Uneven heating by the sun. The amount of solar radiation that strikes the Earth varies with latitude. This is what causes the seasons.

The height of the Sun in the sky varies throughout the year. At the poles, the Sun never rises during the long winter and it does not rise very high in the sky even in the summer At the equator, day length varies very little by season. The Sun is nearly overhead at midday throughout the year.

The height of the Sun in the sky determines the amount of sunlight that reaches the ground. The Sun's rays travel in a straight parallel lines. Where the Sun is directly overhead, the rays strike perpendicular to the ground and the energy is concentrated on a relatively small area.

Because the Earth's surface curves, a location that is not directly beneath the Sun will have the same amount of solar radiation spread over a broader area. The same amount of solar radiation is spread over a larger area of the ground near the poles than near the equator.

In addition, solar rays are filtered through a great wedge of atmosphere before they strike the ground.

The amount of solar energy that a region receives determines its average air temperature. For this reason, the equatorial regions are warmer than the polar regions. The most important factor that determines a region's climate is its latitude, which is directly related to the amount of solar energy it receives.

Solar flux. The solar flux (the amount of radiation per area) is much greater at lower latitudes than at higher latitudes.

Atmospheric circulation. The difference in the amount of solar energy received at the equator versus at the poles drives atmospheric circulation.

Convection cells originate when heat at the surface of the ground transfers to the atmosphere. At the equator, hot, moist air rises, and expands. This creates a zone of lower atmospheric pressure. As the air rises, it cools, causing it to release its moisture as precipitation. The air then moves either north or south away from the equator. It continues to cool, eventually becoming dense enough to sink. As it descends it contracts, creating higher atmospheric pressure. The air also warms; it arrives at Earth's surface warm and with relatively low humidity. The air then flows back to the equator, completing the cycle.

There are three such convection cells between the equator and the poles, resulting in six cells around the globe.

Atmospheric convection cells. Earth has three major convection cells in each hemisphere, making six cells around the planet. In these cells, warm air rises, and cold air sinks.

Pressure zones. Air rises at a low pressure zone and sinks at a high pressure zone.

The general pattern of rainfall on Earth's surface matches the vertical limbs of the convection cells. Where the air is rising, such as at the equator, it cools and loses moisture, and so regions under ascending air have wet climates.

Where air sinks, such as around 30° north and south, it collects moisture, causing more evaporation than precipitation.

Wet climates are found in bands at around 60° and dry climates again at still higher latitudes.

Pressure zones. Air sinks at a high pressure center and then moves outward in all directions. Air rises at a low pressure center; air is sucked form the surroundings into the rising column of air.

Heat Distribution. Convection currents distribute heat.

Heat moves vertically in a convection cell as warm air rises and cold air sinks.

Heat moves horizontally as warm air at the equator works its way toward the poles and cold air at the poles works its way toward the equator.

Although convection currents do not come close to evening out the heat around the planet, they do distribute the heat more evenly than if there were no convection. So the equatorial regions are somewhat cooler and the polar regions are somewhat warmer than they would otherwise be.

Earth's heat distribution. Convection cells move warm air from the equator towards the poles and cold air from the poles towards the equator. The result is that average temperatures found around the globe are more moderate than they would otherwise be.

Energy balance. The total solar energy coming into and leaving the Earth system is nearly constant, and all the energy gains and losses are in balance.

As a result, Earth's climates are relatively stable for thousands of years, and temperature ranges and weather patterns can be broadly forecast.

Global warming is due to extra heat being trapped in the atmosphere due to an increase in heat-trapping greenhouse gases. This means that the amount of energy entering the Earth system is slightly greater than the amount leaving the Earth system, causing the planet to warm. The increase in greenhouse gases is due to human activities, primarily burning fossil fuels and deforestation.

Fun fact: Methane is a greenhouse gas and methane levels in the atmosphere are rising from animals passing gas. While this is a natural occurrence, the exploding human population has greatly increased the number of animals that are raised for meat, so methane gas in the atmosphere is rising. (Okay, not so fun a fact.)

Earth's energy budget. The amount of solar energy coming into Earth's system is about equal to the amount that leaves the system.