The earth’s atmosphere

Composition/properties

● The atmosphere is composed of 78% nitrogen, 21% oxygen.

● The remaining 1% is composed of argon, carbon dioxide, other gases and water vapour.

● Although water vapour is less than 1%, it is responsible for fuelling the weather in the atmosphere by transporting heat.

● The capacity for expansion, mobility, and compression are the properties of our atmosphere.

Vertical Structure 

● The gaseous area surrounding the planet is divided into several layers or “strata.”

● Each strata is separated by narrow transition zones known as “_____ pause.

● About 99% of the earth’s atmospheric mass is concentrated in the troposphere

● The atmospheric layers are characterized by differences in chemical composition. These produce variations in temperature and lapse rates.

● The major layers of the atmosphere:

➢ Troposphere

➢ Stratosphere

➢ Mesosphere

➢ Thermosphere

➢ Exosphere

Troposphere

● This is the atmospheric layer closest to the planet.

➢ Most of the weather takes place in this layer.

➢ Air is much denser here than it is in the higher atmospheric layers.

● In the Troposphere the temperature decreases at a constant rate with height.

➢ On average it decreases by 1.98 o C per 1,000 feet.

➢ We call this the Standard Lapse Rate

● Over 99% of atmospheric water vapour is found in the troposphere.

● This water plays a major role in regulating the air temperature.

● Water absorbs both the incoming solar energy and the thermal radiation that is re-emitted by the planet's surface

Tropopause

● This is the boundary layer between the troposphere and the stratosphere:

○ Average temperature is -56°C

○ Marks the top of the weather layer (except for CBs)

○ Often indicated by a layer of haze

○ Temperature remains relatively isothermal (constant)

○ Sudden change in the lapse rate (from positive to zero change)

○ Generally smooth flying conditions with little moisture

● Over the poles the tropopause is nearest the earth at approximately 8 km or 25,000 ft.

● Over the equator, the tropopause sits much higher at 18 km or 54000 ft.

● The height also varies seasonally, being higher in the summer than in the winter.

● Average height overall is 36,000 ft throughout the globe (7 miles or 11 km)

Stratosphere 

● This is a 50,000 ft thick layer above the tropopause.

● The stratosphere extends up to 50 km or 180,000 ft high.

● The atmospheric pressure continues to decrease.

● The air temperature in the stratosphere starts to INCREASE beyond the tropopause. It is called a Negative Lapse rate

● Because the air temperature increases with altitude in the stratosphere it does not permit convection.

● Weather that is able to “punch through” from the tropopause is not able to rise any further.

● This lack of convection has a stabilizing effect on thunderstorms and other weather which is unable to penetrate any further.

Stratopause

● The temperature gradually increases to a value of 0° C or 273°K at the stratopause.

➢ This happens approximately 50 km or 180,000 ft above the earth’s surface.

● Why does the temperature increase?

➢ The ozone layer absorbs much of the incoming solar radiation creating this rise in temperature.

Mesosphere

● This layer is from 50 km or 180,000 feet and extends up to 80 km or 275,000 ft.

● Once again we have a shift in the lapse rate, and the temperature decreases.

● This temperature decrease is much more rapid than in the troposphere and temperatures get much colder.

● Temperature in this layer decreases to -100°C or 173°K at an altitude of 80 km or 275,000 ft at the Mesopause

● Concentrations of ozone and water vapour are negligible within the Mesosphere.

● As distance increases from the earth's surface, the chemical composition of the atmosphere becomes strongly dependent on altitude－the higher altitude is where lighter gases are more likely to be found.

● At very high altitudes these gases begin to form into layers according to their molecular mass (weight).

● Gravity has a greater “pull” on the heavier molecules so they tend to be found at lower heights rather than the lighter molecules

Thermosphere 

● The thermosphere starts at 100 km and is located above the mesosphere

● The temperature in the thermosphere generally increases with altitude up to 727-1227°C or 1000-1500°K

● These high temperatures are caused by the intense solar radiation that is found at these heights.

● This is where the aurora or “Northern Lights” are formed.

● At an altitude of 100-200 km the major atmospheric components are still nitrogen and oxygen.

● At this extreme altitude the gas molecules are widely separated

Exosphere 

● The exosphere is the most distant atmospheric region from the Earth's surface starting at 500 km.

● Pressure is a little more than a vacuum

● The upper boundary of the layer extends to heights of perhaps 960 to 10,000 km and is relatively undefined.

● At an altitude of 140 to 160 km one is entering the realm of satellites and aerodynamic lift can no longer be used for maintaining height.

● The exosphere is a transitional zone between Earth's atmosphere and interplanetary space.

The Standard Atmosphere ICAO

● As pilots, we use it as a baseline to calculate airplane performance and to visualize nonstandard weather and atmospheric conditions.

➢ At Sea Level

➢ 29.92 inches of mercury, 14.7 lbs per sq inch, or 1013 millibars or hectopascals.

➢ 15°C or 59°F.

➢ 1 inch of mercury drop for a 1 000 feet increase.

➢ 1.98°C drop for a 1 000 feet increase, Standard lapse rate.

➢ Dry air mass, no precipitation

ISA Conditions

● Sometimes temperature is given in relation to standard conditions.

● For example if the conditions are ISA + 3 this means that the temperature is 3 degrees warmer than standard.

● So if the aircraft is flying at 10,000 ft with an ISA + 3, the temperature is -2°C.

➢ Rounding off standard lapse rate to 2 degrees per 1000 ft.

➢ Then at 10,000 ft it would be 20 degrees colder than standard conditions at sea level (15°C).

➢ So 15°C - 20° = -5°C + 3 = -2°C