Chapter 2 - Theory and Aircraft Performance

Snow

19.  The meteorological approach to estimating snow rate has always been based on visibility alone. Scientific research has indicated that the use of visibility in snow as the sole criteria for establishing snowfall rate/intensity is invalid. The evidence indicates that a visibility and temperature pair needs to be used for establishing more accurate snowfall rates. The highest snowfall rates occur near 0°C. The Visibility in Snow versus Snowfall Intensity Chart contained in the Transport Canada HOT Guidelines document is based on Research sponsored by Transportation Development Centre (TDC) Transport Canada.

20.  For example, based upon the 2003 Transport Canada Visibility in Snow vs. Snowfall Intensity Chart, assume that the daytime visibility in snowfall is 1 statute mile and that the temperature is -7°C. Using the "Visibility in Snow vs. Snowfall Intensity Chart" (Table 1), for this example, we conclude that the snowfall rate is light. This snowfall rate will be used to determine which HOT Guideline value will be appropriate for the fluid in use.

21.  The snow column in the HOT tables indicates the range of holdover times for light to moderate snowfall rates. The maximum snowfall rate covered by the HOT guidelines is moderate.

TABLE 1 VISIBILITY IN SNOW VS. SNOWFALL INTENSITY CHART1

Lighting Temperature Range Visibility in Snow (Statute Miles)
ºC ºF Heavy Moderate Light Very Light
Darkness -1 and above 30 and above ≤ 1 1 to 2½ 2½ to 4 4
Below -1 Below 30 ≤ ¾ ¾ to 1½ 1½ to 3 3
Daylight -1 and above 30 and above ≤ ½ ½ to 1½ 1½ to 3 3
Below -1 Below 30 ≤ 3/8 3/8 to 7/8 7/8 to 2 2
 

HOW TO READ THE TABLE

Assume that the daytime visibility in snowfall is 1 statute mile and the temperature is -7°C. Based on these conditions, the snowfall intensity is light. This snowfall intensity is used to determine which holdover time guideline value is appropriate for the fluid in use.

1  Based on:  Relationship between Visibility and Snowfall Intensity (TP 14151E), Transportation Development Centre, Transport Canada, to be published in November 2003; and Theoretical Considerations in the Estimation of Snowfall Rate Using Visibility (TP 12893E), Transportation Development Centre, Transport Canada, November 1998.