Simply sticking a yardstick into the snow to measure
its depth is not the best way to gauge how much water runoff that snow
will produce in spring. The best way to deduce the amount of water in the
snow is to use a low-flying aircraft to measure the natural gamma radiation
emitted from the upper eight inches of soil.
For about two decades, that’s exactly what the National Operational Hydrologic
Remote Sensing Center in Chanhassen, Minnesota, has done around Lake Superior.
Usually in March, the National Weather Service’s NOHRSC crews fly along
50 to 60 established flight lines out of Sault Ste. Marie and Marquette,
Michigan, Duluth, Minnesota, or Thunder Bay, Ontario.
It works this way. A steady release of radiation comes from the upper eight
inches of soil. Once radiation readings are determined for a flight line
(usually in the driest part of fall), they remain valid for about a billion
years, says Thomas Carroll, director of the center. “The radiation changes
from place to place, but not from time to time.”
Water in all its forms - liquid or solid, rain, snow or ice - blocks radiation.
By measuring radiation in winter from their Aero Commander or Turbo Commander
aircraft, the National Weather Service’s crews can determine how much water
is in the snow and ice below them. Then they produce a “snow water equivalent”
map.
“I think it’s pretty amazing,” Tom says of judging snow’s water content
from the air. Such surveys done elsewhere help to alert regions to flood
dangers. In 2002, the center took Lake Superior readings in late February.
Here is the latest “snow water equivalent map.”
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