Measuring snowfall a complex procedure
By Jan Null
You hear that your favorite Sierra ski area just received 16 inches of new
snow and you're raring to try out that new snowboard. But was there really
16 inches? How is it measured, and by whom?
Unlike many weather elements, measuring snowfall is more than just reading a
number off a dial. It is probably the most difficult weather parameter to
measure accurately and consistently.
We usually hear only about the depth of snow, but this is just one of the
parameters measured to characterize snowfall. As any powder skier will tell
you, the snow in the Sierra is often a heavy, wet accumulation of snow, not
so fondly referred to as ``Sierra cement'' when compared to fluffy powder in
This is a matter of the water content of the snow, an important factor for
water managers and dam operators when it begins to melt.
The recommended method of measuring snow depth is to use ``snowboards.''
Unlike the object you strap to your feet before careening down a snowy
slope, this type of snowboard is simply a flat wooden surface upon which
snow can accumulate. Snowboards are preferable to uneven surfaces or an
object with heat that might affect the snow's characteristics.
The National Weather Service standard for measuring snow is to take a
measurement every six hours, starting with a snow-free board, with four
six-hour amounts added together for a 24-hour total. If measurements are
taken less frequently, the weight of the snow causes it to compact, and the
depth that is measured will be smaller.
The location of snow measurements is also a crucial factor. Ideally,
readings should be taken away from structures or terrain features that cause
drifts or valleys of snow. Multiple readings should also be taken in the
same area, with an average value being representative of the overall depth.
New technologies to measure snow depth include ultrasonic sounders. These
devices are mounted on poles above the snow and bounce a sound wave off of
the top of the snow to determine its depth.
The water content of snow is also measured using a variety of methods. One
of the simplest ways involves a heated rain gauge that melts snow as it hits
the gauge and then is measured as liquid. On another device, large platforms
called snow pillows respond to the weight of snow upon them. The density of
the snow is measured as an antifreeze liquid is displaced in proportion to
the weight of the snow.
Cosmic rays are sometimes used. Sensors that measure cosmic radiation are
placed above and below the snow. The water content of snow attenuates these
rays, and the difference in the amount of radiation reaching the top and the
bottom sensors is calibrated to the density of the snow water.
The Sierra Nevada snowpack furnishes about 35 percent of California's water
supply, or approximately 15 million acre-feet of water. To monitor the
amount of water available, snow surveys are conducted in late winter and
spring to determine the depth and water content of the California snowpack
at more than 300 mountain locations.
This is done using core samples. A hollow tube is driven into the snowpack
until it hits the ground below; this is the depth of the pack. The
snow-filled tube is then weighed. By subtracting the weight of the empty
tube from the weight of the full tube, the weight of the snow -- and thus
the amount of water it contains -- can be calculated. Based on these monthly
measurements, water managers and utility companies can determine how much
water will be available for the remainder of the year.
Q In early October I saw an
unusual (to me) cloud formation, like waves, over the hills east of
Interstate 280 near Sand Hill Road. Jack Cole - San Jose
A As horizontal winds flow over
mountains, waves can form downwind of obstructions. When clouds are already
present or when the air is close to saturation, the waves of clouds that you
noted can form. The rows will be parallel to the mountains and can extend
downwind for miles. Sometimes called ``cloud streets,'' they are usually
below 3,000 or 4,000 feet in the Bay Area, and are in the form of stratus or
stratocumulus clouds. Higher level winds usually do not form wave clouds
over the Bay Area, but downwind of the Sierra Nevada there can be some
spectacular lenticular, or lens-shape, wave clouds.
Q How do I read my rain gauge?
At the top of the meniscus? The bottom? Split the difference? Clark Beck -
A On a rain gauge, the meniscus
in a tube of water is the concave shape on the top surface caused by surface
tension and its adhesion to the sides of the tube. To accurately read it,
the bottom of the curve should be read.
However, gauges using more viscous liquids, such as mercury, create a convex
meniscus, and the top of the meniscus should be read.