Atmosphere

The earth is surrounded by an envelope of gases, which is termed as an atmosphere. Its pressure, density and temperature highly depends on the altitude level, along with geographical location around the world. The air is made up of approximately 21% oxygen, 78% nitrogen and 1% other gases. The speed of sound at sea level is related to the temperature as shown in the table below.

Standard atmosphere

Standard atmosphere is an atmospheric model that is used to calculate the changes in the temperature, pressure, density and viscosity of the earth’s atmosphere. It allows standardization and calibration of engines except taking the speed of sound into consideration. Instruments such as pitot static operated instruments can be calibrated using the standard atmosphere using the same parameters.

    The ICAO (International Civil Aviation Organization) standard atmosphere or ISA (International Standard Atmosphere) deals with pressure, temperature, density and viscosity of the atmosphere with average values observed at a latitude of 40^oN. The air is assumed to be dry and the temperature and pressure at sea level is 15^oC and 101.325kPa, respectively. The gravity remains with the uniform value of 9.81m/sec² at sea level.

    The following figure graphically represents the level of dependency of the parameters against altitude. It also shows the different layers of atmosphere, which would be helpful to assess while working on different sizes aircrafts.

    The additional pressure in a system relative to atmospheric pressure is known as gauge pressure. The absolute pressure would be gauge pressure plus atmospheric pressure and is a part of zero-reference trinity.

    The density can vary depending on the temperature and the relative humidity. The density will increase with the drop in temperature and will decrease with the rise in relative humidity.

    The other atmospheric standards are STP (Standard Temperature and Pressure), NTP (Normal Temperature and Pressure) and SATP (Standard Ambient Temperature and Pressure).

Dew

The heat transfer can occur due to radiation when the air cools down resulting in heat loss from the earth to the surrounding atmosphere. At the same time, the air is saturated with moisture and will condense out to form small water droplets on any cooled surfaces and this wetness is known as dew, which is found on parked aircrafts. The temperature at which the water vapour present in the air is sufficient enough to saturate the air is called as the dew point.

Frost

A deposit of minute ice crystals formed when water vapour condenses at a temperature below the freezing point (0^oC) is known as frost. It forms a white soft crystalline layer, which covers the surface is known as hoarfrost or ground frost.

Fog

These are formed by small water droplets suspended in the air ranging between 1 and 10 microns in diameter. These are often confused with low lying clouds. According to the international definition, fog is when visibility is reduced to 180m or less. When smoke particles combine with fog, it results in smog, which carries chemical waste particles.

Mist

The mist droplets are up to 1 micron in size with visibility remaining above 1.5 miles. All the weather related deposits must be removed from aircraft before flight.