Av. Weather: Atmospheric Pressure and Altimetry (Ch.8)

Volume

• THE MASS OF THE MOLECULES
  IN A GIVEN VOLUME

• IF TOTAL MASS IN A VOLUME
  DECREASES, THE DENSITY
  DECREASES AND VICE VERSA

• THE UNITS OF DENSITY ARE
  EXPRESSED IN TERMS OF
  MASS PER UNIT VOLUME

Pressure

• The force exerted by the moving molecules of the gas/liquid/solid in a given area.

• Pressure at a point acts equally in all directions.

• Example: A typical value of atmospheric pressure at sea level is 14.7 pounds per square inch (1013.25 millibars, 29.92 inches of mercury).

Gas Law (P/DT=R)

• P = pressure

• D = density

• T = temperature

• The ratio of pressure to density and temperature is always the same (BALANCED). If pressure changes, then either density or temperature must also change

Charting Atmospheric Pressure

• Pilots pay special attention to the horizontal distribution of atmospheric pressure

• These pressure patterns are normally identified through
  chart analyses

Sea Level Pressure Patterns

• Pressure gradient – difference in pressure over a given distance

• Strong pressure gradient = closely spaced isobars

• Weak pressure gradient = spaced isobars

• Isopleths spacing displays gradients of many parameters

The Pressure Altimeter

• Indicated altitude – measured by altimeter

• True altitude – height MSL

• Absolute altitude – height AGL

• Three specific errors caused by nonstandard atmospheric conditions:

  • Sea level pressure different from 29.92 in. Hg

  • Temperature warmer or colder than standard

  • Strong vertical gusts

Pressure Variation Error

• Standard atmosphere is based on a fixed pressure (1013.25 mb)

• This value provides important reference because different locations vary due to:

  • Heating and cooling of the earth

  • Movements of high and low pressure areas

Nonstandard Temperature Error

• Temperature variations expand or contract the atmosphere and raise or lower pressure levels that the altimeter senses

• On a warm day – pressure level is higher than standard...altimeter indicates lower than actual
  

• On a cold day – pressure level is lower than standard...altimeter
  indicates higher than actual
  

• REMEMBER: High to Low, Look out Below!

Vertical Gust Error

• Caused by large and rapid changes in vertical movements of air

• The changes upset the balance of forces or the hydrostatic balance

• These errors can be expected in the extreme updrafts of thunderstorms and in strong mountain waves

Density

• Lower atmospheric densities yield LOWER aircraft performance

• Fix with special aircraft and power plant designs

• Density altitude – pressure altitude corrected for nonstandard temperature

• Note: Humid air is less dense than dry air at the same pressure and temperature