03 Atmospheric Pressure Flashcards
What is the formula for pressure?
Pressure = Force (N) / Area (m2)
ISA Surface Pressure?
1013.25 hPa/29.92 insHg/760mmHg
Features on barometric chart?
Isobars - black lines connecting points of equal pressure.
High
Low
Col
Trough - associated with stronger surface convergence, unstable air, strong winds and gusts, heavy showers, rain or hail.
Ridge - extension of a high pressure region.
QFE?
Pressure measured at field elevation.
Elevation/30 = QFE
Current QNH - QFE = QFE
QNH?
Pressure at MSL calculated from QFE using elevation divided by ISA lapse rate (30ft/hPa)
e.g. The airfield elevation is 600ft and the measured QFE on the airfield is 980hPa. What is the QNH?
Airfield Elevation is 600ft. 600/30 = 20 (hPa difference between MSL and field elevation. QFE 980hPa + 20hPa = 1000hPa at MSL.
QFF
Pressure at MSL calculated from actual conditions (varies) using actual lapse rate by weather forecasters.
Comparing QFF to QNH?
Only possible if temperature given.
If AMSL is warmer than ISA, QFF will be lower value than QNH.
If AMSL is colder than ISA, QFF will be higher value than QNH.
Pressure measurement?
Surface pressure varies depending on the weights of air at different places on the Earth’s surface.
E.g. Greater mass of air above a given area on the Earth’s surface = more pressure on that area.
Less mass of air = less pressure.
Simple Mercury Barometer?
Atmospheric pressure presses down on the mercury causing it to rise up an evacuated tube.
Simple Aneroid Barometer?
Increases in pressure causes capsule to contract.
Decreases in pressure causes capsule to expand.
Changes indicated on scale.
Vertical Pressure Variation?
The greatest pressure will be at the surface.
Pressure reduces with altitude.
The actual rate of decrease (lapse rate) varies with altitude.
In ISA, the lapse rate is 30ft or 9m per hPa.
Cold air: pressure decreases rapidly with height.
Warm air: pressure decreases slowly with height.
