How is atmospheric pressure defined?
Force per unit area exerted against a surface by the weight of the air molecules above that surface.
In the vertical, what is hydrostatic equilibrium?
Gravity balances against the pressure gradient force
What is the hydrostatic equation?
dp/dz = -ρg
What is the equation for calculating pressure at scale height?
p=p0e^(z/He)
For every change in height of z=He. By what factor is pressure reduced?
2.72
What is the He value for dry air at T=273K?
8km
What is the H10 value (ln base 10) for dry air at T=273K?
16km
What are some units of pressure?
Inches of mercury, millibars, atmospheres, hecto-pascals
What is 1 atmosphere in mbar?
1013.25mbar
At what altitude is half the atmosphere pressure lost?
5.5km
What does it mean if the atmosphere is ‘isothermal’?
Pressure is directly proportional to density
What’s the density at sea level?
1.2 (290K) -1.3kg(273K) m^-3
What is the barometric or altimeter equation?
z/He = ln(p0) - ln(p)
What does the barometric equation do?
Allows us to measure the altitude above a given reference pressure level (usually the Earth’s surface) if we know the change in pressure moving to that level
What are the different reference pressure levels that are used for defining altitude for aircraft?
Aerodrome Elevation Pressure, Mean Sea Level Pressure and Standard Pressure Setting
What are the advantages of Aerodrome Elevation Pressure as a reference?
Simple for local operations
What are the advantages of Mean Sea Level Pressure as a reference?
Matches charts and terrain
What are the advantages of Standard Pressure Setting as a reference?
Consistent global separation
How can we calculate the ‘lapse rate’ for the lower atmosphere?
First law of thermodynamics, for an ideal gas the internal energy depends only on T, combine with the ideal gas law and the hydrostatic equation to get: dT/dz=-gM/1000Cp (change in temp with height).
What is the Dry Adiabatic Lapse Rate?
-9.81 C km^-1
Why are observed lapse rates smaller than DALR?
Process not always adiabatic, heat is exchanged as air is not always dry as it contains water vapour.
How can we relate temperature and volume at two different altitudes using gamma (g below) as a constant?
T1V1^g-1 =T2V2^g-1, assuming the atmosphere is an ideal gas of nitrogen and oxygen.
If temp is constant, what is the relationship between density and pressure?
Directly proportional
What are typical temp, pressure and density values for the South pole?
-18C to -76C, 675hPa and 1.5kgm^-3, respectively
