Moderate snow?
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Moderate snow?
humour me but coming into LHR the other day (and not adviSed of a late change of rwy) we were advised of moderate snow falling....jeez... That to me is buckets of snow falling, braking action comes to mind etc etc.....in reality it was a sprinkle of wet snow....what is defined as moderate snow, as giving reports of same rachets up a situation where uncalled for. I was'nt with impressed with them at all.........
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From what I recall of the training I did - which was many years ago the qualifiers light, moderate and heavy were supposed to be used in the context of what should be expected in the local environment. Hence, what might be moderate snow in the UK could be considered little more than an insignificant flurry to a Scandinavian.
It's also rather subjective - each individual doing an observation has to decide on the qualifier. Remember also that the report is based on what was happening at the time of the observation - and for the preceding 10 minutes - and unless a significant change has occurred, that report will stand. Admittedly, though, I'm pretty sure the cessation of snow is classed as a significant change.
And then there's the 'automatics'. I'm not sure of how met obs are done at LHR these days but many large airports in the UK now use automatic observing stations to prepare an ob. The ob is then validated by a human (maybe someone else can tell you how much training a validating observer gets) - and that human, of course, makes a subjective assessment on which any adjustments are made. When I last worked with any of these automatic observing stations the sensing systems for precipitation were notoriously easily fooled and inaccurate.
Put all this together and maybe you can see why the ob you received did not accord with your mental picture of what was there. Or, to put it another way, it's surprising that obs are not misleading more often!
It's also rather subjective - each individual doing an observation has to decide on the qualifier. Remember also that the report is based on what was happening at the time of the observation - and for the preceding 10 minutes - and unless a significant change has occurred, that report will stand. Admittedly, though, I'm pretty sure the cessation of snow is classed as a significant change.
And then there's the 'automatics'. I'm not sure of how met obs are done at LHR these days but many large airports in the UK now use automatic observing stations to prepare an ob. The ob is then validated by a human (maybe someone else can tell you how much training a validating observer gets) - and that human, of course, makes a subjective assessment on which any adjustments are made. When I last worked with any of these automatic observing stations the sensing systems for precipitation were notoriously easily fooled and inaccurate.
Put all this together and maybe you can see why the ob you received did not accord with your mental picture of what was there. Or, to put it another way, it's surprising that obs are not misleading more often!
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Use reported Met vis as a back up.
From transport Canada. This is in reference to Hold Over Times but could be used for determining snowfall intensity for landing.
VISIBILITY IN SNOW vs SNOWFALL INTENSITY CHART
LIGHTING TEMPERATURE RANGE VISIBILITY IN SNOW(metres)
˚C HEAVY MODERATE LIGHT
Darkness -1 and above ≤ 1600 >1600 to 4000 > 4000 to 6400
below -1 ≤ 1200 >1200 to 2400 > 2400 to 4800
Daylight -1 and above ≤ 800 >800 to 2400 > 2400 to 4800
below -1 ≤ 600 >600 to 1400 > 1400 to 3200
CAUTION: No holdover guideline exists for Heavy Snow. Takeoff not permitted in Heavy falls of WET Snow (FOM) NOTE: data based on Reported Visibility, rather than RVR
Example how to use; during Daylight, reported temperature is -2 and reported Visibility is 1300 metres. Using the table above result
in determination of Moderate Snow intensity. Crew would use “Snow” rather than “Light Snow” values on Holdover table
VISIBILITY IN SNOW vs SNOWFALL INTENSITY CHART
LIGHTING TEMPERATURE RANGE VISIBILITY IN SNOW(metres)
˚C HEAVY MODERATE LIGHT
Darkness -1 and above ≤ 1600 >1600 to 4000 > 4000 to 6400
below -1 ≤ 1200 >1200 to 2400 > 2400 to 4800
Daylight -1 and above ≤ 800 >800 to 2400 > 2400 to 4800
below -1 ≤ 600 >600 to 1400 > 1400 to 3200
CAUTION: No holdover guideline exists for Heavy Snow. Takeoff not permitted in Heavy falls of WET Snow (FOM) NOTE: data based on Reported Visibility, rather than RVR
Example how to use; during Daylight, reported temperature is -2 and reported Visibility is 1300 metres. Using the table above result
in determination of Moderate Snow intensity. Crew would use “Snow” rather than “Light Snow” values on Holdover table
Braking action?!?Join Date: May 2008
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Both photos in Yamagata (northern Honshu). The second is the road to Gassan. It opens for skiing in June and runs till August. It is closed in the winter, too much snow.
We get an average of 12 metres (fresh accumulation) snow per winter. 10cm overnight is trivial. Yamagata airport, manages to stay open all year round. Very rarely closed by weather.
We get an average of 12 metres (fresh accumulation) snow per winter. 10cm overnight is trivial. Yamagata airport, manages to stay open all year round. Very rarely closed by weather.
Billy Madrid suggests that visibibility might be used as an indication of snow intensity. It seems to me that this would work well in the case of very dry (cold) snow in otherwise low humidity, i.e., an absence of mist over the snow field. Unfortunately, these conditions are almost unknown in the south of England. We just don't experience Canadian weather here.
It seems strange that the description of precipitation intensity on data for international operations should be relative to local norms, as Spitoon states, rather than a worldwide standard. Having grown up in the Tropics and worked long-haul, I do think that European Met forecasters tend to exaggerate precipitation probabilities and intensity, and suspect that some observers may also be guilty of it. Rather like my dear wife... Not helpful for young aviators when first abroad.
It seems strange that the description of precipitation intensity on data for international operations should be relative to local norms, as Spitoon states, rather than a worldwide standard. Having grown up in the Tropics and worked long-haul, I do think that European Met forecasters tend to exaggerate precipitation probabilities and intensity, and suspect that some observers may also be guilty of it. Rather like my dear wife... Not helpful for young aviators when first abroad.
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If 'moderate snow' is going out on the ATIS, its the intensity of the current precipitation that is being reported, not the physical amount/volume of snow. The report is created by the use of a combination of semi-automated weather monitoring system, plus a good ol' Mk1 Eyeball observation by the VCR ATSA - all done as per the local instructions/Met Office reporting criteria.
When there really is 'buckets of snow' on the ground, then it is the responsibility of the Airfield Operator to advise the VCR ATSA of the constitution and depth of snow, and the braking action, both of which will then be included in the ATIS broadcast.
"I was'nt with impressed with them at all.........
"
shame. you might wish to reflect on the fact that some days you guys do things which dont exactly 'impress' us, either
When there really is 'buckets of snow' on the ground, then it is the responsibility of the Airfield Operator to advise the VCR ATSA of the constitution and depth of snow, and the braking action, both of which will then be included in the ATIS broadcast.
"I was'nt with impressed with them at all.........
"shame. you might wish to reflect on the fact that some days you guys do things which dont exactly 'impress' us, either
