Scientists who study the air recognize that it has different characteristics at different altitudes throughout the atmosphere. Understanding that the atmosphere isn’t uniform around the globe helps them study and understand how weather develops. Scientists have identified four distinct layers of Earth’s atmosphere. Take a moment to study the various parts of the diagram below. The two lowest layers of the atmosphere are the most important for the study of weather. These layers are called the troposphere and the stratosphere.

The layer of atmosphere closest to Earth is the troposphere, starting at Earth’s surface and extending 8 to 14.5 km (5 to 9 mi) high. Look at the graphs on this page and find the part of each one that represents the troposphere. The troposphere is where the most air, water, and water vapor mix to produce our weather. 

 The layer just above the troposphere is the stratosphere, extending to about 50 km (31 mi). Find the part of each graph that shows a portion of the stratosphere. Together, the troposphere and stratosphere contain 99 percent of the air in Earth’s atmosphere. Between these two layers, temperature and humidity change dramatically. Earth’s weather occurs in the troposphere, and its boundary with the stratosphere keeps storms from growing upward indefinitely. Often, this boundary can be inferred by the anvil-shaped cloud at the top of a thunderstorm.

Scientists have determined a set of average atmospheric conditions for the air within the troposphere. They compare local measurements with these averages. The comparison helps them understand what the weather may do. Let’s look at how air pressure, temperature, and the amount of water vapor in the troposphere change on average as you ascend. Changes from these averages help scientists predict changes in the weather. 

First, consider air pressure. As altitude increases, gravity exerts less pull on the atoms and molecules of air. In addition, there are fewer atoms and molecules “pushing down” from above. As a result, air pressure decreases as you ascend.

A second condition to consider is air temperature. Air temperature tends to decrease with increasing altitude in the troposphere. Air near Earth is warmed by the transfer of heat from Earth’s surface and begins to rise. As it rises it loses heat and its temperature decreases. At some point, this warm surface air that is rising and cooling begins to sink. This produces a cycle of rising and falling air, setting up convection currents in the atmosphere. If enough heat is released in this cycle, strong storms can develop.

Finally, humidity tends to decrease as altitude increases. This is because air at Earth’s surface is warmer and has more energy to maintain water in its gaseous form.