elongated areas of low pressure. Half right and therefore wrong. A trough is indeed an elongated low, but a ridge is the opposite feature — it belongs to a high-pressure system.

elongated areas of high pressure. The same error the other way. A ridge is an elongated high, but a trough is never one; a trough is where pressure is lowest along its axis.

elongated area of high pressure and of low pressure respectively. Exactly reversed. Anyone who recalls only that the two words are opposites, without fixing which is which, has a one-in-two chance and lands here half the time.

Trough and ridge are

Q: What is the answer to CAPF 2022 Paper I Q6?

A. Correct — A, elongated area of low pressure and of high pressure respectively. On a weather chart the isobars sometimes bulge outward from a low without closing around it; that extended tongue of relatively low pressure is a trough. The mirror feature, an extended tongue of relatively high pressure reaching out of an anticyclone, is a ridge, also called a wedge. Only option (a) keeps the pair the right way round. India's best-known example is the monsoon trough, the seasonal low-pressure axis that runs from the northwest of the country to the head of the Bay of Bengal during the southwest monsoon and whose position governs where the rain falls.

(a)elongated area of low pressure and of high pressure respectively
(b)elongated areas of low pressure
(c)elongated areas of high pressure
(d)elongated area of high pressure and of low pressure respectively

Answer

Why

Correct — A, elongated area of low pressure and of high pressure respectively. On a weather chart the isobars sometimes bulge outward from a low without closing around it; that extended tongue of relatively low pressure is a trough. The mirror feature, an extended tongue of relatively high pressure reaching out of an anticyclone, is a ridge, also called a wedge. Only option (a) keeps the pair the right way round. India's best-known example is the monsoon trough, the seasonal low-pressure axis that runs from the northwest of the country to the head of the Bay of Bengal during the southwest monsoon and whose position governs where the rain falls.

Why the others are wrong

(b)elongated areas of low pressure — Half right and therefore wrong. A trough is indeed an elongated low, but a ridge is the opposite feature — it belongs to a high-pressure system.
(c)elongated areas of high pressure — The same error the other way. A ridge is an elongated high, but a trough is never one; a trough is where pressure is lowest along its axis.
(d)elongated area of high pressure and of low pressure respectively — Exactly reversed. Anyone who recalls only that the two words are opposites, without fixing which is which, has a one-in-two chance and lands here half the time.

Concept

A synoptic weather chart maps atmospheric pressure with isobars, lines joining places of equal pressure. Where the isobars close around a centre of low pressure the feature is a cyclone or depression, and where they close around a centre of high pressure it is an anticyclone. A trough and a ridge are the open, elongated versions of the same two things — a trough is an elongated area of low pressure, a ridge an elongated area of high pressure.

The two features behave as opposites in weather as well as in pressure. Air converges into a trough at the surface and rises, cools and condenses, so troughs bring cloud and rain; air sinks in a ridge and diverges at the surface, warming as it descends, so ridges bring settled, clear weather. That is why forecasters watch the axis of a trough. The trap in the option list is a language one: 'trough' is also the low point of a wave and 'ridge' is also a line of hills, and either association can invite the reversal offered in option (d). Anchor it instead on the meteorological pair — a trough goes with a low, a ridge with a high. As of the 2022 exam and still today, the India Meteorological Department uses exactly this vocabulary in its daily bulletins, and the monsoon trough remains the standard term for the seasonal low-pressure axis over northern India.

Key facts

A trough is an elongated area of relatively low pressure; its isobars do not close around a centre.
A ridge, also called a wedge, is an elongated area of relatively high pressure.
Surface air converges into a trough and rises, giving cloud and precipitation; in a ridge it subsides and diverges, giving fair weather.
The monsoon trough of the southwest monsoon season extends from northwest India to the head of the Bay of Bengal, and its northward or southward shift changes the rainfall pattern.
Closed isobars mark a cyclone or an anticyclone; troughs and ridges are the open, elongated forms of low and high pressure.

The two elongated pressure features

Trough→An elongated area of low pressure. Isobars bulge out from a low without closing. Surface convergence, rising air, cloud and rain.

Ridge, or wedge→An elongated area of high pressure. Isobars bulge out from a high. Subsiding air, surface divergence, clear and settled weather.

Cyclone or depression→The closed form of low pressure — isobars form a complete loop round the centre.

Anticyclone→The closed form of high pressure, with a complete loop of isobars round the centre.

Indian example→The monsoon trough runs from northwest India to the head of the Bay of Bengal in the southwest monsoon season.

Elongated versus closed is the shape; low versus high is the pressure. The question asks only for the second pairing.

Study next

Reading isobars — spacing, pressure gradient force and wind speedCyclones and anticyclones and their circulation in each hemisphereThe monsoon trough and its role in the distribution of Indian rainfallWestern disturbances and upper-air troughs in the westerliesFronts, depressions and the weather sequence of a mid-latitude cyclone

Common traps

Remembering only that the two words are opposites and then pairing them the wrong way round.
Carrying over the wave sense of 'trough' or the landform sense of 'ridge' into a weather question.
Treating a trough as a closed low; a trough has no closed isobars, which is what makes it elongated.

Asked as a two-term definition item where three of the four options either merge the pair or reverse it, so precision on which term goes with which pressure is the whole test.

Related PYQs

UPSC_1997_GS1_Q871997Open & attempt →

Daily weather map showing isobars is an example of

(a) Choropleth map
(b) Isopleth map
(c) Chorochromatic map
(d) Choroschematic map

Answer(b) Isopleth map

The same weather chart, approached from the cartography side. Isobars are the isolines whose shape defines a trough or a ridge, and a map drawn from such isolines is an isopleth map.

NDA_GAT_2022_II_Q1282022Open & attempt →

Consider the following statements about anticyclones: 1. Anticyclones are high pressure systems. 2. Air in the centre of the system must be subsiding. 3. Anticyclones are characterized by converging winds. How many of the above statements is/are correct?

(a) 1
(b) 2
(c) 3
(d) None

Answer(b) 2

The closed cousin of the ridge. That item confirms the two properties of a high — it is a high-pressure system and its air subsides — while rejecting convergence, since surface winds spread outward from a high rather than into it.

Practice

practice — not a real PYQ
In meteorology, an elongated area of relatively high pressure extending from an anticyclone is called
- (a)a trough
- (b)a ridge
- (c)a front
- (d)a depression
Answer(b) a ridge — also called a wedge, and associated with subsiding air and fair weather.
practice — not a real PYQ
During the southwest monsoon season, the monsoon trough extends from northwest India to
- (a)the Arabian Sea off Kerala
- (b)the head of the Bay of Bengal
- (c)the Rann of Kachchh
- (d)the Andaman Sea
Answer(b) the head of the Bay of Bengal — the axis of the trough, whose shift changes the rainfall distribution.