Atmospheric Pressure and Winds

ICSE Class 9 · Geography · Chapter 14

What this chapter covers

Meaning of atmospheric pressure
Factors that affect atmospheric pressure
Major pressure belts of the world
Factors affecting the direction and velocity of wind
Permanent winds
Periodic winds
Local winds
Variable winds
Jet streams

Key definitions

Atmospheric pressure is the force exerted by the weight of the column of air above a unit area of the Earth's surface.

Air pressure is another name for atmospheric pressure, and the two terms are used interchangeably in weather reports.

Normal atmospheric pressure is the average pressure at sea level, equal to 1013.25 mb or the pressure that supports a column of mercury 76 cm (760 mm) high.

Barometer is the instrument used to measure atmospheric pressure.

Isobars are lines drawn on a map joining places that have the same atmospheric pressure, after the readings have been reduced to sea level.

Pressure belts are broad zones of high or low atmospheric pressure that run roughly parallel to the lines of latitude around the Earth.

The Equatorial Low Pressure Belt is the zone of low pressure lying roughly between 5° North and 5° South of the Equator.

The Inter-Tropical Convergence Zone (ITCZ) is the zone near the Equator where the trade winds of the two hemispheres meet and the air is forced upward.

The Subtropical High Pressure Belts are two zones of high pressure lying roughly between 30° and 35° latitude in each hemisphere.

Horse latitudes is the name sailors gave to the calm subtropical high pressure belts, especially near 30° to 35° North.

The Sub-polar Low Pressure Belts are two zones of low pressure lying roughly between 60° and 65° latitude in each hemisphere.

The Polar High Pressure Belts are the zones of high pressure around the North and South Poles, roughly beyond 80° latitude.

Key points

Atmospheric pressure is the weight of the air column on a unit area; it is measured in millibars with a barometer, and normal sea-level pressure is 1013.25 mb.
Pressure falls with altitude, about 1 mb per 10 m near the ground, and is lower where air is warm or humid.
Seven pressure belts alternate high and low: equatorial low, two subtropical highs, two sub-polar lows and two polar highs; they shift with the overhead Sun.
The equatorial low and polar highs are thermally induced; the subtropical highs and sub-polar lows are dynamically induced.
Wind blows from high to low pressure; the pressure gradient sets its speed, the Coriolis effect deflects it, and friction slows it near the ground.
Ferrel's law: deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Permanent winds are the trades, westerlies and polar easterlies; the Southern westerlies are the Roaring Forties, Furious Fifties and Shrieking Sixties.
Periodic winds are land and sea breezes, which change daily, and monsoons, which reverse with the seasons.
Local winds named in the syllabus: the hot Loo of north India, the warm Chinook and Foehn on leeward slopes, and the cold Mistral of the Rhone valley.
Cyclones are low-pressure systems with inward, anticlockwise (Northern Hemisphere) winds and stormy weather; anticyclones are high-pressure systems with outward, clockwise winds and fair weather.
Jet streams are fast westerly winds in the upper troposphere that affect the monsoon, weather systems and air travel.

Questions answered

Define atmospheric pressure and name the instrument used to measure it.

Atmospheric pressure is the force exerted by the weight of the column of air above a unit area of the Earth's surface. It is measured with a barometer, either mercury or aneroid.

Why does pressure decrease with altitude?

The column of air above is shorter at a height, and the upper air is thinner, so there is less weight pressing down.

Give a reason why humid air exerts less pressure than dry air.

Water vapour molecules are lighter than nitrogen and oxygen molecules, so moist air is less dense than dry air.

Why are the subtropical high pressure belts called dynamically induced?

Their high pressure results from air that rose at the Equator sinking and piling up near 30°, because of the Earth's rotation, and not from low temperature.

What are the Doldrums and why are they calm?

The Doldrums are the equatorial low pressure belt; the heated air there mostly rises, so horizontal surface winds are weak.

Calculate the pressure gradient between two places 250 km apart with readings of 1012 mb and 1002 mb.

(10)/(250) × 100 = 4 mb per 100 km.

State Ferrel's law.

Freely moving winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Why do the trade winds of the Northern Hemisphere blow from the north-east?

They move from the subtropical high towards the Equator, that is, southward, and the Coriolis effect turns them to the right, so they arrive from the north-east.

Past board exam questions

ICSE 2019 sample paper · 2 marks

(i) What are the Jet streams? (ii) State any one importance of Jet streams.

Show answer

(i) Jet streams are narrow belts of very fast-moving winds (often 150-300 km/h or more) blowing from west to east in the upper troposphere, near the tropopause, at heights of about 9-12 km. (ii) Any one: they influence the weather of the regions below, e.g. the sub-tropical westerly jet stream shifting north of the Himalayas helps the onset of the south-west monsoon in India (and the easterly jet affects monsoon rainfall); aircraft flying eastwards use them to save time and fuel.

ICSE 2019 sample paper · 2 marks

(i) What is Coriolis Effect? (ii) How does it affect the planetary winds?

Show answer

(i) The Coriolis effect is the deflection of freely moving objects such as winds and ocean currents caused by the rotation of the Earth: to the right in the Northern hemisphere and to the left in the Southern hemisphere (Ferrel's law). (ii) Planetary winds do not blow straight from high to low pressure. The trade winds become North-East trades in the Northern hemisphere and South-East trades in the Southern hemisphere; the westerlies become South-Westerlies and North-Westerlies respectively; the polar winds become north-easterlies and south-easterlies.

ICSE 2019 sample paper · 2 marks

Name the region in which the following local winds blow: (i) Chinook (ii) Mistral (iii) Foehn (iv) Loo

Show answer

(i) Chinook - eastern slopes of the Rocky Mountains (USA and Canada; the Prairies). (ii) Mistral - the Rhone valley in southern France (towards the Gulf of Lions). (iii) Foehn - the northern slopes (valleys) of the Alps in Switzerland. (iv) Loo - the plains of northern India and Pakistan (Punjab to Bihar).

ICSE 2019 sample paper · 2 marks

(i) What are ‘doldrums’? (ii) Why is it so called?

Show answer

(i) The doldrums is the equatorial low-pressure belt lying roughly between 5°N and 5°S, where the trade winds of the two hemispheres converge and the air rises. (ii) It is called the doldrums (belt of calm) because there are no steady surface winds - the air mostly rises vertically, so the region is calm and sailing ships were often stranded there.

ICSE 2019 sample paper · 3 marks

Draw a well labelled diagram of sea breeze.

Show answer

Diagram (daytime): land and sea with the sun shining. Land heats faster, so warm air over the land rises (low pressure); the air over the cooler sea has high pressure. Cool air (sea breeze) blows from the sea towards the land near the surface, and an upper return current flows from land to sea, completing the circulation. Labels: Sun/day, land (hot, low pressure), sea (cool, high pressure), rising warm air, sea breeze, return current.

ICSE 2019 sample paper · 3 marks

Give a reason for each of the following: (i) Roaring forties and furious fifties are found in the Southern hemisphere. (ii) Low atmospheric pressure prevails over the circum polar region. (iii) Cyclones are always followed by anticyclones.

Show answer

(i) In the Southern hemisphere between 40° and 60°S there is almost uninterrupted ocean, so there is no land or mountain to cause friction; the westerlies blow there with great force and roar, hence the roaring forties and furious fifties. (ii) Over the circum-polar region (around 60°-65°) the rotation of the Earth throws the air outwards (away from the poles towards the lower latitudes) and the air rises where the westerlies meet the cold polar winds; this is a dynamically produced sub-polar low-pressure belt. (iii) In a cyclone air converges and rises at the centre; this rising air must descend somewhere, and the descending air builds up high pressure from which winds blow outwards. Hence …