Ch 3 - Atmospheric Pressure Flashcards
Pressure
Pressure = Force / Area
Force = mass x acceleration
(Mass of molecules / Surface)
More air molecules = higher mass = higher pressure
Less air molecules = less mass = lower pressure
High Pressure System (Anti - cyclones)
More air in than out
- High pressure on surface (Increases mass = Increased Pressure)
- Low pressure at tropopause
Better weather, poor visibility
Lots of air converging at the top to fill the space left by other molecules
Molecules getting trapped at the bottom due to friction creating the increased mass and the increased pressure
Converges at the top, diverges at the bottom
High pressure air moves in a clockwise direction (N. Hemisphere)
Low Pressure System (Cyclones and Depressions)
More air out than in
- High pressure at the tropopause
- Low pressure on the surface 2/3000ft (low mass)
Converges at the bottom
Diverges at the top
Poor weather
Anti-clockwise direction (N. Hemisphere)
Measuring Pressure
- Mercury Barometer
- Aneroid Barometer
- Much more reliable - Used on aircraft in altimeter
Horizontal Pressure Variation
Horizontal Pressure Chart
- Lines and pressures are Isobars
- Indicate pressure at roughly 2/3000ft
- How close the lines are together representing the pressure gradient force (closer together = greater gradient = Increased Force)
Vertical Pressure Variation
Pressure decreases with altitude at a non-linear rate due to gravity.
Levels Pressure(hPa). Rate of change of P
MSL 1013.25. 27
10,000. Roughly 700. 37
18,000. Roughly 500. 47
30,000. Roughly 300. 71
40,000. Roughly 200. 111.
ISA
Pressure and Temperature
Cold air= Rate of change of pressure much quicker
Warm air = rate of change of pressure much slower
Constant Pressure Charts
Colours Represent High/Low Pressure
- Known as Isohypes
- Jet Streams are parallel to the isohypes
QNH
Question Nautical Height (above sea level)
QFE adjusted to MSL in ISA conditions (altimeters work off this hence temp errors)
QFE
Question Field Elevation
- Pressure at the station so that altimeters read 0 on the ground.
Elevation
Height of ground above MSL
QNE
Question Nautical Elevation - Indicated height on landing at an airfield with 1013hPa set (high airfields)
QFF
= QFE adjusted to MSL in actual conditions
- Do not use in aircraft
- Is used on surface weather charts as it takes into account temperature
- Works off actual temperatures
Pressure Altitude
Altitude indicated when standard pressure 1013hPa set.
Flight Level
Used when above the transition altitude (usually 3/6000ft in the UK)
FL200 = 20,000ft
Flight Levels
More dangerous when flying from high to low
High to Low, watch about below - clearance issue
Use the lowest useable one and the highest negative temperature deviation from ISA
- ensure clearance from terrain
Warmer Than Isa Conditions
QNH and QFF
AMSL. BMSL
QNH>QFF. QFF>QNH
Colder than ISA Conditions
QNH and QFF
AMSL. BMSL
QFF>QNH. QFF
Datum Conversions
Wind off pressure, wind off altitude
Barometric Error
Pressure set on altimeter sub scale, flying into an area of different pressure will lead to an error in true and indicated Altitude
High P -> Low P (look out below) Hot to cold, don’t be bold
IA will over read
TA will be lower
Low P -> High P
IA will under read
TA will be higher
Temperature Error Correction (TEC) Formula
TEC = ( (ISA Dev/10) x 0.04)) x Altitude
(ft)
*Barometric Error first -> ISA and ISA deviation -> Temp error
Effect of Temperature of Flight Level Separation Distance
If conditions are colder than ISA, Flight Level separation distance reduced
If conditions are warmer than ISA, Flight Level Separation distance increased
