Atmospheric Pressure

Pressure falls with a gain in height.

Can be measured with a mercury barometer or aneroid capsule.

Measured in hectopascals (hPa) – used to be called a millibar.

A given reduction of pressure with height is referred to as a pressure lapse rate.

Pressure lapse rate in warm air is less than in cold air.

Average pressure lapse rate is 1hPa per 30ft increase in height.

Lines called isobars join all the points of equal pressure at a given level (i.e. sea level).

An anticyclone or high is when the wind rotates anti-clockwise in the southern hemisphere and has the greatest pressure at the centre.

A cyclone or low or depression is when the wind rotates clockwise in the southern hemisphere and has the lowest pressure at the centre

Pressure gradient is the difference in pressure per horizontal distance.

Wind strength is directly proportional to pressure gradient.

Very high pressure next to low pressure = strong winds
A not very high pressure high next to a shallow low = less strong winds

Isobars that are close together = greater difference in pressure over a small distance = strong winds = strong pressure gradient

Further apart isobars = small difference in pressure over a larger area = light winds = slack pressure gradient

Pressure gradient is a force – always acts at right angles to the isobars – always from high to low

Dense air is tightly packed = abundance of air = good aircraft performance
Lower density air = less air = poor aircraft performance

warm air leads to low density
cold air leads to high density

if pressure increases, density increases
if pressure decreases, density increases

if temperature increases, density decreases
if temperature decreases, density increases

so both temperature and pressure affect air density

ISA – International Standard Atmosphere

If conditions better than ISA, good performance
If conditions worse than ISA, poor performance

Sea level pressure 1013.2hPa, +15 degrees C, 1.98 degrees C temperature lapse per 1,000ft, above 36,090ft constant temperature of -56.5 degrees C

ISA allows calculation of pressure altitude and density altitude

Lower QNH (sea level pressure) = overread of altimeter, shows higher than you are – Lo-Hi-Hi

Higher QNH = underread of altimeter, shows lower than you are – Hi-Lo-Lo

1hPa per 30ft