This lesson will discuss the basic building blocks of the atmosphere, beginning with the lower layers in which most flight activity occurs. These layers have particular temperature characteristics that affect many aspects of weather, and thus are important to the understanding of later chapters. Pressure is another characteristic of the atmosphere which enables meteorologists to track weather phenomena as they move across the surface of the earth. Additionally, pressure is important to the aviation community since one of the most basic flight instruments, the barometric altimeter, operates from the action of atmospheric pressure upon its sensors. In order to gain a complete understanding of the altimeter, the effects of temperature and pressure variations on altimeter readings will also be discussed.