BenGriffiths 0 #1 November 24, 2003 As part of my Uni project I need to know what sort of air pressure ranges I'll being experiencing. I know the pressure at ground/sea level vary either side of 1000 millibars (i.e. 29.53 inches). But how much will height and weather affect these values.---------- Ben G Still Sinking :-( Quote Share this post Link to post Share on other sites
Hooknswoop 19 #2 November 24, 2003 Standard air pressure is 29.92 or 14.7 psi. 18,000 feet MSL, is about 1/2 an atmosphere or 7.35 PSI. So, figure 13,500 back to sea level, depending on the DZ. Derek Quote Share this post Link to post Share on other sites
kallend 1,635 #3 November 24, 2003 QuoteAs part of my Uni project I need to know what sort of air pressure ranges I'll being experiencing. I know the pressure at ground/sea level vary either side of 1000 millibars (i.e. 29.53 inches). But how much will height and weather affect these values. www.booty.demon.co.uk/metinfo/isa.htm also try Googling "ICAO standard atmosphere"... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
jrpayne 0 #4 November 24, 2003 Standard air pressure is 29.92 inches HG at sea level. The pressure decreases as you increase altitude at rate of 1 inch per thousand feet. Weather systems rarely change the air pressure more than one inch. Quote Share this post Link to post Share on other sites
billbooth 10 #5 November 24, 2003 For our jumps at the North Pole, I calculated the barometric pressure at the surface of the ice cap. At sea level and minus 40 C, it worked out to 31.50 " mercury. That's roughly equivalent to a density altitude of MINUS 7,200 feet. Even tiny canopies fly "big" in that very "thick" air. You need special aircraft altimeters to fly in the polar regions, because "normal" altimeters only adjust to 31.00 " mercury. Jumping altimeters, and AAD's seem to handle it without any problem though. Quote Share this post Link to post Share on other sites
kallend 1,635 #6 November 24, 2003 QuoteStandard air pressure is 29.92 inches HG at sea level. The pressure decreases as you increase altitude at rate of 1 inch per thousand feet. Weather systems rarely change the air pressure more than one inch. That is an oversimplification for the change of pressure with altitude. A winter storm in Chicago dropped the pressure by 1.64 inches Hg the first year I came here. Here's another useful site: aero.stanford.edu/StdAtm.html... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
jrpayne 0 #7 November 24, 2003 No its not an oversimplification, it's the standard pressure lapse rate. Quote That is an oversimplification for the change of pressure with altitude. Changes of that magnitude are rare. Even in that storm I'm sure the pressure didn't drop 1.64 inches below the standard station pressure it probably was the total change between the high and the low. Pressures changes larger than one inch from the standard pressure for a given station are not very common, and usually involve very significant weather such as hurricanes. Quote A winter storm in Chicago dropped the pressure by 1.64 inches Hg the first year I came here. Quote Share this post Link to post Share on other sites
kallend 1,635 #8 November 25, 2003 Quote That is an oversimplification for the change of pressure with altitude. QuoteNo its not an oversimplification, it's the standard pressure lapse rate. OK, use your "standard lapse rate" to compute the pressure at 29,920 ft. Pressure is not a linear function of altitude.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
masher 1 #9 November 25, 2003 Doesn't anyone use SI? A standard atmosphere is 101.3 kPa. Of course YMMV.-- Arching is overrated - Marlies Quote Share this post Link to post Share on other sites
jrpayne 0 #10 November 25, 2003 I agree. However in the lower levels of the atmosphere you can figure pressure decreases roughly one inch per thousand feet. QuotePressure is not a linear function of altitude. The original post ask what range of air pressure would be experienced during a normal skydive. Would you disagree that the pressure would drop lightly less than 10 inches during a skydive from 10,000 ft.? Quote Share this post Link to post Share on other sites
BenGriffiths 0 #11 November 25, 2003 QuoteDoesn't anyone use SI? A standard atmosphere is 101.3 kPa. Of course YMMV. I wish but companies that seeling pressure sensors use everything form Inches of water, Inches of mecury, psi, psia, psig, milliBars, bars etc...---------- Ben G Still Sinking :-( Quote Share this post Link to post Share on other sites
kallend 1,635 #12 November 25, 2003 QuoteI agree. However in the lower levels of the atmosphere you can figure pressure decreases roughly one inch per thousand feet. QuotePressure is not a linear function of altitude. The original post ask what range of air pressure would be experienced during a normal skydive. Would you disagree that the pressure would drop lightly less than 10 inches during a skydive from 10,000 ft.? My "normal" skydives are from around 14,000ft msl, at which the ICAO pressure is 17.58 in Hg. Your formula gives 15.92 in Hg. So setting or calibrating an altimeter by your formula would give an error of over 1,500ft. I don't think FAA would approve for IFR.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
kbell 0 #13 November 26, 2003 QuoteAs part of my Uni project I need to know what sort of air pressure ranges I'll being experiencing. I know the pressure at ground/sea level vary either side of 1000 millibars (i.e. 29.53 inches). But how much will height and weather affect these values. From the data recorded by my Pro-Track, the ground level ranges between 993 and 1016 mbar with an average of 1002 mbar over the 87 jumps I have in my current database. The data is from DZs in southcentral Texas with ground level between 200-400 ft MSL. I've also attached a file showing a freefall profile with the pressures listed along with the altitudes. Hope this helps, Kevin Quote Share this post Link to post Share on other sites
BenGriffiths 0 #14 December 2, 2003 Quote Hope this helps, Kevin Thanks. P.S. Sorry for the Late Reply (Damm you UNI & Work)---------- Ben G Still Sinking :-( Quote Share this post Link to post Share on other sites
