National Weather Service United States Department of Commerce


The importance of snow to the nation’s water budget cannot be overstated. In the Western U.S., 85% of freshwater runoff originates as snowmelt. In the Eastern US, nine of the most significant floods of the 20th century were directly related to snowmelt.

The National Water Center’s Chanhassen Office provides a National Snow Analysis that gives daily updates to the state of the nation’s snowpack. The Airborne Snow and Soil Moisture surveys are part of a data suite that also include satellite and ground observations. The products of these observations include maps, analyses, and interactive visualization tools that are used by both government and private-sector applications in water resource management, flood forecasting, and disaster preparedness.

Airborne gamma detection is an operational program that uses naturally occurring background radiation from the soil in order to measure either snow-water equivalent or soil moisture content. The amount of radiation measured by the aircraft can be used to interpret how much water there is in the snowpack below.

The survey aircraft are frequently deployed to an area immediately following a significant rain or snow event.  In order to maximize the strength of the radiation signal, the aircraft have to be operated at 500 feet above the ground. Therefore flights are only conducted during daytime, visual conditions.



There are currently over 2500 flight lines spread across 44 states and eight Canadian provinces, with new lines installed every year as the program continues to grow. The advantages of using aircraft to measure snow-water equivalent are enormous. A single survey flight can consist of up to 20 survey lines, and cover areas up to 40,000 square miles.

Data processing is instantaneous, and the measurements are usually available to the public on the same day. The measurements are used to provide ground truth to the snowpack models. The data is also used in the daily Standard Hydrometeorological Exchange Format message, which contains the snow water equivalent data for each line, along with pilot remarks for each line, and a pilot summary of hydrological conditions for the survey area.  River Forecasters can use this information to compare against historic values in order to make better estimates about flood or drought conditions.

In addition to the gamma data collected, the pilots will typically take photographs of significant hydrologic features or points of interest, such as rivers overflowing banks (photo 1), low water levels in reservoirs (photo 2) or snow at high elevations (photo 3).  

The primary aircraft used by the program include a DeHavilland DHC-6 Twin Otter and a Rockwell Collins JetProp Commander, operated and supported by NOAA’s Aircraft Operations Center, based out of MacDill AFB in Tampa, Florida. The aircraft are deployed year-round, collecting background data in the Spring and Summer, soil moisture surveys in the Summer and Fall, and snowpack analyses in the Winter. A single flight crew consists of two NOAA Corps aviators who both fly the aircraft and operate the gamma detection system. A typical survey day consists of 1-2 flights, each about 3-4 hours in duration.

These surveys help improve the overall picture of how much water there is in the snowpack, oftentimes well in advance of a major flooding event, saving lives, protecting property, and promoting commerce and agriculture by providing accurate and timely information about both flood and drought conditions.