How the Uneven Heating of Earth's Surface Affects Climate

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be exploring how the uneven heating of the Earth causes characteristic climates in different parts of the globe. We'll cover why the Earth doesn't heat evenly and the climates it causes in tropical, temperate and polar regions.

Why is the Earth Heated Unevenly?

Think about where you live. What is the climate like? If you live near the equator most likely it's warm and wet. But, if you live farther north the temperature and precipitation depends on the season. It can be hot and humid, or freezing cold! Other areas of the globe reliably get very little rain, creating vast expanses of desert. Why don't tropical areas get cold? Why do deserts get so little rain? The answer is due to uneven heating of the Earth by the Sun.

The Earth is a sphere, and so is the Sun. When the Earth orbits the Sun, the center of the Earth gets more direct sunlight than the poles. This is exacerbated by the Earth's tilt. Since the center of the Earth gets more sunlight, it is consistently hotter than other parts of the Earth.

Sunlight hits the equator with a more direct angle leading to increased temperatures
sunlight angle

When air is hot, it rises. It creates low-pressure areas that draw air from other areas in, creating wind. This heating and cooling of the air on Earth causes all the climate and weather patterns we know. Today, we're going to look at how this uneven heating causes different climate zones on Earth.


Picture the Amazon rainforest. Trees fight for access to light in the canopy, while millions of species of insects, reptiles, and mammals scurry through the forest. With over 70 inches of rain each year, this warm, wet biome earns its name. The Amazon rainforest is part of the tropics, the area between 23.5 degrees north and 23.5 degrees south in latitude.

The tropics are warm and wet due to differential heating of the globe. With direct sunlight, the area is extremely warm. Hot temperatures cause increased evaporation of the vast expanses of ocean at the tropics. As the water evaporates, it rises with warm air into the atmosphere. As it rises, the air cools, and the water vapor condenses into clouds and eventually precipitation. Many areas in the tropics consequently receive large amounts of rainfall. Most rainforests, including the Amazon, are found in the tropics due to this heating pattern.


Extending north and south from the tropics are temperate regions, about 30 degrees latitude north and south from the equator. If you look at a map, you'll notice a band of deserts in this region wrapping around the globe. This is no coincidence that the world's greatest deserts exist around 30 degrees north or south latitude. It has to do with the differential heating and air patterns extending out from the tropics.

Major deserts exist at 30 degrees latitude because of uneven surface heating

Once air in the tropics begins to cool, water vapor condenses and is released as precipitation. The cool, dry air now spreads out from the equator towards the temperate regions. As the air descends, it increases in temperature once again due to heating from the land. Most deserts are covered in rock and sand. These materials have a very low specific heat value, meaning they increase temperature quickly when exposed to heat.

The tropics, however, are covered in water and plants, which tend to change temperature much more slowly. This is why air in the desert is much hotter than air in the tropics, even though deserts get less direct sunlight. The increased temperature of rock and sand heats the surrounding air, leading to extremely hot, dry conditions.

Not all of the temperate zone is desert, however. Extend further north and south from 30 degrees latitude, and you will find a more moderate climate. Most of America lies in the temperate zone. As air warms in the desert, it once again rises and spreads out picking up more moisture as it leaves the desert.


Polar regions extend north of the Arctic circle and south from the Antarctic circle, about 66.5 degrees latitude north or south. These are the coldest regions on the planet. They have the least direct path of sunlight and thus receive the least amount of heat compared to other parts of the globe. The extremely cold air cannot hold water vapor, and thus there is little precipitation in these regions. Few plants and animals survive here.

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