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According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will accelerate still air to 80 mph in three hours. Increase the distance to 1000 miles and the windspeed will be 40 mph after three hours. Don't know about you, but 80 mph surface winds are beyond my meager skills. I think your .5 inch figure is overstated. Bob Gardner "O. Sami Saydjari" wrote in message ... Well, I thought it was 0.5 inches, but I am not absolutley certain...it was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. My experience is that ATC does not give me settings of nearby airports. For example, I recall entering Chicgo Center airspace from the south. I was just north of Lafayette, Indiana. The controller gave me the altimeter setting for Chicago Midway. That is a long way away. Lafayette airport was at least 0.1 inches different. I used ATC's setting, because I figure they must have some standard reference point in a region, or sub-region, so that all the planes are at least on the same relative setting. -Sami N2057M, Piper Turbo Arrow III A Lieberman wrote: On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote: 0.5 inches is a HUGE altimter setting change. Are you sure about that? I have to agree with Roy here. I just finished coming back from Ohio to Mississippi, and I experienced a very large altimeter setting adjustments in my journey southbound north of Bowling Green KY, but I sure did not experience .50 inches adjustments en route. I did end up .25 inch less over the whole trip though. If I remember correctly on one ATC hand off, I went from 29.96 to 29.88 which lowered my altitude by 80 feet. This really alarmed me as I sure don't like deviating more then 20 feet from assigned altitude. Needless to say, I made a quick climb to my assigned altitude! There was a rather steep pressure gradient even in my trip I just did this week, but as Roy says, .5 inch altimeter change in a flight range distance in planes I fly is probably indicative of weather I wouldn't / shouldn't be flying in anyway. Allen |
#2
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OK, I am convinced that 0.5 inches was overstated.
So are we are concluding that it is best to switch to ATC altimeter setting as soon as they give them to you, and that differences from local airport settings, except when you are on approach to your destination (where you use that airport's setting). Still, I am curious to hear from a controller, how they pick their altimeter settings for their region of control. Is it always the same airport? Is the fix somewhere in the center of their region. Just curious. -Sami Bob Gardner wrote: According to "The Weather Book," by USA Today weather editor Jack Williams, a one-half-pound pressure difference between places 500 miles apart will accelerate still air to 80 mph in three hours. Increase the distance to 1000 miles and the windspeed will be 40 mph after three hours. Don't know about you, but 80 mph surface winds are beyond my meager skills. I think your .5 inch figure is overstated. Bob Gardner "O. Sami Saydjari" wrote in message ... Well, I thought it was 0.5 inches, but I am not absolutley certain...it was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. My experience is that ATC does not give me settings of nearby airports. For example, I recall entering Chicgo Center airspace from the south. I was just north of Lafayette, Indiana. The controller gave me the altimeter setting for Chicago Midway. That is a long way away. Lafayette airport was at least 0.1 inches different. I used ATC's setting, because I figure they must have some standard reference point in a region, or sub-region, so that all the planes are at least on the same relative setting. -Sami N2057M, Piper Turbo Arrow III A Lieberman wrote: On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote: 0.5 inches is a HUGE altimter setting change. Are you sure about that? I have to agree with Roy here. I just finished coming back from Ohio to Mississippi, and I experienced a very large altimeter setting adjustments in my journey southbound north of Bowling Green KY, but I sure did not experience .50 inches adjustments en route. I did end up .25 inch less over the whole trip though. If I remember correctly on one ATC hand off, I went from 29.96 to 29.88 which lowered my altitude by 80 feet. This really alarmed me as I sure don't like deviating more then 20 feet from assigned altitude. Needless to say, I made a quick climb to my assigned altitude! There was a rather steep pressure gradient even in my trip I just did this week, but as Roy says, .5 inch altimeter change in a flight range distance in planes I fly is probably indicative of weather I wouldn't / shouldn't be flying in anyway. Allen |
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"O. Sami Saydjari" wrote:
So are we are concluding that it is best to switch to ATC altimeter setting as soon as they give them to you That's what I'm concluding. You? and that differences from local airport settings, except when you are on approach to your destination (where you use that airport's setting). I'll always favor an altimeter setting ATC gives me over anything I hear on the ATIS. The ATIS could be up to an hour old; the controller is looking at the most current reading right now. |
#4
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Roy Smith wrote:
"O. Sami Saydjari" wrote: So are we are concluding that it is best to switch to ATC altimeter setting as soon as they give them to you That's what I'm concluding. You? and that differences from local airport settings, except when you are on approach to your destination (where you use that airport's setting). I'll always favor an altimeter setting ATC gives me over anything I hear on the ATIS. The ATIS could be up to an hour old; the controller is looking at the most current reading right now. And you do not think the controller is getting his information as reported by that station's ATIS / AWOS/ ETC? Ross |
#5
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![]() "Roy Smith" wrote in message ... I'll always favor an altimeter setting ATC gives me over anything I hear on the ATIS. The ATIS could be up to an hour old; the controller is looking at the most current reading right now. Which could also be up to an hour old. |
#6
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In article , Bob Gardner wrote:
According to "The Weather Book," by USA Today weather editor Jack Williams, a one-half-pound pressure difference between places 500 miles apart will accelerate still air to 80 mph in three hours. Increase the distance to 1000 miles and the windspeed will be 40 mph after three hours. Don't know about you, but 80 mph surface winds are beyond my meager skills. I think your .5 inch figure is overstated. I had a 0.05 difference about 30 miles apart last weekend and thought that was unusually high. Morris (what's an order of magnitude among friends?) |
#7
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![]() "Bob Gardner" writes: According to "The Weather Book," by USA Today weather editor Jack Williams, a one-half-pound pressure difference between places 500 miles apart will accelerate still air to 80 mph in three hours. [...] Does that refer to surface or upper winds? [...] I think your .5 inch figure is overstated. Well, today it's not hard to find two places on the continent with a 0.50 difference in altimeter settings: any place under the big storm in the north east, and another place far enough, like KORD. That's only a couple hours' flight in our bugsmashers. - FChE |
#8
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![]() "Frank Ch. Eigler" wrote in message ... "Bob Gardner" writes: According to "The Weather Book," by USA Today weather editor Jack Williams, a one-half-pound pressure difference between places 500 miles apart will accelerate still air to 80 mph in three hours. [...] Does that refer to surface or upper winds? [...] I think your .5 inch figure is overstated. Well, today it's not hard to find two places on the continent with a 0.50 difference in altimeter settings: any place under the big storm in the north east, and another place far enough, like KORD. That's only a couple hours' flight in our bugsmashers. Yes, Detroit to JFK would be just about .5 this evening. That translates to 500 feet down-error, even if you ignore the "fly toward the low, look out below" mantra and never adjust the altimeter. Allowing 200 feet altitude-maintenance error, you will still have 300 feet buffer from the adjacent flight level. Unless, of course, someone is flying in the opposite direction ALSO ignoring the mantra, in which case his error will be UP, while yours is DOWN, and interesting things may result. So know your weather situation before takeoff, and get QNH updates frequently along the route in such extreme situations. |
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