Flashcards in Ch 3 - Atmospheric Pressure Deck (39):
What causes pressure?
Force exerted on a unit of surface area (i.e. 1 hPa = 100 newtons per square meter)
What causes atmospheric pressure?
Weight of air acting in all directions
What causes pressure in the atmosphere?
What is the standard unit of pressure?
Hectopascal (hPa) = 100 Pa
What happens to pressure with altitude?
(due to a lower mass of air above)
What deviced measure pressure?
- Mercury barometer
- Aneroid barometer
- Aneroid barograph
- Solid-state pressure sensors
What causes changes in pressure?
Changes in mass of the air above (i.e. as it moves around)
What is QFE?
Pressure at the airfield/fixed datum point
Altimeter will read 0 on the ground
What is QNH?
QFE adjusted to MSL
Altimeter will read airfield elevation on the ground
What is QFF?
QFE adjusted to MSL, taking temperature into account
What is SPS/QNE?
Standard pressure setting 1,013.25
What the altimeter will read when set to 1,013.25
What are pressure levels?
Horizontal expanses of equal pressure. Pressure levels change vertically.
(imagine a blanket which drapes around the planet at a particular pressure; it will be contoured as that same pressure differs in altitude across the planet)
What happens to pressure levels in warmer air?
Vertical gaps between them expand
What happens to pressure levels in colder air?
Vertical gaps between them contract
At the same level, will pressure in a column of warm air be higher or lower than pressure in a column of cold air?
(cold air is denser and more of it falls, so pressure is lower because there is not as much air above the level)
Pressure lapse rate at MSL?
27ft / hPa
Pressure lapse rate at 500 hPa?
50ft / hPa
How do you calculate the pressure lapse rate?
96 x Temp (k)
Are pressure and density at MSL high or low?
Are pressure and density at high altitude high or low?
What happens to true altitude when going from high pressure to low pressure?
FROM HIGH TO LOW, LOOK OUT BELOW!
What are isobars and what pressure reading do they connect?
Lines joining areas of equal pressure (QFF)
What to isobars enclose?
Areas of low or high pressure
In the northern hemisphere, which way does wind flow around isobars which enclose high pressure areas?
In the northern hemisphere, which way does wind flow around isobars which enclose low pressure areas?
How much of the atmosphere's mass lies in the first 11 km / 36,000 ft?
What is a Pressure Gradient Force (PGF)
A force driving air from an area of high pressure to an area of low pressure
What is wind caused by?
Horizontal differences in pressure
What can you predict from isobars close together?
What can you predict from isobars far apart?
What are troughs?
Channels of low pressure between isobars
What are ridges?
Channels of high pressure between isobars
What are isohypses?
Lines joining the same true altitude of equal pressure
What are isohypses usually labelled in?
What do lower isohypses usually indicate?
What do higher isohypses usually indicate?
When will QFF be higher than QNH?
When the outside air temperature is not hotter than ISA
What weather can be expected from a low-pressure system?
- Good visibility
- Significant cloud
- Significant rain / precipitation
- Small diurnal changein temperature