Atmospheric Pressure

Pressure Measurements

• Inches of mercury (Hg) >> for altimetry
• Hectopascals (hPa) and millibars (mb) >> for weather map analysis
• measured using a barometer (digital or aneroid type)

METARs give both an altimeter setting and a sea level pressure.  What's the difference?

• Station Pressure
• measured at the airport; the weight of the air above the station
• Altimeter Setting
• station level pressure reduced to MSL assuming ISA conditions
• MSL Pressure
• station pressure reduced to MSL using the average surface temperature for the last 12 hours
• useful in weather map analysis when patterns across different observation stations must be compared

And just for fun, here's 'Under Pressure' by Queen:

Pressure Levels

In my January 2 post about the Earth's atmosphere, I said "some things you just have to memorize" with regards to which pressures correspond to which levels of the ICAO standard atmosphere.  As it turns out, memorization is not required if you have a CX-2 flight computer (and possibly others, but this is the one I have)!   Here's how:

• press 'FLIGHT'
• select (1) Altitude
• select (3) Std Atmos
• enter the altitude
• read the OAT, as well as pressure in "Hg and mb!

Small wins :)

The Earth's Atmosphere

Being a complete aviation nerd, I have no shame in admitting that Meteorology was one of my favourite subjects in flight school. And whereas I'd like to jump into things like lapse rate and stability, I'll save those for later and start with the basics.  Our trusty Study and Reference Guide starts with these topics about the Earth's Atmosphere:

Properties - from ACWM Chapter 2

• mobility
• capacity for expansion
• capacity for compression

Vertical Structure - in ascending order

• troposhere
• contains all the elements of the weather
• temperature decreases with altitude
• tropopause
• height varies from about 8 km over the poles up to 17 km over the equator
• there is an abrupt change in height over each frontal surface
• higher in the summer than in the winter
• In ISA conditions its height is 36 090 ft and temperature is -56.5 C
• stratosphere
• temperature remains steady and then increases with altitude
• stratopause
• temperature starts to decrease with altitude
• mesosphere
• temperature decreases with altitude to about 275 000 ft
• mesopause
• lowest temperature
• thermosphere
• where aurora are seen
• temperature increases dramatically with altitude

ICAO Standard Atmosphere (ISA)

• At sea level: 15 C and 1013.2 hPa / 29.92"Hg
• Average lapse rate is 1.98 C / 1 000 ft

Some things you just have to memorize:

Pressure Level (mb)	Altitude (ft)
700	10000
500	18000
400	24000
250	34000
200	39000
150	45000

And now... time for a POP QUIZ!

(1) What are lines drawn on weather charts joining places having the same temperature called?

(2) Does TAS increase or decrease with temperature?

(3) If the temperature at FL300 is -50, how would this be expressed in terms of an ISA deviation?