## International Standard Atmosphere

ISA Atmosphere:

It is the International standard atmosphere , known as  ISA (International Standard Atmosphere) .

The most important measures are:

– Temperature at sea level = 15º.

– Pressure at sea level of 1013mb, or 29.92 “.

– Decrease in temperature 2º per 1000 feet (1.98º), or 6.5º per 1000 meters.

– Decrease of pressure 1 “every 1000 feet, or 1mb every 28 feet ( approximately)

TEMPERATURE:

The heat of the sun goes through the atmosphere without raising its temperature significantly, but the Earth nevertheless absorbs this heat, raises its temperature, and gradually gives way to the layers of air in contact with it.

In this continuous cycle, the farther away the layers of air from the earth the less heat they receive from it.

Because of this, the second  air layer  is colder as  the height increase , “the higher the height the lower the temperature”.

At I.S.A the temperature decreases 2ºC every 1000 ft or 6.5ºC every 1000m

PRESSURE: Pressure is defined as the amount of force applied per unit area. According to this definition, atmospheric pressure is the force exerted by the atmosphere on a surface unit, this  force is  due to the weight of the air contained in an imaginary column based on a specific surface.

The height of this column and therefore the weight of the air it contains depends on where we are. At sea level the column we have above is larger than at the top of the Everest.

This circumstance indicates that a first quality of the air is that the pressure decreases with the height, “to a greater height less pressure”. At I.S.A it Decreases exponentially

DENSITY: Density is the amount of mass per unit volume.

If it is compressed, the same mass of gas will occupy less volume, or the same volume will contain more gas;  the density increases or decreases in direct relation to the pressure. Since the pressure decreases with the height, according to this relation will also the density, that is to say that “higher height lower density”.

We also know that if heat is applied to a body it dilates and occupies more volume, so that in the same volume there will be less mass, or what is equivalent its density will be lower. Thus, increasing the temperature of the air decreases its density “at a higher temperature lower density”.

There is now a dilemma, because if the height increases, on the one hand decreases the pressure (decreases the density) and on the other decreases the temperature (increases the density), how is the density ?. Well, it influences to a greater extent the pressure change than the temperature, resulting in “higher height lower density”. At I.S.A it Decreases exponentially

Summary: height increases, temperature decreases, pressure decreases, density decreases.

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