Chapter 3 - Atmospheric Pressure

Question 1

What are the main features of a mercury barometer and how does it measure pressure?

Answer 1

Evacuated tube in a pool of mercury.

Atmospheric pressure pushes mercury up the tube and the height of the mercury is equivalent to a pressure reading in inHg.
29.92inHg = 760mm = 1013.25 hPa

Question 2

What does aneroid mean?

Answer 2

Partially evacuated

Question 3

What are isobars?

Answer 3

Lines of equal pressure on a surface pressure chart.

Question 4

What happens to pressure levels/altitudes when air is colder than ISA?

Answer 4

The drop in altitude.

“High to low, look out below!”

Question 5

What effect does air that is colder than ISA have on the pressure lapse rate?

Answer 5

Pressure in a colder column of air falls quicker with height, therefore the lapse rate (ft/hPa) is faster (a lower value).

Question 6

How do you calculate the pressure lapse rate?

Answer 6

Lapse rate = (96*T)/p
T (K)
P(hPa)

Question 7

Define QFE and what an aircraft altimeter would display when at an aerodrome with the QFE set.

Answer 7

Pressure at field elevation. The altimeter would read 0.

Question 8

Define QNH and what and aircraft altimeter would read with the correct QNH reference set.

Answer 8

QFE adjusted down to MSL using ISA pressure lapse rate.

The altimeter would read the altitude above MSL, assuming ISA conditions.

Question 9

Define SPS and when would an aircraft use it?

Answer 9

Standard pressure setting, 1013.25hPa.

An aircraft uses SPS when above transition altitude to maintain a specific flight level.

Question 10

Define QFF.

Answer 10

QNH but with actual pressure and temperature conditions (so not ISA).

Question 11

When is QNH larger than QFF?

Answer 11

QNH is larger when temperatures are hotter than ISA at MSL, unless the reference is below MSL then it is when temperatures are colder than ISA at MSL.
“Not hotter not higher”
“Now hotter now higher”