Snow and Weather Glossary for Skiers

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Area Forecast Discussion: A plain-text write-up from a National Weather Service office explaining the reasoning behind its forecast: what the models show, how confident the forecasters are and what could change. US resort pages include it under “The brains behind the forecast.”
Atmospheric river: A long, narrow band of moist air flowing in from the ocean. When one hits the mountains it can bring heavy precipitation for days, and whether that falls as snow or rain depends on the snow level.

Base depth: How deep the settled snow is on the slopes, as reported by the resort. It is different from new snow: a resort can have a deep base and no fresh snow.
Blizzard: To the National Weather Service, a blizzard is about wind and visibility, not snowfall: sustained winds or frequent gusts of 35 mph or more with falling or blowing snow that cuts visibility below a quarter mile for three hours or longer.

Chance of precipitation: The probability that measurable precipitation falls at a given spot during a forecast period. A 40% chance means it may or may not happen. It says nothing about how much.
Cold smoke: Very light, dry powder that billows up around you like smoke when you ski or ride through it. It comes from cold storms with a high snow-to-liquid ratio.

Estimated: A snow total label on SnowBrains Weather resort pages. It means the number is modeled snowfall from a weather model: a solid estimate, but not a measurement.

Freezing level: The height where the air temperature reaches 32°F (0°C). Snow usually keeps falling for about 1,000 feet below it before turning to rain. Canadian resort pages show the freezing level because Environment Canada doesn’t publish a snow level.

GEM: The Global Environmental Multiscale model, Environment and Climate Change Canada’s weather forecast model. SnowBrains Weather uses it for Canadian resort forecasts.
GFS: The Global Forecast System, NOAA’s global weather model. Where the National Weather Service doesn’t publish a snow level, we calculate one from the GFS forecast.
Graupel: Soft, small pellets of snow that form when supercooled water droplets freeze onto falling snowflakes. It often falls in spring showers and can feel like skiing on tiny foam balls.
Gust: A brief burst of wind stronger than the sustained wind. Gusts are often what slows or stops exposed lifts, so check them along with the average wind speed.

Inversion: A layer where the air gets warmer as you go up instead of colder. In winter, cold air can pool in valleys under milder air, so the base can be colder than the summit.

Lake-effect snow: Snow made when cold air crosses a large, warmer lake, picks up moisture and dumps it downwind. It falls in narrow bands, so one town can get buried while the next stays dry. It is common near the Great Lakes.
Lapse rate: How fast temperature changes with height. SnowBrains Weather adjusts forecast temperatures to each resort’s mid-mountain elevation using 3.5°F per 1,000 feet.

Mid-mountain: The middle elevation of a ski area, between the base and the summit. Our forecast temperatures are set to mid-mountain, so the summit is usually colder and the base warmer.

New snow: Snow that has fallen recently, usually reported for the past 24 hours. Resorts report it each morning in their snow reports.
NOHRSC: NOAA’s National Operational Hydrologic Remote Sensing Center. It makes the national snow depth and snow water equivalent maps on our Snowpack page.
NWS: The National Weather Service, the US government’s official weather forecaster. SnowBrains Weather uses its forecasts for every US resort.

Observed: A snow total label on SnowBrains Weather resort pages. It means the number was measured at the nearest SNOTEL station (US only), which may be a few miles away and at a different elevation than the resort.
Orographic lift: Air forced up and over a mountain range cools, and its moisture condenses into clouds and precipitation. It is why mountains get far more snow than the valleys around them.

Percent of median: How today’s snowpack compares with the middle value for the same date over a reference period. NRCS snowpack maps use 1991 to 2020, so 100% means a typical year to date.
Powder day: A day with fresh, untracked snow to ski or ride. There is no official threshold: ask ten skiers and riders and you’ll get ten answers.

Rain shadow: The drier area on the downwind side of a mountain range. The air drops much of its moisture on the windward side, so resorts in a rain shadow often get less snow.
Reported: A snow total label on SnowBrains Weather resort pages. It means the number comes from the resort’s own snow report, the most authoritative word on what fell on the hill.
Rime: Ice that builds up when supercooled fog or cloud droplets freeze on contact with trees, lift towers and signs. It is what turns summit trees into “snow ghosts.”

Sierra cement: A nickname for the heavy, wet snow common in California’s Sierra Nevada. It has a low snow-to-liquid ratio and builds a deep, sturdy base.
SNOTEL: Short for snow telemetry: a network of automated snow stations run by the USDA Natural Resources Conservation Service in the western US and Alaska. Read our SNOTEL guide to see how we use it.
Snow depth: How deep the snow on the ground is at one spot. It changes as new snow falls, settles and melts.
Snow level: The elevation where snow turns to rain. Above it, expect snow; below it, rain; near it, a wet mix. It sits below the freezing level, because falling snow cools the air around it as it melts.
Snow pillow: A flat, fluid-filled bladder on the ground that weighs the snow sitting on it. SNOTEL stations use one to measure snow water equivalent.
Snow-to-liquid ratio: How many inches of snow one inch of water makes. Cold storms make fluffy snow with a high ratio, and warm storms make dense snow with a low one.
Snow water equivalent (SWE): The amount of water in the snow, if you melted it. It is the main number on snowpack maps, because it compares years and places better than depth does.

Trace: A little snow, too little to measure as a full inch. SnowBrains Weather shows a trace as <1″.

Upslope snow: Snow made when wind pushes moist air uphill against the mountains. On the Colorado Front Range, an easterly upslope storm can bring big totals to the east side of the Divide.

Wet-bulb temperature: The temperature air would cool to if water evaporated into it until it was saturated. It is a good guide to whether precipitation falls as snow or rain. Where we calculate a snow level, we use the height where the wet-bulb temperature reaches about 33°F.
Wind chill: How cold it feels on exposed skin when wind is added to the air temperature. A chairlift ride in strong wind can feel far colder than the thermometer says.
Wind loading: Wind picking up snow from one side of a ridge and dropping it on the other. Loaded slopes can ski deep, but they can also build unstable slabs, so check your local avalanche forecast before heading into the backcountry.
Winter Storm Watch and Warning: National Weather Service alerts for significant winter weather. A watch means a storm is possible in the next few days. A warning means it is expected or happening. The exact snowfall thresholds vary by NWS office.
Winter Weather Advisory: A National Weather Service alert for winter weather that is a nuisance but less severe than a warning, like light to moderate snow or slick roads. The Storm Tracker on our homepage shows active winter alerts from the National Weather Service.