Atmospheric Pressure

Question 1

What is Static pressure

Answer 1

Pressure caused solely by the effect of gravity on a mass of air.
Atmospheric pressure=static pressure of atmosphere. Force created felt in all directions as air is a fluid.

Question 2

Pressure Measurement

Answer 2

Mercury Barometer-atmospheric pressure drives mercury up tube. If pressure drops mercury column will drop.
Aneroid Barometer-capsule measuring pressure increase in static pressure compresses. If atmospheric pressure decreases then capsule expands. Linked to needle which indicates pressure.

Question 3

Unit of measurement for pressure

Answer 3

Millibars/Hectopascal

1mb=1hpa.

Question 4

What factors determine atmospheric pressure

Answer 4

Earths gravity acting upon mass of atmosphere.

Mass of air determines variability of atmospheric pressure. Greater mass=Greater atmospheric pressure.

Question 5

What are surface pressure charts

Answer 5

Diagrams of variation of surface pressure.
Also known as Synoptic charts.
Use a common datum which is MSL.

Question 6

What are low pressure area

Answer 6

Known as depressions/lows. At centre pressure is lowest. Pressure increases with distance from centre.

Question 7

What are high pressure areas

Answer 7

Known as Anti-cyclones/Highs. Centre pressure is highest. Pressure decreases with distance from centre. In N.Hem isobars (winds) move clockwise. Reverse in S.Hem

Question 8

What are extensions/protrusions

Answer 8

Protrusion of high pressure is called ridge. For lower called trough. Weather in such features more significant than in parent system. E.g at trough potentially bad weather.
A col occurs between 2 high/low and is neither high nor low pressure.

Question 9

What is a pressure gradient force

Answer 9

Whenever a pressure variation exists a force created that tries to drive air from area of high to area of low pressure. Primary force driving flow of air and determines wind speed. Greater difference in pressure between 2 points greater pressure gradient.

Question 10

What are isobars

Answer 10

Lines of equal pressure. Spacing of isobars can show strength of PGF and speed/strength of wind. Areas of high pressure (flat pressure pattern) have widely spaced isobars and very light winds.

Question 11

Vertical Pressure Variation

Answer 11

Pressure decreases with height. However rate at which pressure decreases decreases with height. MSL PRESSURE IS 1013.25 Lapse rate 1hpa/30ft.
At 18000ft lapse rate 1hpa/48ft.

Question 12

What is the effect of temperature on pressure with regards to height

Answer 12

Cold air causes pressure to change more rapidly with height. As a consequence above cold air pressure is lower than at the same level in warm air.

Question 13

What is QFE

Answer 13

QFE is a the barometric pressure set from an aerodrome. If an altimeter is set to QFE its altitude will be zero at the aerodrome.

Question 14

What is QNH

Answer 14

Barometric pressure from mean sea level.

Question 15

What is standard pressure setting. (SPS)

Answer 15

Mean sea level pressure of 1013.25. Essential for a common datum of pressure to be used for all aircraft not in close proximity to terrain. Heights above SPS pressure level are known as Flight levels (FL).

Question 16

What is QFF

Answer 16

Calculated from MSL. However it takes into to account actual pressure change depending on actual temperature rather than ISA conditions like QNH does.

Question 17

What is the relationship between QNH and QFF

Answer 17

Airfields above Sea level. In warm air QFFQNH. Airfields below sea level these are reversed.
At same sea levels QFF=QNH.

Question 18

What are pressure lapse rates in standard conditions

Answer 18

30ft per hpa at MSL, 50ft per hpa at 20000ft, 73ft per hpa at 30000ft.

Question 19

What is atmospheric pressure in ISA conditions

Answer 19

at 18000ft pressure is 50% of MSL and density is approximately 50%of MSL at 22000ft and 25% at 40000ft.