The Northern and Southern Hemispheres

The Northern and Southern Hemispheres are the most familiar of the several “hemispheres” Earth is divided into. The equator splits the planet into a northern half and a southern half; the /learn/the-equator pillar covers the line itself and the /learn/the-eastern-and-western-hemispheres support covers the longitude-bisected pair. This article covers what distinguishes the two equator-bisected halves: the asymmetric distribution of land, water, and people; the opposite seasons; the different stars; the opposite-rotating cyclones; and the climatic consequences of having most of the planet's landmass in one half.

Definition

A hemisphere is half of a sphere — any plane through the centre divides the sphere into two hemispheres. “Northern” and “Southern” specifically refer to the two halves separated by the equator (the plane perpendicular to Earth's rotation axis through the centre). Latitude is positive in the Northern Hemisphere (from 0° at the equator to +90° at the North Pole) and negative in the Southern (from 0° down to −90° at the South Pole). The /learn/parallels-of-latitude support covers the family of circles in each hemisphere; the /learn/what-is-latitude pillar covers how latitude is measured.

The equator itself is conventionally assigned to neither hemisphere or to both. In ordinary speech the distinction is meaningless because the equator has zero area; in formal coordinate systems the convention is usually that latitude 0° is its own boundary.

Land and water

The most striking property of the two hemispheres is how unequally land and water are distributed.

| Hemisphere | Land area | Ocean area | Land fraction | |---|---|---|---| | Northern | ~100 million km² | ~155 million km² | ~39% | | Southern | ~49 million km² | ~206 million km² | ~19% |

The Northern Hemisphere holds about 68% of Earth's land. Continents in the Northern Hemisphere: North America entirely, Europe entirely, Asia almost entirely, and most of Africa (the equator runs through Gabon, Republic of the Congo, Democratic Republic of the Congo, Uganda, Kenya, and Somalia, with most of Africa's land north of the line). Continents in the Southern Hemisphere: Australia entirely, Antarctica entirely, South America almost entirely (only a thin band of Colombia, Ecuador, and Venezuela sits north of the equator), and the southern sliver of Africa.

The Southern Hemisphere is ocean-dominated: the Pacific, Atlantic, Indian, and Southern Oceans together make up roughly 80% of its surface. The Northern Hemisphere's land-water ratio is closer to 60-40. The ocean dominance of the South has substantial climatic consequences, covered below.

The asymmetry is not a permanent feature of the planet. Over hundreds of millions of years, plate tectonics have redistributed the continents: roughly 250 million years ago, the supercontinent Pangaea sat largely across the equator. The current asymmetry is a snapshot of a slow geological cycle.

Population

The asymmetry in habitable land and the position of major civilisations gives the Northern Hemisphere an even more lopsided share of humanity than of land. Per UN population data, roughly 88% of Earth's ~8 billion people live in the Northern Hemisphere, leaving about 12% (roughly 950 million) in the Southern.

The largest national populations entirely or almost entirely in the Northern Hemisphere include China, India, the United States, Indonesia (majority south but capital and majority population in southern islands straddling the equator — split is roughly 50-50), Pakistan, Nigeria, Bangladesh, Russia, Mexico, Japan, the Philippines, and Egypt. The largest national populations entirely or majority Southern: Brazil (majority), Indonesia (about half), Australia, Argentina, South Africa, Madagascar, Chile, and Peru.

The largest entirely-Southern city is São Paulo at about 12 million in the city proper. Buenos Aires, Jakarta (just south of the equator), Sydney, Melbourne, Lima, and Johannesburg round out the major Southern metropolises. None reach the size of Tokyo, Delhi, Shanghai, Mumbai, Cairo, or New York.

Seasons

Earth's rotation axis is tilted about 23.44° relative to its orbital plane, per the NASA Earth Fact Sheet. At the June solstice the Northern Hemisphere is tilted toward the Sun (northern summer, longer days, higher Sun) and the Southern is tilted away (southern winter, shorter days, lower Sun). Six months later, at the December solstice, the geometry reverses.

The seasonal opposition has many consequences:

• Northern Hemisphere summer holidays in July–August are Southern Hemisphere winter; Southern Hemisphere school years typically start in late January and run through November.
• Christmas (25 December) is winter in the Northern Hemisphere and summer in the Southern; the cultural iconography of snow, evergreen trees, and warm hearths is mapped onto a beach context in Australia, South Africa, and Argentina.
• Hurricane and typhoon seasons run northern-summer (June–November in the North Atlantic) versus southern-summer (December–April in the South Indian Ocean and South Pacific).
• The astronomical seasons (solstice-to-solstice, equinox-to-equinox) align: the June solstice is the simultaneous start of summer in the North and winter in the South.

The two tropics — covered in the /learn/the-tropic-of-cancer and /learn/the-tropic-of-capricorn supports — bound the latitudes at which the Sun can pass directly overhead. The polar circles, covered in /learn/the-arctic-circle and /learn/the-antarctic-circle, bound the latitudes at which 24-hour daylight or darkness occurs.

Climate asymmetry

The land–water imbalance drives most of the climate asymmetry between the hemispheres:

• Thermal stability. Oceans have far greater heat capacity than land, so the ocean-dominated Southern Hemisphere has smaller seasonal temperature swings than the land-dominated North. Average Northern Hemisphere temperature varies by about 14°C between summer and winter; Southern by about 7°C.
• Storm tracks. Both hemispheres have a mid-latitude westerly storm track between roughly 40° and 60°. The southern track sits over open ocean — the “Roaring Forties”, “Furious Fifties”, and “Screaming Sixties” — with high sustained wind speeds undamped by friction over land. The northern track crosses North America, Europe, and Asia and produces characteristic land-margin storms.
• Albedo and clouds. The two hemispheres receive almost identical total solar radiation across the year, but the Southern Hemisphere reflects more of it because of greater cloud cover and the bright Antarctic ice sheet. The two albedo balances roughly cancel through cross-equatorial atmospheric heat transport.
• Hadley cell. The Hadley circulation crosses the equator twice per year, lifting the Intertropical Convergence Zone with the seasons. The ITCZ shifts more in longitude than in latitude over land than over ocean, producing the monsoon-season migrations of South Asia and West Africa.

Coriolis and rotation direction

The Coriolis parameter f = 2Ω sin φ, where Ω is Earth's rotation rate and φ is latitude, is positive in the Northern Hemisphere and negative in the Southern. The sign difference reverses the rotation direction of large-scale atmospheric and oceanic systems:

• Northern Hemisphere cyclones rotate counter-clockwise (cyclonic in the positive sense); anticyclones (high-pressure systems) rotate clockwise.
• Southern Hemisphere cyclones rotate clockwise; anticyclones counter-clockwise.

The same rotation difference applies to ocean gyres. The North Atlantic and North Pacific gyres rotate clockwise (driven by the wind, which follows the Coriolis-set atmospheric circulation); the South Atlantic, South Indian, and South Pacific gyres rotate counter-clockwise.

The toilet-flush myth — that water drains in opposite directions in the two hemispheres — is wrong at toilet scales (the Coriolis force is overwhelmed by basin shape and initial conditions) but correct at hurricane scales. The /learn/the-equator pillar covers the scaling argument.

The celestial sphere

The two hemispheres see different stars because Earth's rotation brings them under different parts of the celestial sphere. The Celestial South Pole in Octans has no bright nearby star, so Southern navigators historically used the Southern Cross (Crux) to estimate the pole's position. The Celestial North Pole, currently very close to Polaris, has been the Northern navigator's anchor for centuries.

Several iconic constellations are visible only or primarily from one hemisphere: the Southern Cross, Centaurus, Carina, and the Magellanic Clouds (which are nearby dwarf galaxies, naked-eye objects only from the South) are Southern-Hemisphere features; the Big Dipper, Polaris, Cassiopeia, and Andromeda (the spiral galaxy) are practically Northern-only. The two hemispheres share the equatorial constellations (Orion, the zodiac, Aquila), although they appear inverted relative to each other.

Biogeography

The two hemispheres host substantially different biotas. Wallace's biogeographic realms, defined in the 19th century by Alfred Russel Wallace and refined since, divide Earth into eight realms; the hemispheric distribution is uneven:

• The Palearctic (most of Eurasia and northern Africa) and Nearctic (most of North America) are large temperate-zone realms entirely or mostly in the Northern Hemisphere; together they host the boreal forests, temperate deciduous forests, and grasslands that dominate Northern ecology.
• The Neotropical realm (Central and South America), the Afrotropical realm (sub-Saharan Africa), and the Indomalayan realm (South and Southeast Asia) straddle the equator with rich tropical biodiversity.
• The Australasian realm (Australia, New Guinea, and surrounding islands) and the Antarctic realm are entirely or mostly Southern, with distinctive endemic faunas (marsupials, monotremes, cold-adapted seabirds and seals).

The Southern Hemisphere's lower land area and oceanic isolation have produced unusual endemism: about 80% of Australian mammals, reptiles, and amphibians are found nowhere else; New Zealand was bird- and bat-dominated with no native land mammals before human arrival. The Northern temperate zone has more total mammal species but lower endemism, reflecting the connected landmasses of Eurasia and North America that allowed species to spread.

Equator-straddling countries

Thirteen sovereign states have land territory the equator crosses, straddling the two hemispheres. The same list appears in the /learn/the-equator pillar: Ecuador, Colombia, Brazil, São Tomé and Príncipe, Gabon, the Republic of the Congo, the Democratic Republic of the Congo, Uganda, Kenya, Somalia, the Maldives, Indonesia, and Kiribati. Of these, Indonesia and Brazil have the largest equator-spanning populations; the rest are dominated by territory in one hemisphere.

For most administrative purposes, equator-straddling states are treated as belonging to the hemisphere where their capital and majority population live. Ecuador (capital Quito just south of the equator) is conventionally treated as a Southern Hemisphere country; the same for Kenya (Nairobi just south), Uganda (Kampala just north), and the Republic of the Congo. Indonesia (Jakarta in southern Java) is Southern; the centre of the Indonesian archipelago straddles the equator with no clean assignment. The treatment of these countries in statistical aggregates affects the hemispheric population figure modestly but does not change the broad 88/12 split.

Sources

• UN DESA, World Population Prospects — population distribution.
• USGS, How much water is there on Earth — land and water area.
• NASA, Earth Fact Sheet — axial tilt and seasons.
• NOAA JetStream — atmospheric circulation and storm tracks.
• USNO — celestial-sphere references.

For the longitude analogue, see /learn/the-eastern-and-western-hemispheres; for the line that separates the two hemispheres, see /learn/the-equator.