Circulation around lows and highs

Detail

1. Two basic patterns: cyclonic and anticyclonic circulation

• Cyclonic circulation: wind blowing around a low-pressure centre (a "low", or depression). The lowest pressure is at the centre and it rises outward.
• Anticyclonic circulation: wind blowing around a high-pressure centre (a "high", or anticyclone). The highest pressure is at the centre and it falls outward.
• Why winds spin instead of blowing straight in or out:
• The pressure gradient force (the push from high pressure towards low pressure) starts the air moving.
• The Coriolis force (a turning effect caused by the Earth's rotation) bends moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Near the ground, friction slows the wind. The air then crosses the isobars at an angle: it spirals into a low and out of a high.
• Higher up, where there is little friction, winds blow almost parallel to the isobars (geostrophic wind).

System	Centre	Northern Hemisphere	Southern Hemisphere	Surface air	Vertical motion	Weather
Cyclonic circulation	Low (L)	Anticlockwise	Clockwise	Converges (flows in)	Rises	Cloud, rain, unsettled
Anticyclonic circulation	High (H)	Clockwise	Anticlockwise	Diverges (flows out)	Sinks (subsides)	Clear, dry, settled

• Exam trap: the direction flips across the equator. A Northern Hemisphere low turns anticlockwise (NCERT Class 11 exercise 1(iii)). A Southern Hemisphere low, such as a cyclone near Mauritius or northern Australia, turns clockwise.
• The Coriolis force is zero at the equator. Organised spinning is therefore weak within a few degrees of the equator. This is why tropical cyclones rarely form there.

2. How to draw them in an answer

• Low (NH): draw closed, roughly circular isobars (lines joining places of equal pressure), with values rising outward, for example 996, 1000 and 1004 hPa. Mark "L" at the centre. Draw arrows that cross the isobars inward and curve anticlockwise.
• High (NH): draw closed isobars with values falling outward, for example 1032, 1028 and 1024 hPa. Mark "H". Draw arrows that cross the isobars outward and curve clockwise.
• SH versions: draw the same isobars but reverse the curl of the arrows.
• Add a small side-view sketch (cross-section): arrows going up over L with an outward spread at the top, and arrows coming down over H with an outward spread at the ground. This matches NCERT Fig. 9.5.

3. Convergence and divergence (NCERT Fig. 9.5)

• Convergence: air flowing together into an area from different sides. Divergence: air spreading out from an area.
• Over a low (cause → process → result):
• Surface air flows in from all sides (surface convergence).
• The air has nowhere to go but up. It rises.
• At the top of the troposphere it spreads out (divergence aloft).

• Rising air expands and cools. Its water vapour condenses into cloud. This gives cloud and rain.

• Over a high:

• Air flows together high up (convergence aloft).
• It sinks (subsides) towards the ground.
• At the surface it spreads outward (surface divergence).

• Sinking air is squeezed and warms. Cloud droplets evaporate. This gives clear, dry, settled weather.

• World examples: the subtropical high-pressure belts near 30° N and S lie under sinking air. That is one reason the great hot deserts lie near these latitudes: the Sahara, Thar, Arabian and Kalahari deserts. The ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet) is a belt of surface convergence and rising air. It gives heavy convectional rain.

4. Other ways air is lifted

• Rising air is essential for cloud and precipitation. Convergence is only one way to lift air. The others are:

Lifting mechanism	What happens	Rain type / example
Convergence	Air flowing in from all sides is forced up	Tropical depressions; ITCZ rain
Convection	The ground heats the air above it, and the warm, light air rises	Convectional rain: equatorial afternoon showers; pre-monsoon thunderstorms
Eddies	Swirling, uneven flow near the ground lifts air in pockets	Local showers
Orographic uplift	Moist wind hits a mountain and is pushed up. The side facing the wind (windward) gets rain, and the far side (leeward) stays dry	Orographic (relief) rain: Western Ghats; Shillong Plateau. The leeward rain-shadow lies on the Deccan interior
Fronts	Warm air is pushed up over a wedge of colder, heavier air	Frontal (cyclonic) rain: temperate cyclones; Western Disturbances over north-west India

• For the full classification of rainfall types, see atmospheric-moisture-precipitation.

5. Surface and upper-air flows are linked

• What happens near the ground and what happens 8–12 km up are two halves of one circulation.
• Divergence aloft deepens a low:
• If air is taken away at the top faster than it flows in at the bottom, the total mass of the air column falls.
• Surface pressure therefore drops further. The low deepens and the winds strengthen.

• This is a condition for tropical cyclones, together with warm sea surface water, a strong Coriolis force and weak vertical wind shear.

• IMD's categories for cyclonic disturbances (by wind speed) [3]:

• Cyclonic Storm: 62–87 km/h
• Severe Cyclonic Storm: 88–117 km/h
• Very Severe Cyclonic Storm: 118–165 km/h
• Extremely Severe Cyclonic Storm: 166–220 km/h

• Super Cyclonic Storm: more than 220 km/h

• Cyclonic and anticyclonic circulations also exist only higher up. IMD weather bulletins routinely list "upper air cyclonic circulations" and "anti-cyclonic circulations" by height above mean sea level. The IMD press release of 22 April 2026 is an example [3]:

• It reported upper-air cyclonic circulations over north-east Assam, south-west Rajasthan and the adjoining part of Pakistan, and north Tamil Nadu, in the lower troposphere. Upper-air cyclonic circulations of this kind help trigger thunderstorms [3].
• It reported an anti-cyclonic circulation over interior Maharashtra, adjoining Telangana and North Interior Karnataka, between 3.1 and 5.8 km above mean sea level [3].
• It reported a Western Disturbance as a trough in the mid-tropospheric westerlies, with its axis at 5.8 km, along about 52° E, north of 32° N [3].

6. Beyond NCERT: blocking highs and heat domes

• Blocking high: a strong high that stalls and does not move for days or weeks. It blocks the usual west-to-east movement of weather systems.
• Heat dome: a strong, long-lasting high-pressure system in the upper air that traps hot air under it, like a lid on a pot.
• Cause: a stalled Rossby wave pattern (Rossby waves are the large north–south bends in the upper westerlies and the jet stream; see Section 7). The jet stream bulges far towards the pole and stops moving.
• Process: air piles up and sinks. Sinking air is squeezed (compressed) and warms. Sinking air also stops clouds from forming, so the sky stays clear.
• Result: strong sunshine every day, soils dry out, and less of the sun's heat goes into evaporating water. Heat therefore builds day after day.

Case study: Pacific Northwest heat dome, June 2021

• Where: southern British Columbia (Canada) and Washington and Oregon (USA), in the last week of June 2021 [2].
• Mechanism: WMO described "a significant atmospheric blocking pattern which has led to a heat dome, with low pressure either side". The jet stream was not moving it along [2]. A high with a low on each side is the classic omega block.
• Lytton, British Columbia broke the all-time Canadian record on three days in a row [2]:

Date (2021)	Temperature at Lytton
27 June	46.6 °C (first break of the national record)
28 June	47.9 °C
29 June	49.6 °C (Canada's all-time record)

• Previous Canadian record: 45.0 °C, set on 5 July 1937 [2]. The new record was 4.6 °C higher.
• Other records [2]:
• British Columbia set 43 all-time station records by 28 June 2021.
• Seattle reached 40 °C on 27 June and 41.7 °C the next day.
• Portland reached 42 °C on 26 June and 44.4 °C on 27 June.

• More than 14 million Americans were under heat warnings or advisories.

• Climate link: a UK Met Office scientist quoted by WMO said such record June mean temperatures in the western USA would have been "almost impossible" without human-induced climate change. Climate change made them several thousand times more likely [2].

Indian link: anticyclones and heat waves

• Indian heat waves often come with an anticyclone over north-west India:
• Sinking air keeps the sky clear and warms the air by compression.

• Hot, dry winds blow in from the Thar Desert and Pakistan (hot advection: heat carried in by the wind).

• April 2026 example [3]:

• Under the upper-air anticyclone over interior Maharashtra, Telangana and North Interior Karnataka, IMD warned of heat wave conditions in Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh, Rajasthan, Madhya Pradesh, Vidarbha and Chhattisgarh during 22–27 April 2026.
• The highest temperature on 22 April 2026 was 44.2 °C at Banda (Uttar Pradesh).

• Delhi had "mainly clear sky" with westerly to north-westerly surface winds.

• IMD heat wave criteria [3]:

• A heat wave is considered only when a station's maximum temperature reaches ≥ 40 °C on the plains or ≥ 30 °C in hilly regions.
• By departure from normal: Heat Wave = 4.5 to 6.4 °C above normal. Severe Heat Wave = more than 6.4 °C above normal.
• By actual temperature: Heat Wave = ≥ 45 °C. Severe Heat Wave = ≥ 47 °C.

• Coastal stations: the departure must be more than 4.5 °C, provided the maximum temperature is ≥ 37 °C.

• For more, see insolation-heat-budget (the heat budget) and natural-hazards-disasters (heat waves as a disaster).

Prelims Hooks

• A Northern Hemisphere low turns anticlockwise, and a Northern Hemisphere high turns clockwise. Both directions reverse in the Southern Hemisphere.
• Low → surface convergence → rising air → cloud and rain. High → divergence at the surface, convergence aloft → sinking air → clear, dry weather.
• Divergence aloft helps a surface low deepen. It is a condition for tropical cyclone growth.
• Cyclonic spin is weakest at the equator because the Coriolis force there is zero.
• Ways of lifting air: convergence, convection, eddies, orographic uplift, fronts. Sinking air does not give rain.
• Heat dome = a persistent upper-level high. The heat comes from compression of sinking air plus clear skies, not from rising air.
• Lytton, British Columbia: 49.6 °C on 29 June 2021, Canada's all-time record. It replaced 45.0 °C from 1937 [2].
• IMD heat wave on the plains: the maximum must be ≥ 40 °C. Heat Wave = departure of 4.5–6.4 °C, or actual ≥ 45 °C. Severe Heat Wave = departure > 6.4 °C, or actual ≥ 47 °C [3].
• IMD Super Cyclonic Storm: wind speed > 220 km/h. Cyclonic Storm: 62–87 km/h [3].

Mains Points

• Pressure systems control weather (GS-I): lows bring convergence and rising air, which gives rain (monsoon depressions, Western Disturbances, tropical cyclones). Highs bring sinking air, which gives drought-like dry spells and deserts near 30° N and S. Maps of where highs and lows sit explain where rainfall falls in India and the world.
• Surface and upper-air coupling: weather near the ground depends on flow aloft. Divergence aloft deepens cyclones, while upper-air anticyclones suppress clouds. This is why IMD bulletins track upper-air cyclonic and anticyclonic circulations by height (for example, 3.1–5.8 km, April 2026) [3].
• Heat domes as a growing hazard (GS-III, disasters): stalled Rossby waves create blocking highs. The 2021 Pacific Northwest event broke Canada's record by 4.6 °C, and WMO-cited analysis linked it to climate change [2]. Indian heat waves under anticyclones over north-west and central India affect health, crops and power demand. Responses include IMD's impact-based heat warnings and agromet advisories, such as light irrigation and mulching, and Heat Action Plans [3].
• Answer-writing tip: draw NH and SH lows and highs with isobars and arrows, plus a side-view sketch of convergence and divergence. This earns marks quickly in questions on cyclones, monsoons or heat waves.