Winter Weather Awareness
 
 
   

What is CoCoRaHS?

 CoCoRaHS stands for the Community Collaborative Rain, Hail & Snow network. It is a unique, non-profit, community-based network of volunteers of all ages and backgrounds working together to measure and map precipitation (rain, snow and hail). The network originated with the Colorado Climate Center at Colorado State University in 1998 thanks in part to a devastating flood that occurred in the previous year. Since then, the network has expanded rapidly with thousands of volunteer weather observers in an ever ncreasing number of states. Hundreds of observers have signed up across Kansas and Missouri already!

 

Can I participate?

Absolutely! The ultimate goal of the program is to have one observer per square mile in urban/suburban areas and one observer every 36 square miles in rural areas. Everyone can help, young, old, and in-between. The only requirements are an enthusiasm for watching and reporting weather conditions and a desire to learn more about how weather can effect and impact our lives. 

 Once trained, volunteers collect data using low cost measuring tools such as a 4-inch diameter high capacity rain gage (pictured right), snow boards to measure snow, and aluminum foil-wrapped styrofoam hail pads in some states. Information on purchasing a rain gage can be found via the CoCoRaHS main page.

Snow measurements are also an inportant way CoCoRaHS participants can help the NWS.  On the CoCoRaHS main page you will find directions on how to correctly measure and report snow!

 

4 inch Diameter High Capacity Rain Gage

Click Here to Join CoCoRaHS!

How do I join?

Either click on the “Join CoCoRaHS” image to the left or visit the CoCoRaHS home page at www.cocorahs.org to get more information on the program.

 Click on the CoCoRaHs GIS (KML) to view CoCoRaHs data in a GIS viewer.

For a list of GIS viewers and more infomation on GIS data go here.

 


 How Do I Measure Snow?

How to Take the Snow Measurements

 Each season, before the first snow: Review the following instructions for measuring snow. It is easy to forget what needs to be measured. 

  • At the beginning of each snowfall/freezing season, if you have a manual rain gauge which will be used for measuring the water equivalent of the snow, remove the funnel and inner measuring tube of the eight-inch manual rain gauges, (or other type gauges) leaving only the 8-inch diameter overflow can so that it can more accurately catch frozen precipitation.
  • Put your snowboard(s) out and mark their location with a flag or some other indicator so they can be found after a heavy snowfall. They should be located in the vicinity of your station in an open location (away from trees, obstructions, and the north side of structures in the shadows). Each snowboard should be positioned in an area where snow accumulates relatively uniformly. At the same time, it should be in a convenient and accessible location but not in an area frequently disturbed by pets, pedestrians, et. The goal is to achive the most representative measurement of snow accumulation. A snowboard can be a simple piece of plywood, typically 16" x 16" or 16" x 24". Ideally, it should be painted white to minimize heating by sunlight. It serves as a flat open surface where snowfall can be accurately measured.
  • Check your precipitation gauge to make sure there are no leaks. If there are leaks, take appropriate action. Plastic rain gauges often crack and break if water becomes frozen inside. Once your equipment has been readied for winter you are prepared for taking snowfall measurements.
  • Make sure you have a ruler or measuring stick ready to find and grab when it's time to go out and measure the snow. Ideally, the ruler should be calibrated in tenths of an inch or for metric reports, in mm or cm.

Snowfall

Measure and record the greatest amount of snowfall that has accumulated on your snowboard (wooden deck or ground if board is not available) since the previous snowfall observation. This measurement should be taken minimally once-a-day (but can be taken up to four times a day, see note below) and should reflect the greatest accumulation of new snow observed since the last snowfall observation. To maximize the utility of your observations they should be taken at the same time as those taken by NWS Cooperative Observers: around 07:00 AM local time. The resolution of the measurement should be at least inches and tenths, for example, 3.9 inches. (Metric reports should have at least 0.2 cm resolution.) If you are not available to watch snow accumulation at all times of the day and night, use your best estimate, based on a measurement of snowfall at the scheduled time of observation along with knowledge of what took place during the past 24 hours. If you are not present to witness the greatest snow accumulation, input may be obtained from other people who were near the station during the snow event. 


It is essential to measure snowfall (and snow depth) in locations where the effects of blowing and drifting are minimized. Finding a good location where snow accumulates uniformly simplifies all other aspects of the observation and reduces the numerous opportunities for error. In open areas where windblown snow cannot be avoided, several measurements may often be necessary to obtain an average depth and they should not include the largest drifts. In heavily forested locations, try and find an exposed clearing in the trees. Measurements beneath trees are inaccurate since large amounts of snow can accumulate on trees and never reach the ground.

If your daily schedule permits, you may wish to make a snowfall observation every 6-hours, beginning with your regularly scheduled time of observation. This is the procedure followed by National Weather Service Forecast Offices. Follow the same rules for a once-a-day observation, but the snow accumulation reported will be the greatest for the previous six hours instead of 24 hours. If you take your observations at this frequency, make sure that you clear your snowboard (or other measuring surface) no more than once every 6 hours. your report. Never sum more than four, six-hourly observations to determine your 24-hour snowfall total. If you use more than four observations, it would falsely increase snowfall totals.

While sleet or ice pellets should be included in snow amounts, freezing rain (or glaze ice) should never be reported as snowfall.

If you are not going to measure the Snow Water Equivalent (SWE) of snowfall, after you have finished making the snowfall measurement clear the snow off of the snowboard. After completely clearing snow and ice from the board, reposition it on the surface of the existing snow and mark it with a flag or other marker so you can find it at your next observation time. Never leave a snow measurement board in a depression in the snow as it will then tend to drift over and provide unrepresentative data.

Snow Water Equivalent of Snowfall

Measure the water equivalent of snowfall since the previous observation. This measurement is taken at your specified time of observation using a rain gauge that has been left to collect without using the funnel or inner tube. Melt the contents of your gauge inside your home or other heated space. Let the gauge stand OR add water using the inner tube to the large outer tube having first noted the EXACT amount in the inner tube being added. (Note: avoid using HOT water as a plastic gauge may crack). Pour the combined melted snow plus any added liquid into the funnel and smaller inner measuring tube and measure the amount to at least the nearest 0.01 inch (0.2 mm). Subtract any amount of water added from your measured total and record the result.

Do not measure the melted precipitation directly in the large outer cylinder. Make sure the inner measuring tube can't fall over when pouring the liquid back into it. If the melted water equivalent (including any added warm water) exceeds one inch (4-inch gauge) or 2 inches (8-inch gauge) and cannot fit into the measuring tube all at one time, then empty the full measuring tube and pour the remaining liquid from the large outer cylinder into the emptied measuring tube. Then, add and record the water equivalent of the multiple measurements. If you added warm water to the gauge to melt the snow, make sure you accurately measure the amount of warm water added before pouring it into the gauge. Then, when you take your liquid measurement, subtract the amount of warm water added from the total liquid measurement to get your final liquid water equivalent of the snowfall.

As winds increase, gauges collect less and less of the precipitation that actually falls.* Generally speaking, the stronger the wind and the drier the snow, the less is captured in the gauge. If you notice that less snow is in the gauge than accumulated on the ground, or if the snowfall is deeper than the gauge itself, or other condition that caused a non-representative amount to fall into the gauge, you will need to take a core sample of the snow. To take a core:

Empty any existing snow from inside the cylinder which will be used to take a snow sample, sometimes referred to as "taking a core" or "cutting a biscuit". If drifting is a factor, find an area where drifting is minimal. This will usually be a flat area away from obstructions such as trees and buildings, although obstructions at some distance can help minimize drifting. Otherwise take the cutting from on top of your snow board. Invert the overflow can and force it down through the snow. The rim will cut a cylindrical vertical sample. If the snow is very deep, it may be necessary to push the can part way to the ground. Then, remove and empty the snow into a container, and insert the can in the same hole to obtain the rest of the snow. CAUTION: If the snow board was NOT used, do not push the can through snow that was measured at the previous observation, or its water equivalent will be counted in both measurements. Slip a piece of sheet metal or thin wood beneath the mouth of the can to prevent the snow from falling out. Take the snow indoors, melt it, and obtain the water equivalent as described above. If there is a question about the accuracy of the water equivalent of snow measured directly in the can, compare it with the amount determined by a core sample and use the larger of the two readings.

Once you have finished measuring both Snowfall and Snow Water Equivalent of Snowfall, clear off the snowboard as described in the last paragraph of the instructions above for Snowfall.

 

Total Snow Depth

Determine the total depth of snow, ice pellets, or ice on the ground. This observation is taken once-a-day at the scheduled time of observation. It is taken by measuring the total depth of snow on exposed ground at a permanently-mounted snow stake or by taking the average of several depth readings at or near the normal point of observation with a measuring stick. When using a measuring stick, make sure the stick is pushed vertically into the snow until the bottom of the stick rests on the ground. Do not mistake an ice layer or crusted snow as "ground". The measurement should reflect the average depth of snow, ice pellets, and glaze ice on the ground at your usual measurement site (not disturbed by human activities). Observers should always make sure they are measuring just the depth of snow and not the height of the grass. A thick turf can hold several inches of heavy snow up in the air. Measurements from rooftops, paved areas, and the like should not be made.

 

Report the snow depth with precision of at least 1 inch (1 cm). If you are reporting only to 1 inch (1 cm) precision, values of less than 0.001 inches (0.002 cm) should be reported as a trace (T).

Frequently, in very hilly terrain, you will be faced with the situation where no snow is observed on south-facing slopes while snow, possibly deep, remains in shaded or north-facing areas. Under these circumstances, you should use good judgement to visually average and then measure snow depths in exposed areas within several hundred yards surrounding the weather station. For example, if half the exposed ground is bare and half is covered with six inches of snow, the snow depth should be entered as the average of the two readings, or three inches. When in your judgement, less than 50 percent of the exposed ground is covered by snow, even though the covered areas have a significant depth, the snow depth should be recorded as a trace (T). When no snow or ice is on the ground in exposed areas (snow may be present in surrounding forested or otherwise protected areas), record a "0".

When strong winds have blown the snow, take several measurements where the snow was least affected by drifting and average them. If most exposed areas are either blown free of snow while others have drifts, again try to combine visual averaging with measurements to make your estimate.

Snow Water Equivalent of Total Snow Depth

Measure the water equivalent of ALL snow on the ground, old and new. The snow water equivalent is obtained by inverting the rain gauge can and pushing it through the entire depth of snow on the ground that represents the average snow depth. Make sure that snow does not fall out of the can when you remove the snow core. It is very helpful to have a firm, thin sheet of metal to slide between the surface of the ground and the rim of the can to cleanly gather the entire sample of snow. The snow is then melted and measured as described above in the instructions for Snow Water Equivalent of Snowfall.

From the perspective of water resources, flood prediction, and engineering, the most important observation (and one not taken by most weather observers) is the "Snow Water Equivalent of Total Snow Depth". This measurement is very important as it represents the amount of water contained in the total snowpack that is available to melt and run off when temperatures rise. Thus it can be used to forecast future water supplies and assess the potential for flooding. It can also be converted to weight per area. This number is critical for monitoring the weight of snow on roofs. For example, if there are 10 inches of old packed snow on the ground, this may contain 3.0" of water content which equates to nearly 15 pounds per square foot if uniformily distributed on the ground and roofs.

 

Source: Based on ‘Snow Measurement Guidelines (10-23-96)’, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service (NWS), Office of Meteorology and ‘National Weather Service, Observing Handbook No. 2, Cooperative Station Observations’, Observing Systems Branch, Office of Systems Operations, Silver Spring, Md, July 1989, ‘The Snow Booklet’ by Nolan J. Doesken and Arthur Judson, CSU, 1996, ‘MANOBS: Manual of Surface Weather Observations ’, Environment Canada, Meteorological Service of Canada, June 1996, and the Minnesota SnowRules! website.

 


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