Earth’s atmosphere is divided into layers based mainly on temperature, but on other physical properties such as density, air pressure, and temperature as well. These layers are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer closest to Earth and is where all weather occurs. The temperature within the troposphere decreases with altitude because the gases in the troposphere are heated mostly by radiation of solar energy from the ground, not directly from sunlight. This radiation occurs as sunlight hits the ground and heats the ground. This energy then radiates to the air above, which means that the temperature in the troposphere decreases with altitude; since the higher above the ground the air is, the farther it is from the heat source. Meteorology is the study of weather and its causes. Most weather events occur in the troposphere. Convection currents from rising warm air and sinking cool air, occurs in this layer of the atmosphere. However, it is import to note with students that the troposphere, as a layer of the atmosphere, has decreasing temperatures with increasing altitude, which is a trend that exists despite the convection of air currents.

The next layer up is the stratosphere; this is where ultraviolet radiation is absorbed. Within the stratosphere, temperature increases with increasing altitude. This is due to the presence of a large amount of ozone molecules in the stratosphere, which absorb ultraviolet light and give off heat during their formation and destruction.

The mesosphere is above the stratosphere. Within the mesosphere, temperature decreases with increasing altitude and is the layer where meteoroids burn up. The thermosphere lies above the mesosphere. Within the thermosphere, where the aurora borealis occurs, temperature increases with increasing altitude. This is due to absorption of solar radiation by the thinning atmosphere. A “thin” atmosphere refers to the density of air, or quantity of air molecules in the atmosphere. A thinning atmosphere means there are fewer and fewer air molecules with increasing altitude. This also means the air is less dense. In the thermosphere the air becomes less dense with increasing altitude, which makes it easier for solar radiation to be absorbed, and results in increasing temperature with altitude. However, the amount of heat or energy in this layer is much less than in lower layers of the atmosphere because of the thinness of the air. This means that although temperatures increase within the thermosphere, it would still feel very cold as there are few air molecules in this layer of the atmosphere to transfer heat. The outermost layer of the atmosphere is the exosphere. The exosphere is the boundary between the Earth's atmosphere and outer space. Temperatures remain stable, or do not change, within the exosphere. The exosphere is the coldest of the layers of the atmosphere because it is composed of the thinnest, or least dense air.

Air is the mixture of gases that make up Earth’s atmosphere. The atmosphere is dominated by nitrogen gas (78%) and oxygen gas (21%). Earth’s atmosphere has not always had the same composition. Scientists can study the rock record to learn more about the history of Earth’s atmosphere. The primary atmosphere of Earth was very thin and was made up of hydrogen and helium. The temperatures in the atmosphere were very high while the Earth was still forming. As a result, these gas molecules could obtain enough heat energy to escape the gravitational pull of the Earth and to move out of the atmosphere and off into outer space. This movement of gas molecules kept the atmosphere around an early Earth very thin. Early Earth experienced a large amount of volcanic activity as the planet continued to cool. This activity added water vapor (H2O), ammonia (NH3), and carbon dioxide (CO2) to the atmosphere. Carbon dioxide and water vapor are known as greenhouse gases. In the atmosphere, these gases trap heat energy and reradiate the energy back to Earth. On early Earth, this effect from greenhouse gases, caused a warming trend that made the planet more hospitable to living things. Eventually, bacteria in the ocean   evolved to use carbon dioxide and light from the sun to produce energy. One of the waste products from this process was the gas oxygen gas (O2). As the bacteria continued to thrive, more and more oxygen gas accumulated in Earth’s atmosphere. Today, Earth’s atmosphere continues to change. Some of the current changes are from due to human activity. For example, burning of fossil fuels adds a significant amount of carbon dioxide to our atmosphere.

The atmosphere can also be divided into layers based on chemical composition. The homosphere occurs below 100 km and is characterized by a mixing of gases that leads to similar chemical composition throughout the homosphere. The heterosphere occurs above 100 km and is characterized by a separation of gases based on weight.

Radiant energy from the sun (radiation) heats the Earth, which in turn heat the air above it by conduction.  This energy is distributed through Earth’s atmosphere and hydrosphere in wind and water currents. Warm air is less dense than cool air. So warm air rises, forming areas of low air pressure, and cool air sinks, forming areas of high air pressure. This process sets up convection cells. Air moves across Earth’s surface from areas of high pressure to areas of low pressure. Moving air is called wind. Winds drive surface currents in the oceans. Winds and water currents move warm air and water to cooler locations, thus distributing heat energy around the globe.

Meteorologists use a variety of technologies to study weather, including satellites, radar, and special types of aircraft. Meteorologists also use simpler tools, such as rain gauges and thermometers, at ground stations. Data from all these sources is collected and analyzed to show the locations of air masses and fronts on weather maps. By studying the movement of air masses, meteorologists can forecast changes in weather.

Other common materials in the atmosphere come from natural sources. Air contains dust, bacteria, pollen, salt from sea spray, ash from volcanoes, water vapor, and even dead skin cells. An erupting volcano can change the composition of the atmosphere locally and globally as winds carry particles over large distances. The effects of these added materials can cause increased cloud cover, changes in weather, and respiration problems for living things. Water vapor increases in air as warm dry air moves over a body of water, causing evaporation and making the atmosphere a central component of the water cycle. The composition of air, therefore, changes constantly due to environmental factors.

Many changes to the composition of air also originate from unnatural sources. The burning of fossil fuels releases greenhouse gases including carbon dioxide. This is thought to be responsible for a prominent increase in global temperatures over the last century. Burning fossil fuels also causes the formation of ozone near Earth’s surface, a component of smog. Other chemicals, such as sulfur from burning coal and nitrogen compounds from industrial burning and car emissions, readily combine with water vapor in the atmosphere to create sulfuric and nitric acid rain. Finally, CFCs, or chlorofluorocarbons, cause the destruction of beneficial atmospheric ozone in the stratosphere.

The atmosphere is in a constant state of flux as both natural and unnatural processes alter its composition. The rate of change, and its effects on humans, is somewhat unknown. Links to respiratory problems, global warming, and changes in weather patterns are attributed to changes in atmospheric composition. Increased erosion, changes in plant and animal populations, and even increases in skin cancer from the destruction of ozone are all potential hazards due to atmospheric composition changes.