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AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
SEVENTH MEETING
Montréal, 9 to 12 September 2008
Agenda Item 5: Aerodrome observations
SNOWFALL RATE THRESHOLDS FOR LIGHT, MODERATE AND HEAVY
(Presented by Tom Fahey)
1.
INTRODUCTION
1.1
Aviation operations during winter weather conditions require that aircraft be de-iced and
anti-iced to remove ice/snow and protect against re-icing, respectively. Hold Over Times and Allowance
Times are used by pilots and other aviation de-icing/anti-icing staff to support decision making such as
the type of de-icing/anti-icing fluid to be applied or maximum time prior to return for additional
treatment. Hold Over and Allowance Times are specified based on the precipitation intensity, type and
air temperature. Holdover times have been developed by the Society Automotive Engineers (SAE)
International Committee G-12 –Aircraft Ground De-icing. Allowance times were developed by the
Federal Aviation Administration (FAA) and Transport Canada.
1.2
The SAE in developing Hold Over Time tables uses a specific test protocol with defined
liquid water equivalent (LWE) precipitation rates.
1.3
The following is a discussion to further explain how the SAE conducts testing and the
basis for precipitation rate criteria.
2.
DISCUSSION
2.1
Since 1988 the SAE Ground De-icing committee has used the following LWE intensity
definitions for testing during snowfall:
0 – 1 mm/hr: Light
1-2.5 mm/hr: Moderate
> 2.5 mm/hr: Heavy
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2.2
The published holdover times are based on using the thresholds of 1 mm/hr and 2.5
mm/hour for snowfall rate transitions between light to moderate, and moderate to heavy, respectively.
The upper threshold is of particular importance since there are no holdover times defined for heavy snow,
at rates greater than 2.5 mm/hour.
2.3
Under current practices airlines instruct their pilots and other aviation de-icing/anti-icing
staff to use intensity as a surrogate for liquid water equivalent rate for snowfall. The intensity of snowfall
is obtained from the local report or the METAR/SPECI which uses visibility to estimate snow intensity
with the following criteria:
Greater than or equal to 1.0 km (greater than ½ SM [statute mile]): Light
0.4 km< visibility < 1.0 km ( > 1/4SM and less than or equal 1/2SM : Moderate
Less than or equal to: 0.4 km (less than or equal 1/4SM) : Heavy
2.4
The Experts at the WMO 1997 meeting suggested that the LWE intensity for snowfall
could be defined as follows:
0 – 1 mm/hr: Light
1- 5.0 mm/hr: Moderate
> 5.0 mm/hr: Heavy
2.5
The following quote from the 1997 World Meteorological Organization (WMO) final
report summarizes the recommendations of the experts attending that meeting (the thresholds referred to
are the values of 1.0 mm/hr for light to moderate snow and 5.0 mm/hr for moderate to heavy snow):
2.6
“It was agreed that for all forms of precipitation the water equivalent should be the
criterion for determining the intensity thresholds within the existing code. Users may have to define
different thresholds for their applications if the thresholds given below do not meet their needs. This,
however, would significantly limit the availability of equipment with standardized output. Where mixed
precipitation is detected the type of higher importance (higher code number) should be reported. The
determination of the intensity of mixed and solid precipitation should be based on the water equivalent,
noting that the intensity of snow is presently based on visibility and the intensity of freezing and frozen
precipitation; e.g. ice pellets, is subjective. Where possible, definitions should not depend on the method
of measurement.”
2.7
As stated above, the WMO snowfall rate definitions are only suggested definitions, and it
is left it up to individual nations or specific users to define their own appropriate rates. The above WMO
definition has been presented in ICAO Document 9837 - Manual on Automatic Meteorological Observing
Systems at Aerodromes with regard to the thresholds used by automated precipitation systems to
determine light, moderate, or heavy snow intensity. The current information paper advocates for a slightly
revised liquid equivalent intensity categorization based on operational experience from aircraft ground deicing, laboratory and field testing of de-icing fluids and consistency of the visibility and liquid equivalent
definitions of snow intensity and will be further discussed in the following.
2.8
Note that the upper threshold for the WMO criteria is a factor of two higher than the
threshold used for Holdover Tables developed by the SAE (5.0 mm/hr in the WMO criteria vs 2.5 mm/hr
in the SAE criteria).
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Furthermore, the lower threshold between light and moderate, 1.0 mm/hr, is the same for both the SAE
and WMO. Theoretical relations between visibility and snowfall liquid equivalent rate (Rasmussen et al.
1999) show that visibility is inversely proportional to snowfall rate. At a rate of 1 mm/hr rate, the
visibility is 1 km. Thus, the equation relating the visibility to rate can be understood as:
Rate (mm/hr) = 1/(Visibility(km))
2.9
The visibility for the threshold between moderate and heavy is 0.4 km (1/4SM).
Substituting this value into the above equation gives a rate of 2.5 mm/hr. This rate is the same value that
the SAE Ground De-icing committee used, and is consistent with the visibility criteria stated above which
is accepted in both the WMO and SAE communities. In addition, a snowfall depth (unmelted)
accumulation of 1 inch/hr or 25.4 mm/hr has been used by weather observers as a useful threshold for the
onset of heavy snow. Using a typical ratio of depth to melted snow of 10:1, yields a liquid equivalent
threshold of 2.54 mm/hr for the threshold between moderate and heavy snow rate using this methodology
as well. Thus, the threshold of 2.5 mm/hr is a reasonable liquid equivalent threshold rate given a threshold
of 1.0 mm/hr for the threshold between light and moderate and standard practices.
References:
ICAO document 9837, Manual on Automatic Meteorological Observing Systems at Aerodromes.
Rasmussen, R.M., J. Vivekanandan, J. Cole, B. Myers and C. Masters, 1999:
The estimation of snowfall rate using visibility. J. Appl. Meteor.,
38(10),
1542-1563.
World Meteorological Organization Commission for Instruments and Methods of Observation, Expert
Meeting on Automation of Visual and Subjective Observations, Trappes/Paris, France, 14 - 16 May 1997,
Final Report.
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