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.
Static & Dynamic Pressure
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 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
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|
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.
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.
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).
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