Tropical Systems and Cyclones

Q: With no fronts in the tropics, where does the weather come from?

Tropical air masses differ little in temperature, so instead of fronts there are lines of convergence: the ITCZ (where the trades meet), easterly waves along it, the peninsular discontinuity (dry line) in spring, and the monsoon's surges and shear lines.

Q: Where do the trade winds meet, and how far does that line move?

The ITCZ is the narrow zone where the trade winds of the two hemispheres converge, along the equatorial trough. It follows the sun: in July it reaches about 20°N (over India it becomes the monsoon trough); in January it lies south of the equator, down towards Australia, though never south of 5°S over the South Atlantic.

Meteorology · Chapter 18 · DGCA CPL and ATPL

The weather-makers of the tropics, from the ITCZ to the monsoon depressions and the great tropical cyclones of the Bay and the Arabian Sea, and how to fly around one.

Remember

No fronts in the tropics: discontinuities instead. ITCZ = equatorial front, up to 20°N in summer, towards Australia in winter, never south of 5°S in the S Atlantic.
Easterly wave: trough in the easterlies, ~1500 km, best at 500 hPa, weather behind it, moves E to W. PD (dry line): central peninsula, Apr-May, afternoon TS with hail. Surge (velocity convergence) and shear line: vigorous monsoon.
Classes (kt): low <17, depression 17-27, deep 28-33, CS 34-47, SCS 48-63, VSCS 64-119, super ≥120. T numbers 1.5 to 8.0.
Monsoon depression: head Bay N of 18°N (Jun-Aug), 3-5 days, worst SW sector (NE when recurving), slopes SW with height, Assam fair.
TC needs SST > 26 °C to 60 m, RH to 7000 m, instability, Coriolis (5-25°), shear < 20 kt. Eye 30-65 km; wall worst; gales to 400 km; 1000-900 hPa, steep to 100 km; 300-500 km/day.
Worst: right forward sector (NH). Wind from port → centre ahead. Bay 4.5, Arabian Sea 1.1 a year; max Oct-Nov. Indian cyclones short-lived (3-5 days), less intense.
Frontal depressions: polar front wave → warm and cold fronts → occlusion; move W to E; families of 3-4, each to the SW.

With no fronts in the tropics, where does the weather come from?

Where tropical weather starts

Five tropical 'lines' to know. Tap Next.

Where the trades meet The inter-tropical convergence zone is where the air from the north meets the air from the south, along the equatorial trough. It follows the sun: up to about 20°N in summer (where it becomes the monsoon trough), down towards Australia in winter. It circles the globe but is active only in patches 75-150 km long, with CB and heavy showers. Flying from India to SE Asia or Australia in winter, you cross it. Also called the equatorial front.
Ripples in the easterlies Troughs in the tropical easterlies that form on the ITCZ, best seen at 500 hPa and above, about 1500 km long, with most weather behind the trough. In India they come in from the Andaman Sea across the south peninsula (10-15 kt) and help pre-monsoon and monsoon depressions form.
The peninsular discontinuity In April and May a line over the peninsula south of 23°N separates hot, dry air from the north and cooler, moist air from the Bay. Along it: afternoon thunderstorms, hail, NW squalls and heavy showers, for 3-4 days.
A sudden push A surge is a sudden strengthening of the wind within the same air mass: the air arrives faster than it can leave, piles up and rises (velocity convergence). Common over the Arabian Sea in the monsoon: 40 kt behind the surge line, 10-15 kt ahead, with overcast skies, sharp showers and squalls.
Fast beside slow Where the wind speed in the same air mass drops sharply from south to north (or the reverse), there is a shear line, with thick layer cloud up to 500 km across, and CB where a surge arrives. Surges and shear lines are features of a vigorous monsoon.

Takeaway: Tropical air masses differ little in temperature, so instead of fronts there are lines of convergence: the ITCZ (where the trades meet), easterly waves along it, the peninsular discontinuity (dry line) in spring, and the monsoon's surges and shear lines.

Where do the trade winds meet, and how far does that line move?

The ITCZ in 3D

A real NASA picture of the Earth for July and January. Drag to turn it.

Where the trades meet Trade winds blow from the subtropical highs towards the equator: north-easterly in the north, south-easterly in the south. Where they meet, air converges and rises along a narrow zone: the Inter-Tropical Convergence Zone, along the equatorial trough (the low of the doldrums). Another name: the equatorial front.
January: south of the equator In the northern winter the ITCZ lies south of the equator over the Indian Ocean, reaching towards northern Australia. Flying from India to SE Asia or Australia in winter you cross it. Rarely it is pushed into the south Bay and helps a cyclone form.
July: up to 20°N It follows the sun north. In July it reaches about 20°N; over India it merges with the monsoon trough, and the SE trades cross the equator to arrive as the SW monsoon.
Round the whole globe It circles the Earth, but is active only in patches about 75-150 km long, with CB and heavy showers; elsewhere scattered showers from big CU/CB. It never goes south of 5°S over the South Atlantic, which is why no cyclones form there. There are no tropical cyclones in the South Atlantic: the ITCZ hardly ever moves far enough south there.

Takeaway: The ITCZ is the narrow zone where the trade winds of the two hemispheres converge, along the equatorial trough. It follows the sun: in July it reaches about 20°N (over India it becomes the monsoon trough); in January it lies south of the equator, down towards Australia, though never south of 5°S over the South Atlantic.