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#1
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How high is that cloud?
I picked up a cheap infarred themometer a few days ago and discovered
that it will happily tell me the temperature of the clouds if they are low. It seems to me that the temp gradiant from the ground is well known and if its consistant, this device combined with a table could tell me how low the coulds are. For example right now there are clear skys and its reading -27*C. METAR at hte local airport is claiming 230900Z 19011KT CAVOK 14/09 Q1027 so there should be a 41*C difference between ground and whatever the thing is reading the temp of in a clear sky. Today it was reading -8 with a cloud base of at least 5000 ft. Any comments on this? -tim http://web.abnormal.com |
#2
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Tim Hogard opined
I picked up a cheap infarred themometer a few days ago and discovered that it will happily tell me the temperature of the clouds if they are low. It seems to me that the temp gradiant from the ground is well known and if its consistant, this device combined with a table could tell me how low the coulds are. For example right now there are clear skys and its reading -27*C. METAR at hte local airport is claiming 230900Z 19011KT CAVOK 14/09 Q1027 so there should be a 41*C difference between ground and whatever the thing is reading the temp of in a clear sky. Today it was reading -8 with a cloud base of at least 5000 ft. Any comments on this? Clear sky should read 3*A, or about -270*C, the temperature of the cosmic background radiation . Cloud base temps should be surfaceTemp - altinThousands * 2. But I could be wrong. -ash Cthulhu in 2005! Why wait for nature? |
#3
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"Ash Wyllie" wrote in message ... Cloud base temps should be surfaceTemp - altinThousands * 2. But I could be wrong. It will be wrong when the lapse rate is different from standard, itself a significant indicator that something is afoot. When I read a METAR I often compare the observed ceiling to the surface temp to see if it's close to the 2C/1000 rule or not. If they're lower than they "should be" then you can bet there's a good chance of convection, turbulence, and generally fast-changing conditions. At least in New England this really only holds during the warmer months though. -cwk. |
#4
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C Kingsbury opined
"Ash Wyllie" wrote in message ... Cloud base temps should be surfaceTemp - altinThousands * 2. But I could be wrong. It will be wrong when the lapse rate is different from standard, itself a significant indicator that something is afoot. When I read a METAR I often compare the observed ceiling to the surface temp to see if it's close to the 2C/1000 rule or not. If they're lower than they "should be" then you can bet there's a good chance of convection, turbulence, and generally fast-changing conditions. At least in New England this really only holds during the warmer months though. Good point. -ash Cthulhu in 2005! Why wait for nature? |
#5
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"C Kingsbury" wrote in message ink.net... "Ash Wyllie" wrote in message ... Cloud base temps should be surfaceTemp - altinThousands * 2. But I could be wrong. It will be wrong when the lapse rate is different from standard, itself a significant indicator that something is afoot. When I read a METAR I often compare the observed ceiling to the surface temp to see if it's close to the 2C/1000 rule or not. If they're lower than they "should be" then you can bet there's a good chance of convection, turbulence, and generally fast-changing conditions. At least in New England this really only holds during the warmer months though. -cwk. Would you elaborate on this please? Are you talking about comparing the temp/dewpoint spread to the ceiling at 2C/1000? Cheers, John Clonts Temple, Texas N7NZ |
#6
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snip
It will be wrong when the lapse rate is different from standard, itself a significant indicator that something is afoot. When I read a METAR I often compare the observed ceiling to the surface temp to see if it's close to the 2C/1000 rule or not. If they're lower than they "should be" then you can bet there's a good chance of convection, turbulence, and generally fast-changing conditions. At least in New England this really only holds during the warmer months though. -cwk. Would you elaborate on this please? Are you talking about comparing the temp/dewpoint spread to the ceiling at 2C/1000? I think he's suggesting a comparison between the observed ceiling and the number of feet you get by dividing the temp/dewpoint difference by 2 and multiplying by 1000. (AGL in both cases.) If the actual and predicted ceilings are different then the lapse rate is not 2 -- not standard. If the bases are lower, then the lapse rate is deduced to be larger than 2. And vice versa. |
#8
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"Andrew Sarangan" wrote in message 1... Probably the greatest use of your device is for figuring out the lapse rate rather than cloud height. Get the cloud height and surface temperature from METAR, use your device to get the cloud temperature and calculate the lapse rate. A lapse rate higher than 2C/1000' will indicate an unstable atmosphere and bumpy flight conditions. Unlike the temperatures from the winds aloft forecast, yours will be actual temperature, not forecasted, and will be more accurate. Also, winds aloft data forecast is not accurate for calculating stability near the surface, as the temperature is omitted for the first 3000'. Extrapolating between surface and 6000' may not accurately reflect the instability close to the ground. 2C per thousand has nothing to do with stability. In unsaturated air, 2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the standard for calibrating altimeters, it has nothing to do with the real atmosphere or stability. Mike MU-2 |
#9
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On Wed, 24 Nov 2004 15:33:59 GMT, "Mike Rapoport"
wrote: 2C per thousand has nothing to do with stability. In unsaturated air, 2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the standard for calibrating altimeters, it has nothing to do with the real atmosphere or stability. Mike MU-2 Exactly. Stability is a function of the actual lapse rate and the dry (or moist) adiabatic lapse rate. The dry adiabatic lapse rate is 1C per 100 meters or 5 1/2deg F per 1000 feet. If the actual lapse rate is more than this, the air is unstable. In other words, a parcel of air will rise as long as the air around it is cooler than the parcel. The parcel will be 5 1/2 degrees F cooler after rising 1000 ft. If the surrounding air at 1000 feet is still cooler than the cooled parcel, the parcel keeps on rising. It will keep on rising (and cooling) until the parcel is the same temperature as the surrounding air, which has its own (different) lapse rate. That's why its smoother above cumulus clouds. The clouds mark the top of the column of rising air. |
#10
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"Mike Rapoport" wrote in message link.net...
2C per thousand has nothing to do with stability. In unsaturated air, 2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the standard for calibrating altimeters, it has nothing to do with the real atmosphere or stability. Mike MU-2 Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average for somewhat moist but unsaturated air. It is still a useful indicator of stability. I would not discount is as a completely meaningless number. It is a useful reference, just like 29.92" and 15C. Please explain how 2C/1000 is used in altimeter calibration. I did not know altimeters had any temperature corrections. |
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