uwochris
Flightinfo's sexiest user
- Joined
- Dec 21, 2001
- Posts
- 381
Hey guys,
I hope everyone is having a good Christmas so far. Pilots do get Christmas off, right?
I have another question, but this relates to aircraft instruments, particularly the altimeter.
My understanding is that the altimeter functions as a barometer. It displays an altitude for a given pressure, calibrated so that a 1 inch of mercury decrease in pressure equates to 1000ft gain in altitude. Pressure decreases as you climb, and it increases as you descend.
My first question has to do with the rate of pressure decrease. I read that in the lower levels of the atmosphere (below 5000ft), pressure decreases 1 inch of mercury for every 1000ft gain in altitude, but only .3 inches for every 1000ft in the higher levels (all under ISA). Why does the rate of pressure decrease vary? Does it have to do with the density of the air? I figured that since the density of the air decreases, it must be exerting a lesser force (i.e. less weight, less force). Something tells me I am wrong though.
I read elsewhere that warm air has higher pressure than colder air, also, cold air has a higher density than warm air. If cold air is more dense, and it sinks, wouldn't this translate to higher pressure? Maybe the cold air produces (or is a factor in producing) the higher pressure but has lower pressure in itself. Is this right? How is density related to pressure, and how does it relate to varying pressures?
Also, I am confused with temperature errors and altimetry. I do not understand how pressure changes in different temperatures, and what types of errors are related (i.e. why does the altimeter overread in colder than ISA conditions?) I read that pressure decreases more rapidly in colder air than in warmer air, but again I do not understand why (does it have to do with density?). This must be a reason for the altimeter temperature errors, and I hope someone can please clarify this up.
Maybe I'm going over my head with this stuff, but I just want to make sure I understand this stuff. I know how important meteorology is, and I don't just want to memorize it- I want to understand it. Basically, my two questions are about the rate of pressure decrease, and temperature errors. These have been causing me great difficulties, and I want to get this stuff down cold.
I really appreciate you guys taking the time to reply. I hope you all have a great Christmas!
I hope everyone is having a good Christmas so far. Pilots do get Christmas off, right?
I have another question, but this relates to aircraft instruments, particularly the altimeter.
My understanding is that the altimeter functions as a barometer. It displays an altitude for a given pressure, calibrated so that a 1 inch of mercury decrease in pressure equates to 1000ft gain in altitude. Pressure decreases as you climb, and it increases as you descend.
My first question has to do with the rate of pressure decrease. I read that in the lower levels of the atmosphere (below 5000ft), pressure decreases 1 inch of mercury for every 1000ft gain in altitude, but only .3 inches for every 1000ft in the higher levels (all under ISA). Why does the rate of pressure decrease vary? Does it have to do with the density of the air? I figured that since the density of the air decreases, it must be exerting a lesser force (i.e. less weight, less force). Something tells me I am wrong though.
I read elsewhere that warm air has higher pressure than colder air, also, cold air has a higher density than warm air. If cold air is more dense, and it sinks, wouldn't this translate to higher pressure? Maybe the cold air produces (or is a factor in producing) the higher pressure but has lower pressure in itself. Is this right? How is density related to pressure, and how does it relate to varying pressures?
Also, I am confused with temperature errors and altimetry. I do not understand how pressure changes in different temperatures, and what types of errors are related (i.e. why does the altimeter overread in colder than ISA conditions?) I read that pressure decreases more rapidly in colder air than in warmer air, but again I do not understand why (does it have to do with density?). This must be a reason for the altimeter temperature errors, and I hope someone can please clarify this up.
Maybe I'm going over my head with this stuff, but I just want to make sure I understand this stuff. I know how important meteorology is, and I don't just want to memorize it- I want to understand it. Basically, my two questions are about the rate of pressure decrease, and temperature errors. These have been causing me great difficulties, and I want to get this stuff down cold.
I really appreciate you guys taking the time to reply. I hope you all have a great Christmas!