narquespamley wrote:Computer engineer (really) here.
Hahahahaha! Thanks for the reply!
narquespamley wrote:So yes, air in the atmosphere does get colder as you go higher up. However, that's only a concern if you're skydiving, or maybe if you have a habit of opening the window on your airplane for some fresh air.
Oh, au contraire! It's also a concern if - and this is a true story - you want to be a smart@$$ about captions on post cards.
I have a photo post card from the late 1920s or early 1930s showing cars parked at the summit of Pikes Peak, all with their hoods up. The caption says something about a hot day on the mountain.
But even if the temperature had been 100° (which is uncommon) at their starting point of Colorado Springs (~6,500'), by the time they reached the summit (>14,000') they'd have gained 7.5 kilofeet in elevation.
At an average lapse rate of 3.6°/kf, the temperature at the summit would have been no more than 73°. "Hot" for the summit, but not for cars.
No, I believe the engines were overheating because air pressure (and therefore oxygen available to the engines to burn gasoline and therefore power available from the engines) went down down down as they went up up up.
But that part's just a guess.
Also of practical significance is that if you want to escape the city heat (of Denver, especially), you drive up into the mountains.
narquespamley wrote:But to understand the issue, you have to look at why electronic equipment has an altitude spec in the first place.
Yes, good. That's what I wanted to know.
narquespamley wrote:That is because air at high altitude is thinner.
Like, it's on a diet? ;-)
narquespamley wrote:Thinner air doesn't cool electronics as well because there are fewer air molecules to carry away the heat with.
The worst case is when equipment is at high altitude AND at high temperature.
Summertime in the Rockies is heck on aviation. Aspen's airport is at 7,820' while Leadville's (allegedly the highest in North America) is at 9,927' (and has no scheduled commercial traffic). Ain't much of that there Wind Beneath My Wings.
narquespamley wrote:So the worst case is actually somebody like a field engineer in Denver on a hot summer day.
Or maybe someone servicing the slot machines in Leadville, the city of which is at 10,152'.
narquespamley wrote:There is a relationship between temperature specification and altitude specification called a derating curve. A typical derating is 1 degC/1000 ft (1.8 degF/1000 ft). So if a given piece of equipment is specified to operate at 40 degC and 5000 ft, it will also work at 39C and 6000 ft, or 30C and 15000ft.
Cool! Errr ... no pun intended. Thanks very much for the info!