Snow Measurement Uncertainty

Even with perfect knowledge of snow accumulation processes, there is considerable uncertainty associated with the measurement of snow accumulation on the ground. It is usually impossible to measure snow perfectly to a tenth of an inch – or in a major storm, even to the nearest inch! Measurement of snow accumulation after it reaches the ground is modulated by several factors:

Surface type:

Ideally, snow accumulation reports would all be sourced from measurements made on a properly-sited white snowboard. Meteorologists might accept snow reports from other types of surfaces in order to achieve a greater geographic density of reports, at the expense of more uncertainty. A typical source of reporting error is the type of surface on which the snow measurement is made. Snow accumulating on top of thick grass might lead to measurement of both the height of the grass and the snow on top of it, not just the snow. Snow accumulating on asphalt, dirt, a brown wooden deck, or on top of a vehicle, might experience more melting in marginal temperature environments than an adjacent snow board, especially if the snow falls soon after warm temperatures or sunny conditions. Snow measurements taken under trees would cause an artificially low surface measurement. NWP output generally does not attempt to modulate snow accumulation amount by land surface type (though most will accumulate snow on top of ice on bodies of water analyzed to be frozen).

Frequency of measurement:

One critical factor in snow accumulation verification is the frequency of measurement. During heavy snowfall, snow compaction can occur quickly, so summing measurements made every hour for 24 hours could yield a much higher result than a single measurement made at the end of 24 hours. Melting and/or sublimation are also processes which can reduce the amount of snow on the ground if measurements are taken infrequently. Windy conditions might cause snow to drift on or blow away from the measurement site over time. It is important snow accumulation measurements are made as soon as possible at the end of a storm and the user knows how often measurements were made by an observer over the course of a storm.

Compaction:

As falling snow accumulates on the ground, the weight of the new snow on top will compress the older snow below, a process known as compaction. Even after the snow ends, gravity gradually compresses the layer of snow into a thinner layer. Strong winds at the surface can promote drifting and compaction. Measurement frequency, and how quickly after the storm the snow accumulation is measured, are both critical to keep uncertainties associated with compaction to a minimum. NWP and post-processing methods vary on how they handle compaction.

NWS Snow Measurement Guidelines

The official NWS Snow Measurement Guidelines state measurements should be taken on a white snow board away from obstructions. The board should be cleared at least once every 24 hours, and ideally once every 6 hours, but not more frequently. Supplemental measurements may be made without clearing the snow board to determine the maximum depth on the board, as illustrated in the figure.

When verifying forecasts and reporting snow amounts, consideration should be given to whether reports received have followed this standard.

Drifting:

During or after the snowfall, high winds at the surface can cause considerable drifting. This can make precise measurement difficult to impossible for an observer. Snow accumulation reports from untrained observers might not take into account the need to measure the snow on flat ground away from objects and obstructions, nor the need to calculate the average of multiple dispersed measurements when not using a snowboard. NWP output approximates the average snow accumulation amount over wide areas, not attempting to model drifting.

Melting:

Melting of snow after it reaches the ground can occur in several ways. First, snow falling on warm surfaces can melt on contact. Even if snow is accumulating quickly, a warm surface can gradually transfer its heat to the snow at the point of contact, melting the snow layer from below. During compaction, snow at the bottom of a layer can melt as the air it contains is warmed under greater air pressure (ideal gas law). When the air temperature warms above freezing, the snow on the ground will begin a phase change from solid to liquid, with the water either soaking into the ground or running off, decreasing its depth.

Sublimation:

When the air is sufficiently dry and below freezing, a phase change will occur for snow on the ground directly from ice to water vapor in a process called sublimation. This will gradually decrease the depth of the snow. Sublimation is the most efficient on sunny, dry, windy days.