Barometric Pressure at Altitude

[meritflyer]

This may be a dumb question -

Does barometric pressure decrease with altitude? Temperature?

Thanks.

[fish314]

Both pressure and temperature decrease with altitude. "Barometric pressure" is actually redundant. A "barometer" is just a device used to measure air pressure, so what you are really refering to is static pressure, or the pressure measured when airspeed is taken out of the question. (Not the best technical definition, but it will work). Static pressure is the result of the weight of the column of air above you. As go up in altitude, there is less air above you weighing down on you. So that means less pressure.

As for temperature, it decreases until you reach an altitude where it is constant. This is called the "tropopause" (which means "temperature stops"). In general it decreases at something called the standard lapse rate, which is equal to about 2 degrees Celsius (3.6 degrees Fahrenheit) per 1000'.

[todd584]

to add to what Fish said, the pressure level at 18,000ft is aprox. half of what it is at sea level

[desertdog71]

Pressure decreases 1 inch per 1000 feet

[meritflyer]

1" per 1000? You sure?

[Minuteman]

Pressure as a function of altitude is non-linear.



Modeled as an ideal gas, the ambient pressure will be a function of temperature and altitude (i.e., the weight of the atmosphere above that point.). That said, 1" per 1000 feet is an OK rule of thumb from sea level to about 6000 feet.

[AlexF]

Yep, those are the very rough numbers. ~1" of mercury per 1000ft. Based on a Standard Atmospheric Day, that would start with a base of 29.92" (1013.2mb) at Sea Level

[subpilot]

1" per 1,000ft would mean that sea level being basically 30"... at 18K ft the pressure would be 12" and at 30K ft the pressure would be 0".

I understand that the private pilot test teaches the 1" equals 1,000ft thumbrule but a commercial pilot needs to understand that it is more complex than what the student pilot is taught.