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# Heat Transfer on Earth

by Ron Kurtus

Heat is transferred to the surface of the Earth from the hot Earth's core by conduction and from radiation from the Sun.

The atmosphere is heated by absorption of some of the electromagnetic radiation from the Sun, and contact with the warm surface of the land and water. The warm land and water also radiates infrared, some of which is absorbed by the atmosphere, adding to its thermal energy.

The remaining heat on the surface is sent out into space in the form of infrared radiation.

Questions you may have include:

• How is the Earth's surface heated?
• How is the atmosphere heated?
• How is heat lost into space?

This lesson will answer those questions. Useful tool: Units Conversion

## Heating Earth's surface

The Earth's surface can be considered both the land surface and the surface of the oceans and lakes.

### Heat from core

The core of the Earth is very hot. The thermal energy of that core is transferred to the surface of the Earth and lower levels of the oceans by conduction. Water in lakes and oceans transfers heat to the surface by convection.

Effect of conduction of heat from the core is not really significant, as can be seen in winter months.

### Heat from Sun

Electromagnetic radiation from the Sun is absorbed by the soil and bodies of water, thus heating them. This is especially true when the angle of the sunlight is fairly perpendicular to the ground, as in the summer months.

In wintertime, the radiation from the Sun comes in at an angle and is not as readily absorbed. Also the days are shorter and often cloudy, so less sunlight hits the ground. Another factor is when there is snow on the ground, most of the sunlight is reflected back into space.

## Heating the atmosphere

The atmosphere is heated by electromagnetic radiation from the Sun, conduction from contact with warm land and water, convection to even out the temperature and by absorption of infrared radiation from the warm land and water.

### Radiation from Sun

Most of the electromagnetic radiation coming from the Sun passes right through the atmosphere, because the wavelength of visible light is not readily absorbed in air. The infrared portion of sunlight is absorbed, thus heating the air. Convection then spreads and evens out the heat.

### Contact with land and water

Air molecules that come into contact with the warmer land and surface of the oceans and lakes increase their thermal energy through conduction. Those molecules then heat up other air molecules through convection. In the winter, the land and water is cold, thus cooling the air.

### Absorption of infrared radiation

When land and water become warm, the materials emit long-wavelength infrared radiation that is readily absorbed by the atmosphere. This even happens at nighttime. Convection in the air then spreads out the thermal energy throughout the atmosphere. Of course, the ground and water are cold in winter and thus do not emit infrared radiation to heat the air.

Radiation heating the atmosphere

## Heat loss and equilibrium

The warm surfaces of land and water radiate infrared energy. Most of it passes through the atmosphere and is lost in outer space. Some of that energy is absorbed in the atmosphere, thus heating it. Under normal conditions, the amount of thermal energy heating the Earth and its atmosphere from core heat and radiation from the Sun is the same as the amount lost into space in the form of infrared radiation.

Thus the Earth's average temperature stays relatively constant, and there is equilibrium between input heat and heat lost.

### Excess of dust means less heat

One problem that can occur is when there is an excess of dust in the atmosphere, blocking the sunlight. This happened in 1883, when the Krakatoa volcano in Indonesia had a violent eruption, spewing tons of ashes into the air. The ashes blocked out much of the sunlight, as it spread throughout the atmosphere. Average global temperatures fell by as much as 1.2° Celsius temperatures did not return to normal until 1888.

### Excess of gases cause global warming

The present situation is that there has been an increase in infrared-absorbing gases in the atmosphere, such as carbon dioxide (CO2) and methane (CH4). Energy that would normally escape into space is absorbed by these molecules, thus heating the atmosphere and spreading through convection currents. The average temperature of the atmosphere has increased 0.25 °C since 1980, mainly attributed to an increase in infrared-absorbing gases in the atmosphere.

(See Infrared, Gases and the Greenhouse Effect for information on that subject.)

## Summary

Heat from the Earth's core and radiation from the Sun is transferred to the surface of the Earth by conduction. Contact of the atmosphere with these warm surfaces transfers thermal energy, which then heats up the rest of the air through convection.

The atmosphere is also slightly heated by absorption of electromagnetic radiation from the Sun. The warm land and water radiates infrared, some of which is absorbed by the atmosphere, adding to its thermal energy. The remaining heat on the surface is sent out into space in the form of infrared radiation.

Understanding science is amazing

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