Snow
Snow is precipitation that forms as ice crystals in clouds where temperatures are below freezing (0°C or 32°F). Snow appears in various forms, including snowflakes, graupel, and sleet, with snowflakes being clusters of ice crystals generally smaller than 1.3 cm in diameter.
Overview
Once it reaches the ground, snow can be classified by its type and formation. Some examples of snow types and formations include new snow, firn, cornices and sastrugi. Recently-deposited snow in which the original ice crystal form can be recognized is known as new snow. Seasonal snow is snow that lasts for only one season. Firn is dense snow that has persisted for more than a year. Over many years, firn continues to compress, eventually forming glacier ice. Wind-driven snow creates formations like cornices, which are overhanging accumulations on ridges or cliffs, and sastrugi, which are wave-like formations with sharp, irregular ridges and grooves that can grow meters long and up to a meter high.
Seasonal snow cover is present in over 100 countries. Snow covers approximately 44 million square kilometers of the Northern Hemisphere during winter (based on the 1991-2020 average for December reported by Rutgers University Global Snow Laboratory), influencing heavily populated mid-latitude regions and higher latitudes. Snow cover is also present in the Southern Hemisphere, in Antarctica and in mountain regions such as the Andes, Australia and New Zealand.
Impact
Terrestrial snow has the largest geographic extent of the cryosphere components and has a significant impact on Earth's systems. Its high albedo reflects solar energy, lowering surface temperatures, while its low thermal conductivity insulates the ground, reducing energy losses and reducing the severity of soil frost. Additionally, since the albedo of snow is higher than that of sea ice, snow on sea ice reflects more solar energy, thereby mitigating both sea ice melt and ocean warming. Snow smooths the land surface, reduces wind resistance, and modulates energy exchanges with the atmosphere. These properties make snow integral to the land surface energy budget and atmospheric circulation.
Snow is an important store of freshwater, which is released during the snowmelt season each year, providing water resources to downstream populations. As global temperatures rise, snow seasons are shortening and the quantity of water stored in the snowpack is declining. This has consequences for the timing and quantity of meltwater release, with knock-on impacts on drinking water availability, irrigation for agriculture and hydropower generation. Additionally, changing snowmelt patterns impact the magnitude and timing of flood events, endangering downstream communities and infrastructure.
The sensitivity of snow to changes in temperature and precipitation makes it a vital indicator of climate change. Shifts in snowfall patterns, snow cover extent, and snowpack depth provide insights into the accelerating redistribution of water resources. Snow’s role in climate hypotheses highlights its importance in understanding and addressing climate variability and its far-reaching consequences.
WMO's response
The WMO supports the monitoring and analysis of snow through its integrated cryosphere programs. These efforts focus on understanding snow’s role in the climate system, its contributions to the water cycle, and its influence on global and regional weather patterns. Accurate data on snow cover, distribution, and properties are critical for forecasting and modeling that address the challenges posed by changing snow dynamics.
The Snow Watch initiative of WMO’s Global Cryosphere Watch (GCW) provides reliable, up-to-date information and assessments on snow conditions and trends worldwide, through resources such as the GCW Snow Tracker tools and the GCW Snow Assessments. In response to recommendations from the first GCW Snow Watch workshop, a satellite snow product intercomparison and validation project, ESA SnowPEx, was launched. SnowPEx aims to evaluate the quality of present satellite-based snow products and to develop guidelines for their future enhancement.
In collaboration with the International Association of Cryospheric Sciences (IACS) and the Mountain Research Initiative, the WMO GCW contributes to the Joint Body on the Status of Mountain Snow Cover. This Joint Body aims to highlight the significance of mountain snow cover by (i) providing reliable information on global mountain snow cover through the compilation and standardization of existing datasets, (ii) enhancing understanding of snow processes through observations and modeling studies, and (iii) promoting open access to snow data while supporting capacity building for understanding changes in mountain snow cover and their impacts.
In 2022, the WMO published the first globally coordinated guide to the Measurement of Key Snow Variables, within the WMO Guide to Instruments and methods of Observation (WMO-No 8, Volume II), available in its six official languages. It provides an extensive assessment of current practices on monitoring and reporting glacier data, and it supports the definition of standards for data reporting.
In 2023, the GCW Expert Team on Snow Watch (ET-SW) coordinated collaboration between teams of experts at the National Weather Service (NWS), the National Oceanographic and Atmospheric Administration (NOAA) and the European Centre for Medium-Range Weather Forecast (ECMWF) to achieve operational exchange of snow depth data in near-real-time from over 1100 stations in the US to the ECMWF. This operational data exchange will contribute towards enhancing weather forecast accuracy, facilitating improved climate prediction, resource management and disaster preparedness.
The Global Climate Observing System (GCOS), co-sponsored by the WMO, the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization (IOC-UNESCO), the United Nations Environment Programme (UN Environment) and the International Science Council (ISC), has defined a set of key variables that are considered essential for monitoring snow, known as Essential Climate Variables (ECVs). By defining and continually updating these ECVs, GCOS supports the collection of consistent, comprehensive and standardized snow data.
The WMO Third Pole Regional Climate Centre Network (TPRCC-Network) coordinates climate research, monitoring, prediction and data sharing activities across High Mountain Asia, enhancing the capacity of WMO Members to deliver optimized climate services to national and local users and partners.
