Re: [Phys-l] Atmospheric pressure calculus deduction

["LaMontagne, Bob" <RLAMONT@providence.edu>]

Actually, pilots often get this information automatically by tuning their radios to weather observations offered at many airports across the country (These radio broadcasts are found on aviation charts under the acronyms AWOS or ASOS). In the comments section at the end of these broadcasts one often hears the current "density altitude" at the reporting airport. for example, an airport at 100 feet above sea level might report a density altitude of 300 feet on a particular hot day. The pilot then goes to the Operating Handbook for their aircraft to determine if they can safely take off from that length runway on a particular day.
 
The rough rule of thumb for someone to do this without the radio broadcast is to first set the altimeter so that it assumes that the atmospheric pressure at sea level is 29.92 inches (there is a little knob to set this). One then notes the indicated altitude in feet (called the pressure altitude). The standard temperature for that altitude is estimated by assuming a sea level temperature of 15 deg C and using a lapse rate of 2 deg C per thousand feet (e.g., a standard temperature of 5 deg C at 5000 feet). Then look at the actual outside temperature (usually from a thermometer mounted on the windshield) and make an adjustment of roughly 100 ft for each degree that the actual temperature exceeds the standard. That will produce the approximate density altitude.
 
Bob at PC

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Pete Lohstreter
Sent: Wed 6/18/2008 10:15 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Atmospheric pressure calculus deduction



Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
> This looks like a physicist's atmosphere to me.
> Temperature plays a role in determining seal level pressure.
> All the way up!

Actually, a pilots atmosphere.  This is how airlines and private pilots
determine the quality of the atmosphere for safe flight.

Yes, temperature is a factor.  The equation assumes a standard lapse rate.

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