CoCoRaHS in the Cold: Measuring in Snowy Weather
Winter measurements are a little harder and take a little more time, but they are well worth it. Scientists are just as interested in variations of snowfall as they are rain and hail, and the water supply we get from melting snow is extremely valuable.
A snowboard (plywood or water board ½ to ¾" thick cut into a 16" by 16" square and painted white) is helpful to have. For measuring the water content of snow, the 4" diameter CoCoRaHS precipitation gauge can be used. But it does require care. The inner tube and funnel should be removed and brought inside when temperatures go below freezing. The gauge works well for moderate snowfall amounts up to about 6 to 8 inches, especially if winds are light. In windy locations or in very deep snows, special instructions may be necessary. Also, if wet snow builds up on the rim of the gauge it will affect the accuracy of your readings.
Go to the "All About Snow" web site
Winter Precipitation Measurements for CoCoRaHS
There are three different types of measurements you can do in the winter. In increasing order of complexity they are . . .
1. Snowfall Only: this is the measurement of snowfall accumulation with a ruler on a snowboard or equivalent surface.
2. Liquid Equivalent of Snowfall with your Rain Gauge: this is the snow that falls into the gauge and has been melted down.
3. Liquid Equivalent of Snowfall from a "Core Measurement" using your Rain Gauge and Snowboard: this is taking a sample of snow from the snowboard with the gauge and melting the snow.
You may choose to perform only some of these measurements. Any data that you are able to provide is extremely useful.
Snowfall Only: For this you need a snowboard and ruler. A snowboard is typically a flat piece of plywood painted white measuring at least 16" x 16". Decks and other flat wooden structures offer reasonable surfaces for snow measurement. However, decks do not typically offer good exposure because they are usually very close to houses. Because snowfall distribution is very subjective to airflow over and around objects, your snowboard should be placed a distance of 2x the height of your house away from the house and placed at ground level. It should also be away from tall trees if possible.
Snowfall measurement is done with a ruler. We suggest a yardstick since a 12" ruler will be too short in the big storms. Official snowfall measurement is in tenths of an inch, but most rulers are in 1/8" or 1/16" increments. Just enter what you can. Less than a tenth of an inch is considered a "trace". Snow that falls and melts as it hits your measuring surface is a "trace."
One of the difficulties with accurate and consistent snowfall measurements is related to the melting, settling, or evaporation of snow before you have a chance to measure it (especially in the Fall and Spring). Please try to do a measurement as close in time to when the storm ends as possible. Don't wait until 7AM if the snow ended the previous day.
Observers in networks such as CoCoRaHS and the NWS Cooperative Network are required to take snowfall measurements once per day. During long duration events, snowfall may be measured at shorter intervals for the purpose of submitting a Significant Weather Report or reporting to the NWS, but the snowboard should NOT be cleared! The snowboard should be cleared only after your regular 24-hour observation.
Some observers, such as at airports, may be directed by their servicing NWS offices to take 6-hourly measurements. However, CoCoRaHS and U.S. Cooperative stations are to make measurements every 24 hours.
In very big snowfalls (20" or more), you may find yourself averaging several readings around your yard and estimating snowfall to the nearest inch.
Nearest tenth-inch increments: 1/16 = 0.1, 1/8 = 0.1, 3/16 = 0.2, ¼ = 0.3, 5/16 = 0.3, 3/8 = 0.4,
7/16 = 0.4, ½ = 0.5, 9/16 = 0.6, 5/8 = 0.6, 11/16 = 0.7, ¾ = 0.8, 13/16 = 0.8, 7/8 = 0.9, 15/16 = 0.9
Liquid Equivalent of Snowfall (from the rain gauge): Make sure the inner cylinder and funnel of your gauge has been removed for winter. Snow that falls into the outer cylinder of your rain gauge can be melted down for the "liquid equivalent" which is entered under the "total precipitation" entry in CoCoRaHS. Use the inner cylinder and funnel of your rain gauge to measure the liquid to the nearest 0.01 inch.
There are two ways to melt the snow:
1) Place the snow into a saucepan and dip the bottom of the saucepan into hot tap water. The melted snow can then be measured by pouring it from the saucepan to the inner cylinder of the rain gauge. Do not melt the snow by using direct heat from your stove as this could cause significant evaporation and inaccurate measurement. Sometimes there is precipitation frozen to the side of the gauge and you have to let that melt on its own indoors.
2) The other method for melting the snow is to pour hot water into the inner cylinder of the gauge and recorded how much you have in there. Pour the hot water into the outer cylinder with snow and swish it around until it's all melted. Pour the liquid back into the inner cylinder. The total precipitation is the amount in the inner cylinder minus the amount of hot water you poured in.
Note: If you allow snow to melt and refreeze in your gauge, it could crack the gauge. If you are going away for a period of time, I suggest you place your gauge indoors.
Liquid Equivalent of Snowfall from a "Core Measurement": This is done by taking the outer cylinder of your rain gauge and placing it upside-down on the snowboard pushing it down until the rim of the gauge is at the snowboard surface (like a cookie cutter). Turn the snowboard upside-down as you slide the gauge off to the side, keeping that core sample of snow in the gauge. Melt and measure this snow core and enter the liquid equivalent (using the inner gauge cylinder) in the CoCoRaHS entry for "Core Precipitation."
Obviously you will want to do a core measurement after you have measured the snowfall since you will be disturbing the snowboard when you get the core precipitation.
If you would like more information on how to measure snow and its water content, a book is available from the Colorado Climate Center at Colorado State University entitled, "The Snow Booklet: A Guide to the Science, Climatology, and Measurement of Snow in the United States." The Snow Booklet can be downloaded by clicking here: Snowbook
The Climate Center has also produced a DVD for weather observers showing how the National Weather Service weather observers measure snow. It is actually reasonably entertaining. The video can be viewed on our YouTube page here.