Temperature and the Heat Balance

Every bit of warmth on the Earth ultimately comes from the Sun. The energy arrives, spreads across the planet, and is balanced by an equal amount sent back to space. This is the story of temperature and the heat budget. It explains why the Equator is hot and the poles are cold. It also explains why coasts are milder than interiors, and why some nights turn surprisingly cold near the ground.

The energy the Earth receives from the Sun is called insolation, short for incoming solar radiation. It travels as short waves and heats the surface, which in turn warms the air above it. The air is therefore heated mainly from below, by the ground, not directly by the Sun's passage through it.

Insolation is not all visible light. A large part of it arrives as infrared rays, which lie beyond the red end of the spectrum and are invisible to the eye. These infrared waves are largely absorbed by water vapour, which is concentrated in the lower layers of the atmosphere. Visible light, by contrast, passes through fairly freely and reaches the surface.

The amount of insolation a place receives depends most of all on the angle of the Sun's rays. Near the Equator the Sun is high and its rays strike almost vertically, concentrating energy on a small area. Near the poles the rays strike at a slant. They spread the same energy over a larger area and pass through more atmosphere, so they deliver far less heat. This single fact is the root cause of the world's temperature zones. The contrast should not be exaggerated, though. Over the whole year the Equator receives only modestly more total insolation than the poles, nowhere near ten times as much. Long polar summer days partly make up for the slanting rays.

Several factors decide how warm a particular place is.

• Latitude: the further from the Equator, the lower the angle of the Sun's rays and the cooler the place.
• Altitude: temperature falls with height, so hill stations are cooler than the plains at the same latitude.
• Distance from the sea: the sea heats and cools slowly, so coastal places have mild, even temperatures, while interiors have hot summers and cold winters (the continental effect).
• Ocean currents and winds: warm currents and warm winds raise the temperature of the lands they reach; cold currents and cold winds lower it.

Although the Earth keeps receiving energy from the Sun, it does not get hotter and hotter year after year. This is because it sends back to space exactly as much heat as it receives. This balance is called the heat budget or heat balance of the Earth.

Of the insolation arriving at the top of the atmosphere, part is reflected straight back by clouds, dust and the surface. This reflected fraction is called the albedo. The rest is absorbed by the surface and the atmosphere, warming them. The warmed Earth then radiates this energy back out as long-wave heat (terrestrial radiation), which eventually escapes to space. Incoming and outgoing balance out, keeping the planet's average temperature stable. The balance is not even across the globe, though. The tropics receive more heat than they lose, while the poles lose more than they receive. Winds and ocean currents carry the surplus heat from the tropics towards the poles.

Different surfaces reflect very different shares of the sunlight that falls on them. Fresh snow has the highest albedo of any common natural surface. It reflects roughly 80 to 90 per cent of the sunlight it receives. Sand desert reflects much less, and cropland and grassland less still, because darker, rougher surfaces absorb more energy. Taking land, sea, clouds and ice together, the Earth's average albedo is about 30 per cent: roughly three tenths of all insolation is reflected back to space unused.

Albedo also decides how bright an object looks in reflected light. A high-albedo body shines brightly; a low-albedo body looks dull. Dark rocky bodies such as the planet Mercury have a low albedo of about 0.12, so they reflect far less of the sunlight striking them than the Earth does.

The atmosphere does more than let sunlight through. Certain gases in it, above all carbon dioxide and water vapour, absorb the long-wave terrestrial radiation rising from the warmed surface and trap part of that heat. This trapping is called the greenhouse effect, and it is what keeps the planet's average temperature at a comfortable level. Without the atmosphere and its heat-trapping gases, the heat radiated by the surface would escape freely, and the Earth's surface would be well below freezing. The greenhouse effect is therefore natural and necessary. The modern concern is its strengthening as carbon dioxide accumulates in the air.

Normally temperature falls with height. Sometimes, however, the pattern is reversed and temperature rises with height for a short distance near the ground. This is called a temperature inversion.

It happens most often on long, calm, clear winter nights. The ground loses heat quickly by radiation and cools the air just above it. The coldest air then lies at the bottom, with warmer air sitting on top. Inversions trap smoke, fog and pollutants near the surface. This is why valleys can be especially foggy and smoggy on winter mornings.

The temperature of the air over the world is shown on maps using isotherms, lines that join places having the same temperature. By studying isotherms we can see clear patterns. Temperature generally decreases from the Equator towards the poles, and the isotherms broadly run east to west, parallel to the lines of latitude. Isotherms bend where they cross from land to sea, because land and water heat differently. This bending is sharper in the Northern Hemisphere, which has more land.