Other Depressions

A mountain wave has alternating lift and sink. The first strong sink sits immediately downwind of the ridge, so terrain clearance must be planned before the wave is entered.

In humid air, cap cloud, lenticular cloud, rotor or roll cloud, and layers of St, Sc, As and occasionally Ci form continuously on the windward side and dissolve to leeward, so they appear to stand still. Lenticular clouds mark the wave crests. A rotor below the first crest is the most serious visual warning, and ragged wave cloud edges show where the turbulence is. Strong low level flow that weakens or reverses higher up can produce violent rotor streaming.

The hazards are severe up and downdraughts, abrupt turbulence, unexpectedly strong winds, increased icing and deceptive altimeter behaviour. Rapid vertical currents can drive an aneroid hysteresis error, and a non standard temperature adds a further terrain clearance trap. Evening waves over smaller ranges often build after sunset as the low levels cool and stabilise; waves over large ranges are especially common in winter, when the flow is steadier and strengthens with height.

Heating from below is the common mechanism, but the surface matters. A hot continent, a warm lake and a warm winter sea do not produce the same weather on the same scale.

In a thermal depression the heated surface air expands and lifts the pressure surfaces. Outflow aloft removes mass from the column, so surface pressure falls and a cyclonic circulation develops. The low is often shallow, but if the air is unstable it can stay active much higher. Typical weather is Cu and Cb, heavy showers, thunderstorms or hail, good visibility between the showers, and moderate or severe turbulence in the convective cloud.

A polar air depression forms when Arctic maritime air moves south across a warmer winter sea. It gives showery Cu and Cb and can throw off secondary cold fronts. In winter, cold continental polar outflow from the Siberian high crossing the Caspian, Black or Mediterranean Sea produces the same convective thermal low pattern, and the Great Lakes give the North American version. Over hot land in summer, shallow lows form readily and turn thundery where an old frontal zone or general instability is present.

In the tropics the air mass temperature contrasts are weak, so wind and cloud discontinuities are more useful than the classical fronts of higher latitudes.

The ITCZ is the narrow zone where tropical air from the two hemispheres converges, usually near the equatorial trough. It migrates with the sun, holds active Cb and shower segments, and in the rainy season it coincides with the monsoon trough. Easterly waves are troughs in the tropical easterlies, best developed near 500 hPa; most of their weather lies in the rear of the trough, and they can seed depressions. India also has the pre monsoon peninsular discontinuity, dry hot continental air meeting cooler moist Bay air, which produces afternoon thunderstorms, hail and squalls.

A surge is a sudden strengthening within the same air stream, where velocity convergence forces ascent, cloud and squalls. A shear line is a sharp speed change within one air mass; when it is deep it grows broad stratiform cloud, and a fresh surge can embed Cu and Cb in it. Both are features of a vigorous monsoon.

Joshi defines the monsoon depression by two or more closed isobars at 2 hPa spacing, with surface winds of about 17 to 33 kt. The IMD scale runs low pressure below 17 kt, depression 17 to 27 kt, deep depression 28 to 33 kt, cyclonic storm 34 to 47 kt, severe cyclonic storm 48 to 63 kt, very severe cyclonic storm 64 to just under 120 kt, then super cyclone above that.

Indian monsoon depressions usually form over the north or head Bay of Bengal from June to August and then track north or northwest inland; September systems form further south. Each lasts about 3 to 5 days. The upper circulation can reach around 300 hPa and slopes to the southwest with height. The worst weather normally sits in the southwest sector as a broad belt of steady heavy rain; during recurvature the rain belt swings to the north northeast sector, with showers in the strong south to southwest flow behind.

A tropical revolving storm is a non frontal, warm core low containing organised thunderstorms, strong wind and heavy rain. It forms over warm tropical water, and not at the Equator.

The requirements are a warm sea of about 26 to 27°C or more with a deep warm layer, high humidity through the lower and middle troposphere, instability, a pre existing disturbance, enough Coriolis force between about 5 and 25 degrees of latitude, and very low vertical wind shear. Evaporation and condensation release latent heat, lower the pressure and strengthen the convergence, a self reinforcing heat engine.

A mature system has an eye, an eye wall or inner storm, an outer storm and an edge. The eye is relatively warm, holds the lowest pressure, and can be lightly clouded with light wind. The eye wall is the most dangerous ring: towering Cb, violent squalls, torrential rain, a steep pressure gradient and severe turbulence. Spiral rainbands fill the outer storm. In the Northern Hemisphere the right forward sector carries the worst weather, because the rotation and the forward motion add together there.

Oxford labels the stages as tropical depression with a closed isobar and organised circulation, tropical storm above 33 kt, and tropical cyclone, hurricane or typhoon above 63 kt. The eye is typically tens of kilometres across; a storm can last up to two weeks, although Indian systems often last only 3 to 5 days because their sea tracks are short. Over Indian seas the storms are most frequent in May, June, October and November, with the peak in October and November.

Cloud bases can reach the surface and Cb tops can climb to 18 to 20 km. Expect severe convective and frictional turbulence, strong vertical currents, torrential rain and, near coasts, storm surge. The rule in flight is avoidance. In the Northern Hemisphere a strong wind from the port side means the centre is ahead: alter away until the wind comes from starboard and eases.

The last few topics are easy to overlook because they are small or hidden, but each one changes the forecast and each one can be severe.

A secondary depression is a small low within a primary circulation. It can develop on the trailing front of an occluded primary, on a trailing cold front, or near the triple point as the primary fills. It moves cyclonically around the primary and can be more active than the parent, and even a weak secondary can delay a frontal passage. A family of frontal depressions may form one after another, each a little further southwest along the polar front.

A cold air pool is found on a thickness or upper contour chart, not on a surface pressure chart. Low 1000 to 500 hPa thickness marks the cold column. These pools are often slow moving, or drift along behind a cold front, and they turn convective over warm land. Tornadoes are very small, violent, usually cyclonic vortices under a Cb; their extreme wind and sharp local pressure fall make avoidance essential. The West African tornado is different again: it is an east to west moving thunderstorm squall line near the Gulf of Guinea, common in the ITCZ seasons, and not a miniature North American tornado.