Pressure belts, winds and local winds

The first principle

Wind is air moving from high pressure to low pressure, and pressure differences exist because the sun heats the Earth unequally. Three forces then shape the wind: the pressure gradient force starts it, the Coriolis force (the effect of the Earth's rotation) turns it, and friction with the surface slows it. Mountains add a fourth influence, because air forced over a range must rise and cool on one side and sink and warm on the other, which explains most local winds and local climates near mountains.

The core

The forces that make wind

• Pressure gradient force. Pressure falls from high to low over a distance, and the steeper this gradient, the faster the wind; closely spaced isobars on a weather map mean strong winds.
• Coriolis force. Because the Earth rotates, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern; the deflection is zero at the equator and greatest at the poles, which is why tropical cyclones do not form within about 5° of the equator.
• Friction. Near the ground friction slows the wind and weakens the Coriolis deflection, so surface winds cross isobars towards low pressure; above about 1 km the two other forces balance, giving a geostrophic wind parallel to the isobars.

The three cell model and pressure belts

• At the equator intense heating makes air rise, creating the equatorial low, also called the Inter Tropical Convergence Zone (ITCZ). The air spreads poleward aloft, cools, and, turned eastward by the Coriolis force, piles up and sinks near 30° north and south to form the subtropical highs. This loop is the Hadley cell, and the sinking dry air explains why most hot deserts lie near the tropics.
• Near the poles cold dense air sinks to form the polar highs and flows out towards about 60°, where it meets warmer air and rises at the subpolar lows; this is the polar cell. Between the two, the Ferrel cell is driven indirectly, with surface air flowing poleward from 30° to 60°.
• The surface result is seven pressure belts: the equatorial low, two subtropical highs, two subpolar lows and two polar highs.

Planetary winds

• Trade winds blow from the subtropical highs to the equatorial low; turned by the Coriolis force, they become northeast trades in the Northern Hemisphere and southeast trades in the Southern.
• Westerlies blow from the subtropical highs towards the subpolar lows; over the open Southern Ocean, with little land to cause friction, they become the Roaring Forties.
• Polar easterlies blow from the polar highs towards the subpolar lows.

Seasonal shift of the belts

• The belts follow the overhead sun, moving about 5° to 10° north in the northern summer and south in the northern winter. This shift explains Mediterranean climates: they lie under the dry subtropical high in summer and in the rain bearing westerlies in winter. Over India the ITCZ moves far north, to about 20° to 25° north over the heated land, which helps draw in the southwest monsoon (see the note on the monsoon).

Jet streams

• Jet streams are narrow belts of very fast wind near the tropopause, formed where strong temperature contrasts create a steep pressure gradient aloft.
• The subtropical westerly jet lies near 30° at about 12 km; in winter it splits around the Tibetan Plateau, and its southern branch over north India steers western disturbances that bring winter rain to Punjab and snow to the western Himalaya. Its shift north of the Himalaya in early June is linked to the burst of the monsoon.
• The polar front jet lies over the meeting of polar and tropical air and steers mid latitude cyclones.
• The tropical easterly jet blows from east to west over peninsular India and the Arabian Sea at about 14 to 16 km in the summer monsoon, and it is linked to the strength of the monsoon rainfall.

Dimensions

Local winds

• Loo. A hot, dry and dusty westerly that blows over the plains of north and northwest India and Pakistan on afternoons in May and June. It forms because the land under a cloudless sky is baked to very high temperatures, and air moving in from the Thar and beyond is heated further by contact with this ground. Temperatures often exceed 45°C, causing heatstroke, dehydration and deaths, especially among outdoor workers, along with crop and livestock stress, drying of water bodies and a surge in electricity demand.
• Chinook. A warm, dry and gusty wind that descends the eastern slopes of the Rockies in the United States and Canada, mainly in late winter and spring. Moist Pacific air rises over the mountains, loses its moisture as rain and snow, then sinks on the lee side and warms by compression. Because it can melt or evaporate snow within hours, it is called the snow eater, and it opens grazing land early.
• Foehn. The same lee side mechanism on the northern slopes of the Alps in Switzerland and Austria, mainly in spring and autumn. It melts snow and helps ripen grapes on valley slopes, but raises the risk of avalanches and forest fires.

Why lee side winds are warm. Rising air expands and cools at the dry adiabatic rate of about 10°C per kilometre until it becomes saturated; after that, condensation releases latent heat, so it cools at only about 6°C per kilometre. On the lee side the air, now dry, sinks and is compressed, warming at the full 10°C per kilometre. It therefore reaches the foot of the lee slope warmer than it was at the same height on the windward side, and with much lower humidity.

Other local winds in brief

• The mistral (Rhone valley, France) and bora (Adriatic coast) are cold drainage winds falling from mountains. The sirocco is a hot Saharan wind that turns humid after crossing the Mediterranean to southern Europe, and the harmattan is a dry, dusty winter wind over West Africa, called the doctor because it relieves humid heat.

In practice

Alignment of mountain ranges and local weather

• North south ranges lie across the westerlies and so create sharp contrasts between windward and leeward sides. The Rockies and the Coast Ranges catch Pacific moisture on their western slopes and leave dry plains to the east, where Chinooks blow. The Andes create the reverse pattern in the tropics: the southeast trades drop rain on the eastern Amazon side, and the cold Humboldt current plus the rain shadow of the Andes make the Atacama on the western side one of the driest places on Earth; farther south in the westerly belt, southern Chile is very wet and Patagonia in Argentina is dry. The Western Ghats bring very heavy rain to the Konkan and Malabar coasts and leave the interior Deccan in rain shadow.
• East west ranges act as walls between air masses. The Himalaya block cold, dry Siberian air from entering India in winter, keeping north India warmer than places at the same latitude in China, and they trap the monsoon winds, forcing them to shed rain over the plains. The Alps separate the mild Mediterranean from central Europe and produce the Foehn on their northern side. The Atlas Mountains give rain to the Moroccan coast and leave the Sahara to the south.
• Slope and valley winds. By day heated slopes send air upward as an anabatic or valley breeze, often building afternoon clouds. At night cold dense air drains down as a katabatic or mountain breeze and collects in valley floors, causing frost and inversions, which is why hill orchards are planted on slopes rather than valley floors.

Cases

• Heat waves and the Loo. The National Disaster Management Authority (NDMA) issued guidelines on heat waves (2016, revised 2019) that led states and cities to prepare heat action plans. Ahmedabad's Heat Action Plan (2013), the first in South Asia, uses early warnings, cool roofs, drinking water points and changes to working hours.
• Rain shadow in Ladakh. The Great Himalaya block monsoon moisture, so Leh receives only about 100 mm of rain a year and depends on glacier and snow melt.

Issues and way forward

• Hotter and longer heat waves. Warming raises baseline temperatures, so the same Loo now produces more dangerous heat; the Sixth Assessment Report (2021) of the Intergovernmental Panel on Climate Change projects more frequent and intense heat extremes in South Asia. Heat action plans in every vulnerable district, shaded workplaces and rest breaks for outdoor workers, and the heat warnings of the India Meteorological Department (IMD) are the remedy, as the NDMA guidelines recommend.
• Changing jet streams. A wavier polar front jet as the Arctic warms may make heat waves and cold spells last longer, though the evidence is still debated and needs better observation.

Common confusions

• Thermal and dynamic belts. The equatorial low and polar highs come from heating and cooling; the subtropical highs and subpolar lows are dynamic, created by the movement of air in the cells.
• Naming of winds. A wind is named after the direction it comes from, so a westerly blows from the west towards the east.
• Loo and Foehn. The Loo is hot because of the land it crosses; the Foehn and Chinook are warm because they sink and are compressed.
• Katabatic and anabatic. Katabatic winds flow down slopes, usually at night; anabatic winds flow up slopes, usually by day.

For the exam

• Keyword bank: pressure gradient force, Coriolis force, geostrophic wind, Hadley cell, ITCZ, subtropical high, trade winds, westerlies, jet stream, western disturbance, Loo, Chinook, snow eater, Foehn, adiabatic warming, rain shadow, orographic rain, katabatic, anabatic.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): In January 1972 a Chinook raised the temperature at Loma, Montana, by about 57°C in 24 hours, a record that shows how powerful adiabatic warming can be.
  • Introduction (report): The NDMA's heat wave guidelines treat the Loo season as a disaster risk, and Ahmedabad's Heat Action Plan has shown how warnings and simple measures save lives.
  • Conclusion: Global pressure belts set the broad wind pattern, but mountains and local heating decide the weather a farmer or a city actually experiences.
  • Conclusion: As warming intensifies hot winds and alters jet streams, understanding these winds is basic to protecting health, crops and water supplies.