Chapter 2: Field Weather Observations

Objectives

Field weather observations are important to help evaluate slope-specific snow stability and avalanche hazard. In addition, they may be incorporated into an avalanche hazard analysis and forecast for the region. They are done at the beginning of the day, at the high or low points for the day, when changing aspects, at midday, or during changing weather conditions.

Recording

Use a “~” in the notebook when no observation was made. Code as “U” if the observation was attempted but no reliable value could be ascertained. Do not leave blanks. Only write “0” when the reading is zero.

Procedure

Title. Location, elevation.

Date. yyyy-mm-dd.

Time. 24-hour time using local time.

Observers. Initials, with the primary observer first.

Sky Condition. Classify amount of cloud cover with one of the symbols below:

With Valley Fog, estimate the elevation of the bottom and top of the fog layer in meters above sea level. Given the elevation to the nearest 50 m. Data code is VF with bottom and top elevations separated by a hyphen.

When the sky condition features a thin cloud layer, precede the symbol with a dash.

Precipitation Type. Record the type of precipitation at the time of observation.

Precipitation Intensity. Record the precipitation intensity in centimetres of snow per hour.

Air Temperature. Observe the air temperature in the shade about 1.5m above the snow surface. Use a dry thermometer, read after about five minutes, wait another minute and read again.

Depth of Interval Snow (HIN). Estimate the amount of snow that has fallen during a period specified in hours.

Depth of Storm Snow (HST). Estimate the amount of snow that has fallen since the beginning of the storm.

Total depth of Snowpack (HS). Probe in several spots to estimate an average snowpack depth in the immediate area.

Surface Grain Form. Estimate as per the International Standard using the following table: (Source PDF; Downloadable PNG; Downloadable PDF).

Surface Grain Size. Determine the grain size of the surface particles. Disregard small particles and determine the average greatest extension of the grains that make up the bulk of the snow. Record size to the nearest 0.5mm, except for fine or very fine grains which may be recorded as 0.1, 0.3, or, 0.5mm. Where two distinct grain forms exist, record the size of the secondary grain in brackets. Where a range in sizes exist for a single grain form, specify the minimum and maximum size separated by a hyphen. The following table contains terms that describe grain size, however they should not be used in field notebooks:

Wind Speed and Direction. Estimate the wind speed and direction by observing the motion of the trees and snow. If the wind direction is erratic, record as Variable (VAR). Write “Calm” if the wind speed is calm. Canada records wind speed in km/h, wheres SI units are in m/s.

Blowing Snow. Estimate the current extent of blowing snow and note the direction to the closest cardinal point of the compass.

Comments. Make any additional comments or observations as required. These may include ski penetration, intensity of fog, barometric pressure, relative humidity approximation, etc.

Field Weather Summary

Field weather summaries are used to create a clear and concise picture of which weather conditions in the field observations are relevant to creating a final stability and hazard analysis and forecast for the region. The objective is to filter out any extraneous data from the observations. Summaries are generally done once a day, after the field day is complete.

Procedure. The parameters to be recorded in a field weather summary are as follows:

  • Date.
  • Time period.
  • Locations and elevation range.
  • Percent of area observed.
  • Average sky condition.
  • Precipitation types and intensities.
  • High and low temperatures.
  • Field HN24.
  • Field HST.
  • Field HS.
  • Average surface form and size.
  • Average wind speed and direction.
  • Average extent and direction of blowing snow.
  • Comments.