Ice and Snow Measurements in Words

An ALISON ice and snow observatory consists of a 100 m long sampling transect marked by wooden stakes, and one hot-wire ice thickness gauge. Simple measurements at the observatory followed by a little data processing yield geophysically valuable information.

Setting up an observatory involves drilling twenty-one holes through the ice at 5 m intervals along the 100 m transect line. After measuring the ice thickness, a wooden stake is placed in each hole and allowed to freeze into the ice. The stakes mark where snow depth and the temperature at the bottom of the snow are measured with snow depth/temperature probes.

A total of 21 depth and temperature measurements are made each time the observatory is visited. The snow probes are also used to measure the snow surface temperature in the shade at the beginning and end of the transect. Those two measurements give a mean (average) surface temperature value. The snow depth and temperature data allow us to calculate the temperature gradient in the snow cover at each of the wooden stakes. The temperature gradient (the rate of change of temperature per unit length) is the difference between the surface and bottom temperature divided by the snow depth.

A snow sampling tube of known cross-sectional area is used to obtain three snow samples. Each sample is placed in its own plastic bag, on which the snow depth has been written. The snow depth and cross-sectional area of the sampling tube tell us the volume of each snow sample.

The mass of each snow sample is then measured with a balance and the snow density (mass per unit volume) is calculated. A mean snow density value is calculated from the three individual values.

The mean snow density value is converted to a thermal conductivity value. Thermal conductivity is a measure of how well (or how poorly) a material conducts heat. Another way to look at it is to think of thermal conductivity as a measure of how good (or how poor) an insulator a particular material is. Dry snow is a good insulator (a poor heat conductor) because it has a relatively low density, i.e., there is a lot of air trapped between the snow crystals. It is the air that provides most of the insulation offered by a snow cover.

The product of the snow temperature gradient and the thermal conductivity is the conductive heat flow. That is, the snow temperature gradient multiplied by the thermal conductivity tells us how much heat is being conducted through the snow cover and in which direction.

The heat that is conducted out of the snow to the atmosphere is a good measure of the latent heat that is produced as water freezes and the lake ice grows and thickens. We are also interested in the ice thickness at any one time, and we measure it with the hot-wire gauge.

The hot-wire is simply an electrical circuit that includes a resistance wire of known length that has a wooden handle at one end and a metal weight at the other. The wire is frozen into the ice with the weight suspended in the water. When you want to measure the ice thickness you attach a 12V battery to the circuit, the resistance wire heats up and the wooden handle can be raised until the metal weight comes up against the bottom of the ice. The length of the wire from the ice surface to the raised wooden handle is measured and subtracted from the total length of the wire to give the ice thickness. This is a nifty way to measure the ice thickness with minimal site disturbance, something that can’t be said about drilling.

To find out more about ice and snow and thermal conductivity go to Lake Ice and Snow Science page and look under Conductive Heat Flow.

Download the ALISON Handbooks for Site Measurements and Classroom Activities.