Variables of Air
In order to understand the principles of jet propulsion it is necessary to understand the principles of physics which are applicable. Operation of an aircraft gas turbine engine is based on compressing and acclerating air in order to create useful work at the turbine wheel and, eventually, thrust. Depending on engine type, thrust is either created from pure reaction to the flowing gases or from a propeller or fan driven by a turbine. Being a gas, air has a number of unique variables that determine engine performance and play a vital role in the engine cycle. It is therefore necessary to have a look at these first to get a basic understanding

Variables of Air
The pressure of air is defined as the force it exerts upon a surface. Atmospheric pressure varies with altitude, which means that aircraft deal with air of various pressure values as the distance from the ground varies. Pressure itself is influenced by temperature and density.

Temperature can be described as the molecular energy of motion due to heat. As temperature decreases, molecular motion decreases as well while a higher temperature would cause molecular motion to be increased. An increase in temperature causes a problem inside the engines' compressor as it requires more and more work in terms of fuel consumption and compressor speed to increase density.

The figure displayed to the right shows how molecules behave at different temperatures. As you can see, molecular activity at 65 degrees celcius is many times greater than that, for example, at 0 degrees celcius.
Molecular activity due to temperature increase

Density can be described as the amount of material per unit volume and can be illustrated by identical packages containing the same type of material (figure on the right) . You'll see that the weight of these packages will vary if the contents are packed closer together in one of them. The compressor of a gas turbine engine uses this principle by packing more and more molecules of air together into a given space with each compressor stage. This is done in order to increase density and weight of the airflow to create thrust. This property is unique to a gas, since solid and liquid matter generally cannot be compressed. The inverse of density, termed specific volume, is the volume of a gas per kilogram. All of these variables have their respective dimensions.
Density packages example

Pressure acts as a force of motion per unit area. What is meant is that air molecules inside a (limited sized) container bounces off the inner walls with great rapidity causing an even pressure to be exerted over all inner surfaces.

Mass is decribed as the amount of material a particular body contains and is similar to the description of density. The mass of two identical packages having the same volume may have different weights due to the density of the material. Looking at a gas turbine engine, if the density increases the weight will increase as well as the mass thus enhancing thrust.