Chapter 1: Weather and Snow Observations

Objectives

Weather and snow observations represent a series of meteorological and snow surface measurements taken at a properly instrumented study plot. Observational data take at regular intervals provide the basis for recognizing changes in stability of the snow cover and for reporting the weather to the meteorological office. In the long term, observations may be used to improve the ability to forecast the avalanche hazards by statistical and numerical techniques.

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.

Types of Observations

Standard Observations: Taken at regular daily times, usually at 0700 and 1600. If only one standard observation is taken per day, that observation should be made in the morning.

Interval Observations: Taken between standard times, usually when the snow stability is changing rapidly. Interval observations may contain only a few select observations.

Intermittent Observations: Taken at irregular times, usually at sites that are visited infrequently. Intermittent observations may contain only a few select observations.

Equipment

Carried equipment for weather and snow observations typically include:

  • Digital thermometer
  • Probe
  • Ruler
  • Snow density cutter and weight scale
  • Knife or plate for cutting snow samples
  • Field-book
  • Loupe and crystal screen
  • Snow shovel

The following equipment is also used at fixed study plot sites:

  • Stevenson screen for housing thermometers
    • Maximum thermometer
    • Minimum thermometer
    • Present thermometer
    • Hygrothermograph
  • Two snow boards (designated as the 24-hour snow board and the storm board). Additional snow boards may be used (interval, twice-a-day, or shoot).
  • Snow stake with a snow depth marker and a levelling stick
  • Precipitation gauge
  • First section of a Ram penetrometer
  • Barograph
  • Anemometer at a separate wind station with a radio or cable link to the recording instruments
  • Small broom

Procedure

Title. Location, elevation.

Date. yyyy-mm-dd.

Time. 24-hour time using local time. Operations that overlap time-zones should standardize to one 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. Thin cloud has minimum opacity, such that the disk of the sun would still be clearly visible through the clouds.

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. Record the maximum, current, and minimum temperatures. Read all air temperatures to the nearest 0.5 degree.

  • Field Observation: 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.

Air Temperature Change in the Past 3 Hours. Use an arrow symbol to record the temperature trend over the past three hours. If using a thermograph, record the current air temperature to the nearest degree.

At the end of temperature observations:

  • Remove any snow that might have accumulated in the devices.
  • Reset the thermometers.
  • If the Stevenson screen is fitted with a height adjustment, ensure that the screen base is 1.2 to 1.4 meters above the snow surface.
  • Check that the screen door still faces north.

Relative Humidity (RH). Record the relative humidity to the nearest one percent from the hydrograph.

Note: Hygrographs are inaccurate at low temperatures, and the accuracy of any mechanical hydrograph is unlikely to be better than 5%. Humidity measurements are more relevant from upper-elevation sites than from valley-bottom sites.

Snow Temperatures. Allow the thermometer to come to equilibrium and then read the thermometer while the thermistor is still in the snow.

  • Snow Surface Temperature (T0): Measure snow surface temperature by placing the thermometer on the snow surface and shade the thermometer.
  • 10cm Snow Temperature (T10): Insert the thermometer horizontally 10cm below the snow surface and shade the snow surface above the thermometer.

Depth of Snowfall. Use a ruler to measure (in cm) the depth of snow accumulated on the snow boards. Take measurements in several sports on the boards and calculate the average to the nearest centimetre. Record “0.1” when the depth is less than 0.5cm. Do not consider surface hoard on the boards as snowfall. Clear off the hoar layer after observations. Clear the snow boards after sampling the weight of new snow and redeposit the snow in the depression left behind.

Board Naming Conventions:

  • Interval (HIN): Used to measure accumulated snow in interval observations.
  • 24-Hour (HN24): Used to measure accumulated snow over a 24-hour period.
  • Twice-A-Day (H2D): Used to measure accumulated snow between standard observations.
  • Storm (HST): Used to measure accumulated snow since the beginning of a storm. The symbol “C” is appended when the storm board is cleared.
  • Shoot Board (HSB): Used to measure the accumulated snow since the last time avalanches were controlled by avalanches. The symbol “C” is appended when the shoot board is cleared.

Total Depth of Snowpack (HS). Record the total depth of snow cover on the ground to the nearest centimetre using a calibrated stake or probe. If necessary, level settling cones, wells, and drifts around the stake. Snow board and HS values are always measured vertically.

Mass of New Snow. Record the mass of new snow when the depth is 4cm or more. With a snow sampling tube, cut a sample of snow vertically from the designated snow board and weight it. Make a note of the cross-sectional area (cm2) of the snow sampling tube on the title of the field-book.

Water-Equivalent of New Snow (W). Record the water-equivalent new snow using the following calculation:

Density of New Snow (ρ): Record the density of new snow using the following calculation:

Rain. Record the amount of rain that has accumulated in the rain gauge in millimetres and tenths. Empty the gauge at each standard observation.

Note: A manual rain gauge should be placed on the ground or on the snow surface when rainfall is likely to occur.

Precipitation. Record the amount of precipitation that has accumulated in the precipitation gauge to the nearest millimetre. The precipitation gauge collects, snowfall, rainfall, and other forms of precipitation and continuously records their water equivalent.

Surface Penetrability (P). Record the snowpack’s ability to support a given load by using one of the following three tests. Note that Ram Penitration is the preferred observation because it produces more consistent results than ski or foot penetration.

  • Ram Penetration (PR): Let the first section of a standard Ram penetrometer penetrate the snow slowly under its own weight. Do not allow it to accelerate. Read the depth of penetration in centimetres.
  • Foot Penetration (PF): Step into undisturbed snow and gently put full body weight onto one foot. Measure the depth of the footprint to the nearest centimetre from 0-5cm and thereafter to the nearest increment of 5cm. Note that it is recommended that all observers working on the same program compare their foot penetration. Observers who continually produce penetrations more than 10 cm above or below the average should not record foot penetrations.
  • Ski Penetration (PS): Step into undisturbed snow and gently put full body weight onto one ski. Measure the depth of the ski track to the nearest centimetre

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.

Barometric Pressure. Record barometric pressure and the change in barometric pressure over the previous three hours in kPa; specific to two decimal places. Use an arrow to record the pressure tendency over the previous three hours.

Classify the relative pressure when the units of pressure are uncertain or imprecise.

Shear Frame Test. The shear test with tilt board and shear frame is an index observation of the stability of most weak layers, including those in new or partially decomposed or fragmented snow. See Chapter 4: Weak Layer Strength and Stability Tests  on how to perform a Shear Frame Test.

Example