GLACIOLOGY LAB SNOW Introduction Equipment

Transcription

1 GLACIOLOGY LAB SNOW Introduction The objective of this lab is to achieve a working knowledge of the snowpack. This includes descriptions and genetic analysis of features that can be observed on the surface and in vertical sections. Normally, sufficiently thick snow accumulations can be found in the lee of snow fences or other areas where snowdrifts accumulate. Because the snowpack changes with time, a monitoring program should be set up to look at changes over a period of a few weeks. The techniques used in this exercise are similar to those used in the study of avalanches, watershed management and glacier ice although on a much smaller scale. The early stages of snow diagenesis can be seen in the snowpack and quantified fairly easily. Equipment - Thermometers (3) - Snow sampling tube. This is a small, open ended, aluminum or plastic tube of known volume and weight. - Weigh scale. A fairly cheap but accurate kitchen scale is desirable for weight measurements in the field. It should have the capacity to weigh the snow in the sampling tube. - Snow shovels - Large metal spatula or knife - Meter sticks - Compass - Tape measure (30m) - Water soluble dye or ink 1

2 Procedure Snow Stratigraphy 1. Map the study area using tape measures and compasses. 2. Describe the surface features at the site before walking on it. These may include ripples, sediment inclusions, drifts, ice crusts and the like. See Fig. 1 for the suggested symbols to be used in producing a snowcover map. Figure 1. Descriptive categories for snow and suggested mapping symbols. 2

3 3. Using shovels, dig a trench through the snow deposit. Measure and describe the stratigraphic section in detail. Pay particular attention to thin layers of ice, layers with sediment inclusions etc. 4. In the snow pit, measure temperature (air temperature also), hardness, density, type and size of crystals of the snow at various depths. The temperature should be measured first for once the snowpack is opened, the temperature profile may change rapidly. 5. Calculate the water equivalent for the snowpack. This is the type of calculation used for watershed management in areas where accurate prediction of snowmelt is required. 6. Exercises can be performed to evaluate the permeability of the snow and the melting and percolation of water through the snowpack. 7. Analysis can be made of impurities at different levels. 8. Other properties, for instance the petrography of snow and ice can also be examined. Brief explanations of the more important properties that can be used to describe the snowpack are listed below. Water Equivalent Measurements of Snow and Ice (this includes density and thickness measurements). Pits are excavated in the snow and the densities of the recognizable layers are measured in g/cm 3 using a cylindrical snow sampler of known dimension (Fig 2). Layers of snow and ice are differentiated on the basis of color (often associated with impurities), compactness, hardness, clarity, or any other obvious property. The water equivalent of each snow or ice layer is given by: WATER EQUIVALENT = DENSITY x THICKNESS OF THE LAYER density = snow density relative to water density (specific gravity) Example: Depth of snow in sampler = 6.3cm Radius of sampler = 3.15cm Weight of snow in sampler = 55g Thickness of layer = 17cm Density of water = 1g/cm 3 3

4 Figure 2. Snow coring tool and robust kitchen scale suitable for use in cold, wet field conditions. MASS WEIGHT OF SNOW IN SAMPLER Snow Density = = VOL. πr 2 x DEPTH OF SNOW IN SAMPLER 55g = = 0.28g/cm x (3.15cm) 2 x 6.3cm Therefore the specific gravity of the snow in the sampler 0.28 The water equivalent of the snow layer is then: = 0.28 x 17cm = 4.76cm (of water) 4

5 These calculations are repeated for every layer in the section to arrive at an estimate of the water equivalent for the whole snow pack. For rapid surveys over large areas, such as those required for watershed management, measurements of water equivalent are taken using a long sampling tube which is pushed vertically down into the snowpack. On the outside, the tube is marked to measure snowdepth and it can be weighed using a balance which has been calibrated to read in water equivalent depth. Water Percolation. A water soluble dye like food coloring can be placed at various places on the snowpack during the first week. Over the next week or two, snow pits can be opened and the movement of water through the snowpack can be observed. Ice layers may be relatively impermeable to water, causing complex deviations in the fluid percolation. Hardness. The hardness of snow and ice is defined on the basis of how easily it can be penetrated by various objects (Fig. 1). Temperature Profile. Temperature readings should be taken at several depths in the snowpack. The thermometer must be inserted several centimeters into the face of the snow pit so that any effect of the air temperature can be ruled out. A temperature profile of the snowpack can then be generated and the changes in the profile over time can be monitored. For instance during a prolonged cold spell, the top of the snowpack will be cold and the bottom warmer, but if the weather becomes warmer, the temperature near the top and bottom may be higher and the snow in the middle may remain cold due to the insulating properties of snow. Free water content of the snow should be noted. This is normally described as dry, damp or slushy. Sediment inclusions may or may not be present in the section. If they are, they need to be described carefully. Wind erosion of dunes or agricultural land may be significant during dry winters with little snowcover. 5

6 Review of Tasks - Make all of the detailed measurements described and fill out the chart (Table 1) as completely as possible. - Draw a schematic cross section of the pit carefully labeling all the layers. - Draw a site map including the snow surface features (see the list of symbols used for this purpose in Fig. 1) - Try to correlate the features seen in the snow pit with the weather records for the area. For instance a period of freezing rain may be correlated with an ice layer in the snow pit. Try to get weather data from a station as near as possible. Weather records are recorded by a variety of private and government agencies at the local, state and federal level but for local daily observations, airports, park services and conservation authorities are among the best. References Shumskii, P.A., 1964, Principles of Structural Glaciology p ; ; Snow cover and firnification, p A classification with useful information Langway, C.C., 1970, Stratigraphic Analysis of a Deep Ice Core from Greenland. The Geological Society of America, Spec. Paper 125 Robin, G. de Q., 1962, The Ice of the Anatarctic. Scientific American V. 207, p Knight, C. and Knight, N., 1973, Snow Crystals. Scientific American, V. 228, p Keeler, C.M. and Weeks, W.F., 1968, Investigations into the Mechanical Properties of Alpine Snow Packs. Journal of Glaciology, V. 7, p Allen, J.R.L., 1965, Scour Marks in Snow. J. of Sed. Petrology, V. 35, p Bradley, C.C., 1970, The location and Timing of Deep Slab Avalanches. J. of Glaciology, V. 9, p

7 Table 1. Blank sheet for snow data. This should be photocopied and taken into the field. 7