quade 4 #1 April 15, 2002 As spring rapidly turns to summer it's probably a good time to review the concept of density-altitude.Density-altitude is a term to describe how dense the atmosphere is in comparison to the International Standard Atmosphere. The datum point for ISA is 59 degrees Fahrenheit (15°C), with a pressure of 29.92 inches of mercury (1013mb) at sea level. The following chart shows the ISA temperature values for some different altitudes above mean sea level.MSL F C8000 31°F -1°C7000 34°F 1°C6000 38°F 3°C5000 41°F 5°C4000 45°F 7°C3000 48°F 9°C2000 52°F 11°C1000 55°F 13°CSea Level 59°F 15°CAir density decreases as altitude increases. A rough rule of thumb is that with about every 18,000 to 20,000 ft of altitude gained, there is about half as much air density. So at about 40,000 ft, there is about one quarter the amount of molecules per cubic centimeter of air as found at sea level.Air density varies with temperature. For every ±15°F or ±8.5°C variation from the standard temperature at your pressure altitude, the density-altitude is increased or decreased by 1000 ft.To create the same amount of lift, for each 1000 ft increase in density-altitude the true airspeed will increase by 2 percent.Since kinetic energy increases at the square of the velocity, for each 1000 ft increase in density-altitude, the landing rollout (or the energy required to be absorbed by your legs) will increase by 4 percent.Going from a low altitude DZ like Monterey, California in the winter to a high DZ like Mile-Hi Colorado in summer, be aware that your canopy will land quite differently (a lot faster)!These effects can even be seen at a lower altitude DZ like Perris, California when it starts to get, uh, "warm". Landing at Perris (1410 MSL) in the summer with a temperature of 105°F can be the equivalent of landing at a DZ located over 5,000 feet above sea level on a "normal" day.Humidity is not much of a factor in determining the landing effects of density-altitude. Humidity is a factor for pilots during takeoff as it affects engine performance.quadehttp://futurecam.com Quote Share this post Link to post Share on other sites
Spy38W 0 #2 April 15, 2002 Quote As spring rapidly turns to summer it's probably a good time to review the concept of density-altitude.So, are you saying it's swoop huntin season? --~Captain CutawayI can disassemble a rig in less than 5 seconds... Quote Share this post Link to post Share on other sites
quade 4 #3 April 15, 2002 Yeah baby,Warmer temps should mean longer swoops.Ya don't want to push it if the thermals start up -- dust devils and other things.Maybe I should write an article about dust devils.quadehttp://futurecam.com Quote Share this post Link to post Share on other sites
RichM 0 #4 April 16, 2002 Quade,Thanks for the info. I have a question though - you have shown how the density altitude changes for varying altitude, and also for varying temperature, but how does the maths go for varying pressure?Rich M Quote Share this post Link to post Share on other sites
Spy38W 0 #5 April 16, 2002 Quote Maybe I should write an article about dust devils. Hrm, I'm in FL which I don't think lends itself to dustdevils (I could be wrong though), but I am a newbie. I am guessing I should be looking at additional thermals, especially when crossing over transitions from cool river/trees to hot runways/blacktop etc, but could you elaborate on anything I might have missed?I'm still new and dumb--~Captain CutawayI can disassemble a rig in less than 5 seconds... Quote Share this post Link to post Share on other sites
Spy38W 0 #6 April 16, 2002 Quote how does the maths go for varying pressure? Pressure altitude = displayed as the height above the standard datum plane when you adjust your altimeter to the standard sea level atmospheric pressure (29.92'' Hg). 1" HG is inversely proportional to 1000' of altitudeDensity altitude = Pressure altitude corrected for nonstandard temperature. Density altitude is more important in considering airfoil performance.I'm still new and stupid, so check with your local instructor before applying anything you read here.--~Captain CutawayI can disassemble a rig in less than 5 seconds... Quote Share this post Link to post Share on other sites
quade 4 #7 April 16, 2002 Quote . . . but how does the maths go for varying pressure?The pressure is referenced to ISA.Basically, 1 inch less of mercury equals a 1000 foot gain in altitude.Normal variations in local pressure, such as when a high or low passes, normally only varies by about 1 inch of mercury at most. That's about 1000 feet total or maybe about 500 feet either side of standard. In my opinion, not that big of a deal.If you were to do a true density-altitude calculation like pilot's should do before takeoffs at hot or high airports, then you'd want to know the field elevation, local pressure, temperature and maybe the humidity. Pilots can have little do-hickeys that do the math for them installed in their airplanes, but I don't know any of them that take humidity into the calculation -- at least not at my level. Maybe the guys flying the big iron have some more sophisticated instruments, but the average guy flying a C-172 probably doesn't even have a density-altitude meter. He does the math using either a rule of thumb (I gave several) or does an actual calculation using a graph.Some of the more notorious airports, Big Bear in California for example, have a density-altitude indicator right at the end of the runway -- big ol' numbers display -- just to remind folks about it. It's almost guaranteed that every year at the beginning of summer, some guy is going to end up in the lake because he didn't think about density-altitude when he took off out of Big Bear.quadehttp://futurecam.com Quote Share this post Link to post Share on other sites
nacmacfeegle 0 #8 April 16, 2002 Hey Paul, my climbing tent fabric (nylon) actually shrinks in warm temps. Does the same thing happen to canopy fabric, and is it considerable enough to be a factor here as well?CyaD Quote Share this post Link to post Share on other sites
quade 4 #9 April 16, 2002 Good question! Also, one that I don't actually know the answer to.Maybe one of the canopy manufacturers will chime in here.quadehttp://futurecam.com Quote Share this post Link to post Share on other sites
RichM 0 #10 April 16, 2002 Thanks for the further info, just one small obvious hole in my knowledge now - what is the correlation between mercury and millibars. All forecasting that I get is in millibars.Many thanks Quade. I look forward to the thermals (dust devils) post. Dust devils are an obvious stopper to swooping, but are there any other indications that conditions are less than ideal for yanking on the front riser? I noted an earlier post about buffeting, any others?Rich M Quote Share this post Link to post Share on other sites
nacmacfeegle 0 #11 April 16, 2002 Rich, 1 millibar = 0.0295299 inches of mercuryCyaD Quote Share this post Link to post Share on other sites
ltdiver 3 #12 April 16, 2002 Quote Good question! Also, one that I don't actually know the answer to.sorry, just couldn't resist!ltdiver____________________________________________LightDiverCam Quote Share this post Link to post Share on other sites
kallend 2,067 #13 April 16, 2002 <<also for varying temperature, but how does the maths go for varying pressure?Rich M>>>Take a look athttp://aero.stanford.edu/StdAtm.html Quote Share this post Link to post Share on other sites
quade 4 #14 April 16, 2002 You, of all people, should know that I don't have all the answers.quadehttp://futurecam.com Quote Share this post Link to post Share on other sites
ltdiver 3 #15 April 16, 2002 Yeah....ltdiver____________________________________________LightDiverCam Quote Share this post Link to post Share on other sites
lawrocket 3 #16 May 9, 2006 I thought I'd bump this considering that we are entering the season where this becomes more relevant. My wife is hotter than your wife. Quote Share this post Link to post Share on other sites
Avion 0 #17 May 15, 2006 Quote MSL F C 8000 31°F -1°C 7000 34°F 1°C 6000 38°F 3°C 5000 41°F 5°C 4000 45°F 7°C 3000 48°F 9°C 2000 52°F 11°C 1000 55°F 13°C Sea Level 59°F 15°C (29.92" 1013mb) Are there standard pressures for these altitudes as well? BW Quote Share this post Link to post Share on other sites
ZigZagMarquis 9 #18 May 16, 2006 http://wahiduddin.net/calc/calc_da.htm Quote Share this post Link to post Share on other sites
brianfry713 0 #19 May 16, 2006 Pressure drops about 1" of Hg per 1000 feet. Quote Share this post Link to post Share on other sites
mjosparky 4 #20 May 17, 2006 Quote http://wahiduddin.net/calc/calc_da.htm Now tell them where to find a current altimeter setting and dew point. My idea of a fair fight is clubbing baby seals Quote Share this post Link to post Share on other sites
ZigZagMarquis 9 #21 May 17, 2006 Quote Quote http://wahiduddin.net/calc/calc_da.htm Now tell them where to find a current altimeter setting and dew point. Well... that web-site had a link to where you can find that info... right under the gooie where you input the info and get the answer, but here it is... http://weather.noaa.gov/ ... so, an example, for California City Skydive at apx 0950L, 17 May 06 (use North Edwards AFB for current conditions... close enough): Altitude 2454 feet Air Temperature 73.9 degrees F Altimeter Setting 29.97 inches Hg Dew Point 46.9 degrees F ... thus: Density Altitude 4063 feet Absolute Pressure 27.407 inches Hg Relative Density 88.64 % So, this means that although the field elevation is 2454', if you were landing your canopy (or airplane) there at the time / conditions in this example, your canopy (or aircraft) would perform as if it were landing at 4063' on a standard day. Quote Share this post Link to post Share on other sites
mjosparky 4 #22 May 17, 2006 Thank you dear!My idea of a fair fight is clubbing baby seals Quote Share this post Link to post Share on other sites
