The National Weather Service Weather Forecast office in Gray, ME covers CoCoRaHS observers in
all of both Maine and New Hampshire. Data is reported daily for precipitation, snow depth, and new
snow. This data is reviewed daily by the forecast office and used in producing storm total snowfall
reports, verification, and hydrologic outlooks. There has been some confusion about the reporting of
precipitation amounts in the winter. This tutorial addresses how to measure precipitation in the
winter. For more information about CoCoRaHS, or to join, please see the CoCoRaHS website, www.cocorahs.org
2. What is Precipitation?
Precipitation is any water which falls from the sky. This includes rain, snow, hail, freezing
rain, sleet, or any combinations. In the winter we typically see precipitation in the form of snow.
Regardless of the type, precipitation reported to CoCoRaHS is always the melted liquid
equivalent to what fell into the rain gauge. CoCoRaHS observers should always
collect the liquid which fell into a 4" opening in the last 24hrs. This liquid
is the measured in the smaller, inner 1" tube. In the summer, this is simply
accomplished by placing the funnel on top of the rain gauge, allowing the 4" opening to flow
directly into the 1" measurement tube. In the winter, it gets messy, so read on!
3. Measurement methods for winter
At the most basic, the measurement of frozen precipitation can be accomplished by simply
leaving your 4" rain gauge outside(without the 1" inner tube) and allowing it to fill with
snow/sleet/rain/freezing rain. At the end of the 24hr period, bring it inside and allow the liquid
to melt. Pour the resulting liquid into the small inner tube and record the result. Never record the
depth of snow/rain/etc in the outer tube- remember, collect in the big opening, measure with
the small opening! Waiting for snow to melt can be time-consuming, and if the snow is
continuing, you will miss collecting new snow while your gauge is inside thawing out, to avoid this,
try the 'quick melt' method described below.
4. Quick Melt method
Here at the weather office in Gray, we use the 'quick melt' method to quickly determine
the amount of precipitation.
1. Grab your snow-filled gauge and head inside
2. Grab the 1" inner tube for your gauge and fill it about 1/3 of the way with warm tap
3. RECORD how much water you added: Here we added 0.32in of water
4. Add the warm water to your gauge filled of snow, and swirl the water and snow around to
completely melt the snow. Be careful not to splash any of the water out.
5. When the snow is completely melted, pour all the liquid back into the small tube to be
measured - use the funnel and be careful not to spill.
6. Record how much liquid you have in the small tube now - here we have 0.53in
7. Finally SUBTRACT the amount of water you added to get the amount of liquid
precipitation from the snow - here we had
0.53in-0.32in = 0.21in precipitation.
Enter this value as 'precipitation' on CoCoRaHS.
5. New Snow
New snow should be measured with a ruler and for CoCoRaHS is the maximum new snow
depth at any point in the preceding 24hrs. If you receive some snow, followed by rain,
sleet, etc, you should still report the maximum depth that was on the ground before compaction,
melting, etc. To measure new snow, place a 'snow board' - any large flat piece of plywood or
plastic in your yard. Simply put the ruler on the board at the end of the snow event, either when
the snow stops or a switch to freezing rain/rain/sleet occurs. We realize you may not always be
available to measure at the time of maximum new snow depth; simply do your best to estimate the
depth prior to a switch to rain.
6. Snow Depth
Snow depth is the average amount of snow or ice on the ground at the time of observation. Pick
a representative area and measure the compacted snow depth. Included in the 'Snow depth'
measurement is ANY type of frozen precipitation on the ground; hail, freezing rain, sleet, graupel,
snow, etc. Pick a location away from houses and trees which can cause significant drifting. Measure
your snow depth in approximately the same location daily.
7. Tips and Tricks
For more information on measuring snow and precipitation, check out the tutorials at http://www.cocorahs.org/Content.aspx?page=training_slideshows
Thank you for participating in CoCoRaHS!