About the science

Normally, we think of rain falling at air temperatures above 32°F – but in the world of weather forecasting and hydrologic prediction, that isn’t always the case. In some mountainous regions, the shift from snow to rain during winter storms may actually occur at warmer temperatures approaching 39.5°F.

Scientists use temperature thresholds to determine where and when a storm will transition from rain to snow, but if that threshold is off, it can affect our predictions of flooding, snow accumulation, and avalanche hazard.

We launched this project to take on the challenge of enhancing the prediction of snow accumulation and rainfall that may lead to flooding by making real-time observations of winter weather.  Without ground-based observations from community members across the landscape, these curves are obtained from measurements from places like airports (which are typically located on lowland, flat, urban areas) that make them less generalizable to mountainous regions.

Two side-by-side panels in the mountains compare the success of weather stations with human beings to accurately observe whether it is raining, snowing, or mixed at 37F. A green checkmark over the human’s observations represents a correct observation, and a red X over the weather station represents an incorrect observation.
Community observers provide the most accurate observations of winter precipitation near freezing. 

Mountain Rain or Snow research

As the climate continues to warm, we expect a greater fraction of winter precipitation to fall as rain instead of snow. Areas of “at-risk” snow with winter air temperatures near freezing are likely to be most sensitive to losses in snow accumulation, changes in snowmelt rates, shifts in streamflow volume and timing, and shifts in the frequency and magnitude of rain-on-snow events. However, these at-risk snow areas are also most affected by fundamental uncertainties in modeling rain-snow partitioning and observational challenges from both in situ networks and remote sensing platforms.

Our inability to predict rain versus snow at air temperatures near freezing is most pronounced in the very areas expressing the greatest sensitivity and vulnerability to climate warming. This simple fact calls for a project that performs enhanced monitoring of the rain-snow transition zone across the western US.

The primary goal of collecting these observations of rain, snow, and mixed precipitation is to improve satellite-derived estimates of precipitation phase from the Global Precipitation Monitoring (GPM) mission, which will allow GPM to better quantify rain and snow across the western US and snowy regions globally. This will advance our knowledge of the water cycle in mountain regions and improve NASA’s Earth observational capabilities for winter storms, including rain-on-snow events. Project data will also enhance representation of rain vs. snow in land surface and atmospheric modeling systems such as NLDAS-2 and MERRA-2 as well as operational and experimental weather models.

Mountain Rain or Snow is supported by NASA.

The full title of this project is “Using Citizen Science Observations to Monitor the Rain-Show Transition of the Western US and Improve Satellite Estimates of Precipitation Phase”. It is funded by a grant from the  National Aeronautics and Space Administration (NASA) ROSES Earth Science Research Program. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. For more information about NASA citizen science, visit science.nasa.gov/citizenscience or  www.nasa.gov/solve.

Read more about our work:

Meet the team

Headshot of Keith Jennings, PhD

Keith Jennings, Lynker
Data analysis

Headshot of Meghan Collins, MS

Meghan Collins, DRI
Engagement strategy

Headshot of Monica Arienzo, PhD

Monica Arienzo, DRI
Engagement analysis

Headshot of Anne Nolin, PhD

Anne Nolin, UNR

Headshot of Nayoung Hur, MS

Nayoung Hur, Lynker
Data & engagement

Headshot of Katherine Moore Powell, PhD

Katherine Moore Powell

Anne Heggli
Stakeholder & observer engagement

Guo Yu, DRI
Flood hydrology

Headshot of Jessica Garrett

Jessica Garrett, Lynker
Geospatial developer

Sonia Nieminen, DRI

Not pictured:

Dillon Ragar, Lynker

Brian Jenkins, UNR

Angus Watters, Lynker

Mountain Rain or Snow logo with graphics of mountains, a snow crystal, and a rain drop.