Thursday, June 20, 2019

Aptl Integrated Course


For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during flying prior to departure or enroute, earth atmosphere plays a vital role for safety of the aircraft.  To ensure this, pilot has to be well versant with earth atmosphere & other topics related to aviation meteorology.  At Gracious Avatar during commercial pilot training classes for PPL, CPL & ATPL aspirants, meteorology will be covered by the expert ground instructors.   Pressure is one of the important topic which has to be covered during commercial pilot training.

PRESSURE

 

Static & Dynamic Pressure


1.      When the air is at rest its molecules are in random motion all over. The pressure exerted by these molecules is uniform in all directions. This Pressure is called Static Pressure or Barometric Pressure.

2.      If the air is in motion an additional pressure is exerted in the direction opposite to the flow. This is called Dynamic Pressure or Wind Pressure. Wind pressure is used in the design of air speed indicator.

Pressure as Weight of the Air Column


1.      The atmospheric Pressure at any level is the weight of the column of air of unit cross-section extending vertically upwards to the top of atmosphere.

2.         As the weight decreases with height, the pressure also decreases with height.

3.         ICAO has adopted hector Pascal (hPa) as the unit of atmospheric pressure. The other units are mm, and inches.

 

 

 

 

 

Relationship between Pressure & Altitude


1.      Roughly 1 hPa change in Pressure is equivalent to :

at MSL
27 ft
at 20000 ft
30 ft
at 30000 ft
50 ft
at 40000 ft
100 ft

2.      The pressure decreases with height from sea level to 600m at 4%, up to 1.5 km at 3% and up to 3.0 km at 2.5%. At 6 km it reduces to half the value at sea level and is negligible at 100 km.



Variation of Pressure in Warm & Cold Air


1.    Warm Air is less dense than the Cold Air.

2.    Pressure falls at a faster rate over a cold column of air than a warm column of air.

3.    Where upper Air Mean Temperatures are higher upper Air Mean Pressures are higher and where mean Temps are lower mean pressures are lower.



Diurnal Variation of Pressure


1.    Atmospheric Pressure follows a wavy pattern during 24 hours, showing maxima at 1000h & 2200h and minima at 0400h & 1600h local time. Such variations are very small at poles and large at equator.

2.    When the temperature is highest in the afternoon, the density of air close to the ground is low. Hence the pressure is lowest. At about sunrise when the temperature is lowest, the pressure is highest. There is a phase difference of about 3 hours between the temperature and the pressure.

3.    Diurnal variation of pressure is probably a natural oscillation of the atmosphere, having a period of almost 12 hours.

4.    As the air is continuous, if there is a high pressure at one place there has to be a low pressure on the opposite side of the globe. With the rotation of the earth the pressures also rotates. Hence, two maxima and two minima in 24 hours.

 

 

Pressure Systems


1.    Low (Cyclone): It is an area enclosed by an isobar with lowest pressure at the centre. When there are two or more closed isobars at an interval of 2 hPa, it is called a Depression. Winds around a low blow in anticlockwise direction in N hemisphere converging towards the centre. There is convergence and upward motion at a low. Hence it is associated with bad weather.

2.    Trough of Low: A tongue like extension of isobars from a low is called trough of low. Pressure along the trough is lower than on either side. Isobars along the trough are V- shaped and wind direction abruptly changes and backs.

3.    High: It is a region enclosed by isobars with highest pressure at the centre. Wind in a high moves in a clockwise direction. It is associated with fair weather but visibility is poor due to subsidence.

4.    Ridge: It is a wedge like extension of isobars from a high pressure area. Pressure is higher along the ridge than on either side.

 





Height Vs Pressure


Sea Level
1013 hPa
10000 ft
700 hPa
18000 ft
500 hPa
24000 ft
400 hPa
30000 ft
300 hPa
34000 ft
250 hPa
38000 ft
200 hPa
44000 ft
150 hPa
53000 ft
100 hPa

QFE: Pressure measured at the airfield setting point.

QNH: Pressure reduced to mean sea level using ISA specifications.

QNE: 1013 set on altimeter sub scale, will, on landing give an altitude known as the QNE value.

QFF: Pressure reduced to mean sea level using standard met practices (temperature at the station, which is isothermal lapse rate).

For more info :- https://www.graciousavatar.in/Pilot-Training/ATPL.php

Are You Looking For Atp Pilot School


For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during flying prior to departure or enroute, earth atmosphere plays a vital role for safety of the aircraft.  To ensure this, pilot has to be well versant with earth atmosphere & other topics related to aviation meteorology.  At Gracious Avatar during ground classes for PPL, CPL & ATPL aspirants, meteorology will be covered by the expert ground instructors.   

EARTH’S ATMOSPHERE

Atmosphere

The atmosphere of the earth is an envelope of air around earth and it is bound to the earth due to the gravitational pull of the earth. Effectively, it is part of earth and moves with the earth as the earth rotates on its axis. Air is a mixture of gases, and this mixture is quite homogeneous upto about 80km due to turbulent flow, whilst above 80 km the atmosphere is heterogeneous.

Composition & Characteristics

1.         Since atmosphere is made up of air and air has weight, so the atmosphere has weight and it exerts pressure. Atmosphere has no definite shape. It occupies space. Air is a mixture of gases- the most prominent being Nitrogen and Oxygen.
The ratio of gases present in the air is :-
Nitrogen 78.08%, Oxygen 20.95%, CO2 0.035%, Argon (A2) 0.03% and traces of Helium, Methane, Hydrogen, Nitrous Oxide, Ozone, Xenon etc.

2.         Nitrogen And Oxygen making up most of the atmosphere can be said to have a ratio which is :-
Nitrogen: Oxygen= 3:1 (By Weight)
Nitrogen: Oxygen= 4:1 (By Volume)

3.         Even though atmosphere is quite homogeneous, it shows remarkable variable quantities of water vapour in the lower troposphere and of ozone in the lower stratosphere (20-25km).
4.         Water vapour, Carbon dioxide and Ozone, though present in small quantities have significant effect on weather and life. These gases are called Green House Gases. Ozone in the stratosphere absorbs harmful ultra-violet radiation from the sun and protects us.
5.         Air which has no water vapour is called dry air. A mixture of Dry air and water vapour is called Moist Air. The air which has >4% water vapour (=100% RH) is called saturated. Similarly, the air which has <4% water vapour (RH<100%) is called unsaturated. In meteorology, all air that is unsaturated is called Dry Air.

Vertical Distribution of Air


Due to the gravitational attraction of the earth, most of the atmospheric mass is concentrated in the lower portion and the density decreases rapidly with height. As a rough estimate :-
       
·         1/2 of the atmospheric mass is contained in the lower 6 km
·         3/4 of the atmospheric mass is contained in the lower 10 km
·         99% of the atmospheric mass is contained in the lower 35 km

Thermal Structure of the Atmosphere


1.    Sun is the main source of energy for the earth. It gets heated up by the sun’s rays during day and then heats the layers above by conduction, convection, radiation and latent heat.

2.    Sensible Heat (Conduction, Convection & Radiation) – 23%.

3.    Latent Heat (Evaporation, Condensation & Sublimation) 77%.

4.    Due to absorption of solar radiation the atmospheric layers close to ground are warmer than layers above.

5.    The temperature falls with height. This trend however does not continue till the end of atmosphere.

6.    Based on temperature distribution the atmosphere is divided into layers viz. troposphere, stratosphere, mesosphere & Thermosphere (which includes ionosphere).

7.    The top of the troposphere where the temperature stops falling is called tropopause. It is a boundary layer between troposphere & stratosphere.

8.    Higher the temperature at surface higher will be tropopause.

9.    Above (not at) 8km there is a reversal of temperature & density and above this height poles are warmer than equator.

Troposphere


1.    Lowest layer of atmosphere is troposphere.
2.    Extends to 16-18 km over equator and 8-10 km over poles.
3.    Temperature falls with height at a uniform rate of 6° C/km.
4.    In day to day conditions temp may fall (lapse), rise (inversion) or stay constant (isothermal).Troposphere is generally unstable and thus most weather occurs in this layer.
5.    70% of the atmosphere lies within this layer.
6.    Tropopause is top of troposphere and a boundary between troposphere and stratosphere.
7.    At tropopause the temperature stops falling with height. Lapse rate, if any, reduces to just 1-2°/km.
8.    Height of tropopause is 16-18 km over equator and 8-10 km over poles.
9.    There are breaks in Tropopause where jet stream occurs.

10. Polar tropopause- Pole ward of 45-600 N 300 hPa.
11. Tropical tropopause- Equator to 350 N 100 hPa.
12. Jet Stream at 200 hPa between the two tropopause.



Stratosphere


1.    The layer above troposphere which is much stable and in which temperature rises with height is stratosphere.
2.    Nacreous clouds or mother of pearl clouds are seen in upper stratosphere in higher latitudes.
3.    The temperature in lower stratosphere slowly decreases with height in high latitudes in winters, remains nearly constant in the mid-latitudes and increases with height in lower latitudes.
4.    In the upper stratosphere (above 30 km) there is general increase of temperature with height due to absorption of UV rays by Ozone and Oxygen.

5.    The top of stratosphere where once again the temperature has stopped rising and has reached about 0° C is called Stratopause. It occurs at a height of 50 km.

Mesosphere


1.    Mesosphere extends from Stratopause to about 80 km. Lack of absorption of solar radiation and weak vertical mixing causes temperatures to fall with height in this layer.

2.    On rare occasions Noctilucent clouds are seen in upper Mesosphere.

3.    The level at the top of Mesosphere where temperatures after reaching about -90° C stop falling is called Mesopause. It generally occurs at about 80 km.

Thermosphere


1.    The region extends from Mesopause to the outermost fringes of the atmosphere in which temperature increases with height up to 200 km.

2.    In this layer air is very thin and very few molecules are present. However, there are ionized particles that move rapidly and strike each other thus increasing temperature.

3.    The temperatures are about 600° C at 200 km which increases to 2000° C during sunspot maxima.



International Standard Atmosphere

ISA has been defined by ICAO. Its specifications are:-

1.    Air is Dry.

2.    Temp at MSL 15°C.

3.    Pressure at MSL 1013.25 hPa.

4.    Acceleration due to gravity 980.665 cm/sec2.

5.    Lapse rate up to 11km 6.5° C/km.

6.    Temp assumed constant at -56.5° C from 11km to 20 km.

7.    From 20 to 30 km there is a rise of temp at the rate of 1° C/km with a temp of -44.5° C at 32 km.

 For more info :- https://www.graciousavatar.in/Pilot-Training/ATPL.php

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