Ch 3 - Atmospheric Pressure

Question 1

Pressure

Answer 1

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

Question 2

High Pressure System (Anti - cyclones)

Answer 2

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)

Question 3

Low Pressure System (Cyclones and Depressions)

Answer 3

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)

Question 4

Measuring Pressure

Answer 4

• Mercury Barometer
• Aneroid Barometer
  - Much more reliable - Used on aircraft in altimeter

Question 5

Horizontal Pressure Variation

Answer 5

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)

Question 6

Vertical Pressure Variation

Answer 6

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

Question 7

Pressure and Temperature

Answer 7

Cold air= Rate of change of pressure much quicker

Warm air = rate of change of pressure much slower

Question 8

Constant Pressure Charts

Answer 8

Colours Represent High/Low Pressure
- Known as Isohypes

• Jet Streams are parallel to the isohypes

Question 9

QNH

Answer 9

Question Nautical Height (above sea level)

QFE adjusted to MSL in ISA conditions (altimeters work off this hence temp errors)

Question 10

QFE

Answer 10

Question Field Elevation

• Pressure at the station so that altimeters read 0 on the ground.

Question 11

Elevation

Answer 11

Height of ground above MSL

Question 12

QNE

Answer 12

Question Nautical Elevation - Indicated height on landing at an airfield with 1013hPa set (high airfields)

Question 13

QFF

Answer 13

= 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

Question 14

Pressure Altitude

Answer 14

Altitude indicated when standard pressure 1013hPa set.

Question 15

Flight Level

Answer 15

Used when above the transition altitude (usually 3/6000ft in the UK)

FL200 = 20,000ft

Question 16

Flight Levels

Answer 16

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

Question 17

Warmer Than Isa Conditions

QNH and QFF

Answer 17

AMSL. BMSL

QNH>QFF. QFF>QNH

Question 18

Colder than ISA Conditions

QNH and QFF

Answer 18

AMSL. BMSL

QFF>QNH. QFF

Question 19

Datum Conversions

Answer 19

Wind off pressure, wind off altitude

Question 20

Barometric Error

Answer 20

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

Question 21

Temperature Error Correction (TEC) Formula

Answer 21

TEC = ( (ISA Dev/10) x 0.04)) x Altitude
(ft)

*Barometric Error first -> ISA and ISA deviation -> Temp error

Question 22

Effect of Temperature of Flight Level Separation Distance

Answer 22

If conditions are colder than ISA, Flight Level separation distance reduced

If conditions are warmer than ISA, Flight Level Separation distance increased