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AMOFSG/7-IP/4
/ AMOFSG/7-IP/4
27/.6/08

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

1.1Aviation 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.

1.2The SAE in developing Hold Over Time tables uses a specific test protocol with defined liquid water equivalent (LWE) precipitation rates.

1.3The following is a discussion to further explain how the SAE conducts testing and the basis for precipitation rate criteria.

2.Discussion

2.1Since 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

2.2The 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.

2.3Under 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

2.4The 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

2.5The 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.”

2.7As 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 de-icing, 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.

2.8Note 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).

Furthermore, the lower threshold between light and moderate, 1.0 mm/hr, is the same for both the SAE and WMO. [SA1]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))

2.9The 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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[SA1]I would not make any comment such as “we accept”. This begs a question for others to debate the issue. We want straight facts!!!