Atmosphere[PDF]

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The Earth's atmosphere is a layer of gas that covers our planet. Its vertical structure is composed of several layers that are superimposed, that protect us from space and contain moisture (e.g. steam, clouds and precipitation), gases and tiny particles.

COMPOSITION

The Earth's atmosphere is composed of many gases. Nevertheless the first four represent the majority (99.998%). Nitrogen dilutes oxygen and prevents rapid combustion at the Earth's surface. Oxygen is used by all living things and is essential for respiration. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket and prevents heat from escaping to space.

Composition of a dry earth's atomsphere 1.png
Composition of a dry earth's atomsphere 2.png
Composition of a Dry Earth’s Atmosphere - FAA AC 00 6B

VERTICAL STRUCTURE

The Earth's atmosphere is divided into five concentric layers based on the vertical profile of changes in average air temperature, chemical composition, motion and density. Their sizes vary with latitude and weather conditions. Each of the five layers is separated by a pause, where these changes occur.

  • Troposphere:

The troposphere begins at the Earth's surface and rises to an altitude of about 11 kilometers (36,000 feet). This is where we live. As the gases in this layer decrease with altitude, the air becomes thinner. As a result, the temperature in the troposphere also decreases with altitude. As you go up in altitude, the temperature goes from about 15°C (59°F) to -56.5°C (-70°F). Almost all weather conditions occur in this region (cloud formation, precipitation, fog, snow, ...), thanks to the water molecules present in the troposphere. The thickness of the troposphere varies with temperature, which is closely related to latitude and season. It decreases from the equator to the poles, and is higher in summer than in winter. At the equator, the height is about 18 to 20 kilometers (11 to 12 miles), at latitude 50° N and 50° S it is 9 kilometers (5.6 miles), and at the poles it is 6 kilometers (3.7 miles). The transition boundary between the troposphere and the upper layer is called the tropopause.

  • Stratosphere:

The stratosphere rises from the tropopause to 50 kilometers (31 miles) above the Earth's surface. This layer contains 19% of the gases in the atmosphere, but very little water vapor. The temperature increases with altitude as more and more ultraviolet radiation is absorbed by oxygen molecules, resulting in the formation of ozone. The temperature increases from an average of -56.6°C (-70°F) at the tropopause to a maximum of about -3°C (27°F) at the stratopause due to this absorption of radiation. The increase in temperature allows for slow gas movements.

Commercial aircraft often fly in the lower stratosphere to avoid atmospheric turbulence and convection in the troposphere. The disadvantages of flying in the stratosphere can include increased fuel consumption due to warmer temperatures and increased ozone concentration.

  • Mesosphere:

The mesosphere extends from the stratopause to about 85 kilometers (53 miles) above the Earth. On average, the temperature decreases from about -3 °C (27 °F) to -100 °C (-148 °F) at the mesopause. However, the gases in the mesosphere are thick enough to slow down the meteorites as they rush into the atmosphere where they burn up, leaving fiery trails in the night sky.

  • Thermosphere:

The thermosphere starts from the mesopause up to 690 kilometers (430 miles) above the Earth. This layer is known as the upper atmosphere. Ultraviolet and X-rays from the sun are absorbed, allowing the temperature to rise (to a maximum of 2,000 °C (3,600 °F)) near the top of this layer. Despite this high temperature, this layer of the atmosphere would still be very cold to our skin, due to the extreme thinness of the air.

  • Exosphere:

The exosphere is the outermost layer of the atmosphere, and begins at the thermopause 10,000 kilometers (6,200 miles) above the Earth. In this layer, atoms and molecules escape into space and satellites orbit the Earth. The transition boundary that separates the exosphere from the thermosphere is called the thermopause.

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Vertical Structure - FAA AC 00 6B

INTERNATIONAL STANDARD ATMOSPHERE

As explained above, the atmosphere varies continuously (temperature, pressure, wind, chemical composition, ...). Therefore, engineers decided to create an International Standard Atmosphere (ISA).

The International Standard Atmosphere represents an average of the conditions in the whole atmosphere for all latitudes, seasons and altitudes. The ISA is a hypothetical vertical distribution of atmospheric temperature, pressure and density that is considered representative of the atmosphere for the purposes of calibration, calculations and design of aircraft, missiles etc.

Property Metric Units Imperial Untis
Sea level pressure 1013,25 hectopascals 29.92 inches of mercury
Sea level temperature 15 °C 59 °F
Lapse rate of temperature in the troposphere 6,5 °C/1 000 meters 3,57 °F/1 000 feet
Pressure altitude of the tropopause 11 000 meters 36 089 feet
Temperature at the tropopause -56,5 °C -69,7 °F

See also

Reference

  • AC 00-6B - Aviation Weather
  • Pilot’s Handbook of Aeronautical Knowledge – Chapter 12: Weather Theory
  • SKYbrary library – Atmosphere
  • Aircraft Pilot’s Manual – CépaduèsEdition – Chapter 3 – Navigation and Weather
  • ICAO - Document 9365 - Manual of All Weather Operations

Author

  • VID 529524 - Creation

DATE OF SUBMISSION

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COPYRIGHT

  • This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.

DISCLAIMER

  • The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.