The atmosphere can be thought of as a collection of layers. Traveling upward through the atmosphere, they are the troposphere, stratosphere, mesosphere, and thermosphere. There is a region commonly called the ionosphere, but this is not really a layer and we will get into that later. The interfaces of layers are known as pauses (e.g., tropopause, stratopause). Each layer has characteristics that differentiate it from its neighbors, they are not defined simply by height. They stack on top of each other providing us with a roadmap to space.
The troposphere is the lowest and most dense of the layers. It contains most of the mass of the atmosphere, as the Earth's gravitational pull keeps all but the lightest molecules near the surface.
There is not a well-defined boundary dividing our atmosphere from the vacuum of space. As one moves farther from Earth, the molecules that make up the atmosphere become less and less likely of colliding (i.e., the mean free path of a particle gets very long), gradually giving way to the solar wind environment outside the influence of Earth. In the upper reaches of the atmosphere, the composition of the air changes to behave less like a classical fluid, and is strongly influenced by electromagnetic fields, such as the Earth's magnetic field or the solar wind. In this extreme environment, where satellites orbit, the "weather" is described as "space weather" and is governed by magnetohydrodynamics (MHD), which will not be dealt with in this book.
Each layer has its own temperature profile, or the average temperature versus altitude. When you go up in the troposphere from the surface of the Earth, the air around you starts to cool. During the stratopause the air temperature stays constant, but keep going up through the stratosphere and it will increase in temperature. It does the exact opposite in the Mesosphere and then increases in temperature again in the Thermosphere.
As you will learn in the chapter entirely on clouds, they only form in the troposphere and the extreme lower portions of the stratosphere. This is because the moisture content is so little in the upper layers of the atmosphere due to the extreme temperatures. As you will find out, the cloud levels (high, middle, and lower) are quite deceiving as they do not go any higher than the troposphere.