A northwest-moving low is a low-cloud and wind event.
- Night rain maximum Northeast-monsoon rain falls mostly between 2100 and 0300 IST.
- Trans-Bay routes Treat convection as a routing and fuel problem.
Peninsular and Himalayan sectors
Two more Indian route types need their own thinking: trans-peninsular routes across the Deccan, and northern routes near the Himalaya.
Trans-peninsular routes
Mumbai to Chennai, Bengaluru to Kolkata and similar routes cross the Deccan plateau. The plateau is in the lee of the Western Ghats, so it is comparatively dry in the southwest monsoon, but afternoon thunderstorms are common in the pre-monsoon and at the monsoon onset. The tropical easterly jet lies over the south near Chennai at about 15 km in the monsoon, easterly at 60 to 100 kt.
Northern and Himalayan sectors
Routes to Srinagar, Leh, Dehradun and the northeast hills add mountain waves, clear air turbulence near the subtropical jet in winter, and rapid cloud build-up over rising ground. Winter western disturbances give clouding and precipitation over Jammu and Kashmir and Himachal. Terrain means a cloud base or freezing level has a different operational meaning from the same figure at a plains field.
Reading it
- The Deccan is a lee area Drier in the monsoon, but pre-monsoon thunderstorms are the risk.
- Easterly jet in the south Use it for level and groundspeed in the monsoon.
- Mountain waves and clear air turbulence Northern sectors in winter.
- Terrain-referenced minima Read cloud base and freezing level against the ground, not sea level.
Route forecasts and a usable India brief
The seasonal picture becomes a flight plan through two products, the tabular route forecast and the route cross-section, each checked against current charts and reports.
The tabular route forecast, ARFOR
It gives forecast weather section by section along the route: wind and temperature at chosen levels, cloud, weather, visibility, freezing level and significant weather, with a validity period. Read it sector by sector, not as one block.
The route cross-section
This is a vertical slice from departure to destination. Draw the route line, then place each feature by horizontal position and by height, so a cloud top over one sector is not applied to the whole route. Use it for level selection with the freezing level, cloud tops, jet and terrain together.
A disciplined brief
- Season and monsoon set Which season, and which way the low-level wind is set.
- Sector split Coastlines, the Ghats, the trough and the foothills.
- Current evidence METARs, satellite and radar confirm or challenge the seasonal pattern.
- Vertical structure Cross-section and upper-wind chart for the jet, icing, tops and turbulence.
- Decision record Route, level, alternate and fuel consequence.