Important parallels and the heat zones

Detail

Parallels: the basics

• Parallel of latitude: an east–west circle on the globe. Every point on it is the same angular distance north or south of the Equator (0°).
• Latitude runs from 0° at the Equator to 90° at each pole. Parallels never meet. They stay equally spaced. One degree of latitude covers about 111 km on the ground [6].
• Four parallels after the Equator have special names. They are not chosen at random. Each one marks a limit of how the sun's rays fall on the earth.

Parallel	Latitude (rounded)	Hemisphere	What it marks
Tropic of Cancer	23½°N	Northern	Northern limit of the vertical noon sun
Tropic of Capricorn	23½°S	Southern	Southern limit of the vertical noon sun
Arctic Circle	66½°N	Northern	Where the midnight sun and polar night begin
Antarctic Circle	66½°S	Southern	Where the midnight sun and polar night begin

Tropics of Cancer and Capricorn: limits of the vertical sun

• Vertical (overhead) sun: at noon the sun stands straight above your head (at 90°) and casts no shadow.
• The sun can be directly overhead at noon only between 23½°N and 23½°S. It is never overhead anywhere north of the Tropic of Cancer or south of the Tropic of Capricorn.
• How this happens:
• The earth's axis is tilted. As the earth goes round the sun, the belt where the sun is overhead moves north and then south.
• Around 21 June (summer solstice in the north), the sun is overhead at the Tropic of Cancer. This is its northernmost point [4][5]. On 21 June 2022 the solstice came at 5:14 a.m. EDT [4].
• Around 22 December (winter solstice in the north), the sun is overhead at the Tropic of Capricorn.

• Around 21 March and 23 September (equinoxes), the sun is overhead at the Equator.

• So any place between the tropics gets the overhead sun twice a year. A place on a tropic gets it once a year. A place outside the tropics never gets it [5].

• About the names: the tropics are named after the star groups (constellations) the sun stood in at the solstices about 2,000 years ago. The earth's axis slowly wobbles (precession). Because of this, the June solstice sun now stands in Taurus, not Cancer [5].
• Ground markers: some countries put up monuments on the line. One stands in Taiwan, between Hualien and Taitung, on about 23.5°N [5].
• In India: the Tropic of Cancer runs east–west through the middle of the country [7]. It divides India into roughly equal halves. The northern half lies in the temperate latitudes and the southern half in the torrid zone. It crosses eight states, west to east: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.

Arctic and Antarctic Circles: limits of the midnight sun and polar night

• Midnight sun: the sun stays above the horizon for a full 24 hours.
• Polar night: the sun stays below the horizon for a full 24 hours.
• The Arctic Circle (66½°N) and Antarctic Circle (66½°S) mark where these begin.
• On the June solstice, every place north of 66½°N gets at least one full day of 24-hour daylight. Every place south of 66½°S gets at least one full day of 24-hour darkness.

• On the December solstice, the pattern is reversed.

• The effect grows towards the poles:

• On the polar circle itself, there is about one day a year of each.
• At the poles (90°), there is about six months of continuous day and six months of continuous night.
• At the June solstice, the North Pole gets 24 hours of daylight while the South Pole stays dark [4].

Why these exact numbers: the axial tilt

• Axial tilt (obliquity): the angle between the earth's axis and a line at right angles to the plane of its orbit. Today it is about 23.4° (often rounded to 23½°).
• The tropics sit at the same latitude as the tilt. The Tropic of Cancer's latitude matches the tilt of the earth's rotation axis to its orbital plane [5].
• The polar circles sit at 90° minus the tilt, so 90° − 23½° = 66½°.
• The lines are not fixed:
• The tilt changes very slowly, so the tropics and polar circles move slowly too.
• Over a cycle of about 41,000 years, the tilt (and so the Tropic of Cancer) varies between about 22.1° and 24.5° [2].
• The Tropic of Cancer is now at about 23°26′, so the Arctic Circle is at about 66°34′ (NCERT: 23½°N and 66½°N).

Heat zones

• Heat zones (temperature zones): broad belts on the earth based on how much sun heat (insolation) each latitude gets. The angle of the sun's rays and the length of the day set this.
• The rule:
• Near the Equator the rays are vertical or nearly vertical. The heat falls on a small area, so it is hot.
• Towards the poles the rays are more slanting. The same heat spreads over a larger area and passes through more air, so it gets colder.

Zone	Latitudes	Number	Sun's rays	Class 6 climate word
Torrid	0°–23½° N and S (between the two tropics)	1	Overhead at noon at least once a year	Hot
Temperate	23½°–66½° N and S	2 (North and South)	Never overhead; the angle falls towards the poles	Moderate
Frigid	66½°–90° N and S	2 (North and South)	Very slanting; midnight sun and polar night occur	Cold

• Torrid zone:
• The largest single belt. It covers about 36% of the earth's land and holds about one-third of the world's people [3].
• Average temperatures are about 25–28°C all year [3].
• Seasons depend on rain (wet and dry), not on temperature [3].

• Rainfall varies a lot: nearly 3 m a year in the Amazon Basin but only 2–10 cm in the Sahara [3]. Latitude alone does not decide rainfall.

• Temperate zones: clear summer and winter seasons. Most of the world's middle-latitude farmland and big cities are here (for example Europe, China and the USA in the north; Argentina, South Africa and southern Australia in the south).

• Frigid zones: the Arctic Ocean, Greenland, northern Siberia and Canada in the north; Antarctica in the south. Long cold winters, and 24-hour day or night in parts of the year.
• India's position: India lies between about 8°4′N and 37°6′N. The Tropic of Cancer crosses the middle of the country, so India lies partly in the torrid zone and partly in the north temperate zone. Even so, India's climate as a whole is tropical monsoon, because the Himalayas block cold winds from Central Asia.

Heat zones vs. real climate belts

• Heat zones are a simple model based on latitude only. Real climate types, such as the Koeppen types (a scheme that sorts climates by temperature and rainfall), do not follow 23½° and 66½° exactly.
• What else changes the climate:
• Altitude: temperature falls with height (about 6.5°C per 1,000 m). Quito (Ecuador) and Mount Kilimanjaro are both near the Equator but cool, and Kilimanjaro has snow.
• Ocean currents: the warm North Atlantic Drift keeps the coast of Norway ice-free beyond 66½°N. The cold Peru (Humboldt) current makes the coast of Peru and Chile cool and dry within the tropics.

• Distance from the sea (continentality): inland areas have hotter summers and colder winters. Siberia is far more extreme than western Europe at the same latitude.

• How latitude controls heating (the angle of the rays, the length of the day, the path through the atmosphere) is covered under the insolation and heat budget topic.

Prelims Hooks

• The sun is overhead at noon only between 23½°N and 23½°S. It is never overhead at Delhi (about 28.6°N) or Srinagar.
• Places between the tropics get the overhead noon sun twice a year. Places on a tropic get it once a year.
• The polar circles sit at 90° minus the tilt: 90° − 23½° = 66½°. The tropics sit at the same latitude as the tilt.
• The tilt varies between 22.1° and 24.5° over about 41,000 years, so the Tropic of Cancer (now about 23°26′) slowly shifts [2].
• Trap: the June solstice sun is now in Taurus, not Cancer, because of precession [5].
• States on the Tropic of Cancer, west to east: Gujarat → Rajasthan → Madhya Pradesh → Chhattisgarh → Jharkhand → West Bengal → Tripura → Mizoram (8 states).
• Count of heat zones: five belts, made of one torrid, two temperate and two frigid zones.
• The torrid zone covers about 36% of the earth's land and holds about one-third of humanity [3].
• One degree of latitude is about 111 km [6].
• Trap: heat zones are set by latitude only. Altitude, ocean currents and continentality decide the real climate types.

Mains Points

• Latitude sets the base heat budget:
• The angle of the sun's rays and the length of the day decide how much heat each zone gets.
• This gives the torrid, temperate and frigid pattern and, broadly, the sequence of natural vegetation from equatorial forests to tundra.

• Altitude, ocean currents and continentality change this pattern locally. Examples: snow on Kilimanjaro on the Equator, and Norway's ice-free ports beyond 66½°N.

• India's transitional position:

• The Tropic of Cancer splits India into a torrid south and a temperate north.
• The Himalayan barrier and the monsoon still give the whole country a tropical monsoon climate.

• This explains why the north has a wide yearly temperature range while the peninsula stays warm all year.

• Human geography of the tropics:

• The torrid zone holds about one-third of the world's people on 36% of its land [3].
• It has wet–dry seasons rather than temperature seasons, so farming, water stress and disease patterns follow the rains.

• This matters for food security and climate adaptation in tropical countries such as India.

• Polar circles and the cryosphere:

• Long polar nights and very slanting rays make the frigid zones cold.
• These zones hold the sea ice and ice sheets, which are sensitive to warming.
• Changes there affect sea level and weather patterns in the lower latitudes.