As a rule of thumb, the ISA (International Standard Atmophere) looks
Pressure: 29.92" Hg at sea level. Lapses at roughly 1" Hg / 1000'.
Round figures, at 5000': (30 - 5)" Hg = 25" Hg, or
83% of the sea level value.
Temperature: 15 deg C at sea level, lapsing 6.5 deg C per km, or
1.98 deg C / 1000'.
At a constant temperature, the density is proportional to pressure.
Provided you're close to 15 deg C (59 deg F, if I'm correct), you can
accept this proportionality with very little error. A lower
temperature leads to higher density, while a higher temperature leads
to a lower density. To quantify the effect, convert the temperature
deg C + 273 = kelvin
Example: 15 deg C = 288 K
Then calculate the percentage change from this value. For example,
if the lowest expected ambient temperature (worst case) is -10 deg C,
the change is 25 K, or almost 10%. This means the dynamic pressure
will be 10% more than expected on the basis of the pressure alone.
Dynamic pressure (as mentioned before) is
0.5 (density) (velocity) (velocity)
Make sure you check the units.
I hope this helps. Didn't have time to design a well reasoned
treatise, but this should get you started.
Chris R. Burger ZS6EZ
South African Airline Transport Pilot License, Instructor's Rating
US FAA Commercial SEL/MEL, Instrument
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