How is Snow Measured?
There are plenty of snow enthusiasts out there who love to see their backyards turn into a winter wonderland each year. It’s even more exciting learning how much snow accumulated in your area, but how are official snow totals measured? Most think it’s as simple as sticking a ruler in the ground, however, you may be surprised to learn there is a list of do’s and don'ts weather observers follow to ensure accurate measurements.
One of the most fundamental tools observers use to measure snow is a snowboard. A snowboard is a piece of plywood painted white that acts as a surface to collect snow and is placed in an open area (usually on top of grass) away from trees and buildings. Measuring newly accumulated snow directly on grass is not advised because grass blades can falsely heighten totals. Snowboards are painted white since dark surfaces, such as parking lots or driveways, absorb heat and melt newly fallen snow. Official snow totals from the National Weather Service are reported off a snowboard and not from pavement.
Besides having the right equipment, there are certain times snow is measured. Official daily snow observations are rounded to the nearest tenth of an inch and are reported at 7am local standard time in most winter months; however, measuring newly accumulated snow at this time may not necessarily represent an accurate total. For instance, if snow stopped at 3pm and the observer waits until 7am the following day to measure, the snow has time to settle, melt and drift. Therefore, the National Weather Service recommends measuring and clearing off snow from a snowboard immediately following the end of a snow event and reporting that total at the 7am obervation time. When multiple snow events occur over a 24 hour period, the observer reports the overall maxium snowfall that accumulated on the snowboard during that time frame and waits until the 7am observation to clear off the snowboard. See the graph below for an example. Observers never add up hourly snow accumulations as this inflates the total. To find the snow depth (total amount of old and freshly fallen snow), observers average multiple measurements taken around a flat, open area (not off snowboards) at the 7am observation time.
Of course, when the temperature hovers near the freezing mark, measuring snow can be difficult. If snow falls for multiple hours but never accumulates up to a tenth of an inch, observers do not record snowfall as zero but will mark it as a trace. Anytime new snow accumulation is less than a tenth of an inch, obervers will report it as a trace. In cases when snow changes to rain, the highest snow total measured before the transition is reported. During wintry mix events, the snow and sleet totals are measured together.
When strong winds accompany snowfall, snow drifts can introduce obstacles for accurate snow observations. In this situation, multiple measurements are taken in a flat area where the new snow accumulated most uniformly. This is the exception to the rule where measuring in a flat, grassy area rather than a snowboard is recommended as high winds can blow snow off or onto a snowbaord. It's fine to acknowlegde the height of snow drifts in observer comments but these measurements are never official totals.
Obtaining the most accurate snow total requires conscientious observers and the meteorologists at WeatherWorks follow these guidelines to ensure the Past Weather Reports and Certified Snowfall Totals our clients receive adhere to the National Weather Serivce standards.