Graupel
Soft, small pellets of ice formed when supercooled water droplets freeze onto falling snowflakes. Also called snow pellets, graupel is crumbly and opaque, unlike the hard, clear ice of hail. It often occurs in spring and fall storms.
More Precipitation Terms
Freezing Rain
Rain that falls as liquid but freezes on contact with surfaces at or below 32°F (0°C), creating a coating of ice called glaze. Freezing rain is extremely dangerous as it can make roads impassable and cause widespread power outages from ice-coated power lines.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
Hail
Balls or irregular lumps of ice produced in severe thunderstorms. Hailstones form when updrafts carry raindrops into extremely cold areas of the atmosphere where they freeze. Repeated cycles through the updraft can create hailstones larger than softballs.
Virga
Precipitation that falls from clouds but evaporates before reaching the ground. Virga appears as wispy streaks extending from the base of clouds and is common in dry or desert climates where the air below clouds has low humidity.
Snow Squall
An intense, short-lived burst of heavy snow accompanied by strong winds and often causing a sudden drop in visibility to near zero. Snow squalls can create extremely dangerous driving conditions despite lasting only 30-60 minutes.
Drizzle
Very light precipitation consisting of tiny water droplets less than 0.5mm in diameter that appear to float in the air. Drizzle typically falls from stratus clouds and reduces visibility more than light rain of the same intensity.
Thundersnow
A rare phenomenon where thunder and lightning occur during a snowstorm. Thundersnow typically happens in intense winter storms and is often associated with heavy snowfall rates of 2-4 inches per hour.
Atmospheric River
An atmospheric river is a long, narrow corridor of concentrated water vapor moving through the lower atmosphere, typically 250 to 375 miles wide and often more than 1,000 miles long. When one comes ashore and is lifted over mountains, it produces prolonged heavy rain and snow. Atmospheric rivers are rated from AR 1 (weak) to AR 5 (exceptional) by strength and duration.
Pineapple Express
A strong atmospheric river that carries moisture from the tropical Pacific near Hawaii to the West Coast of North America. Because the air is warm, a Pineapple Express raises mountain snow levels, so more precipitation falls as rain and can melt existing snowpack, increasing the flood risk.
Graupel: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Frost Advisory vs Freeze Warning vs Hard Freeze: What Each One Means
A Frost Advisory usually means lows of 33 to 36°F, a Freeze Warning 32°F or colder, and a hard freeze about 28°F or colder. Learn why frost forms above freezing, why the alerts stop in late fall, and what to protect at each level.
Weather Education
Winter Storm Watch vs Warning vs Winter Weather Advisory (Plus Blizzard, Ice Storm, and Snow Squall Warnings)
What a Winter Storm Watch, Winter Storm Warning, and Winter Weather Advisory mean, how many inches of snow trigger a warning where you live, and how Blizzard, Ice Storm, Snow Squall, and Extreme Cold Warnings differ.
Weather Education
What Is an Atmospheric River? The AR Scale, the Pineapple Express, and El Niño
Learn what an atmospheric river is, how the AR 1 to 5 scale works, what a Pineapple Express is, and whether a very strong El Niño means more of them for the West Coast this winter.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Seasonal Hub
Severe Weather Season Guide
A seasonal hub for tornadoes, hail, damaging winds, and storm-prep reading.
Seasonal Hub
Winter Weather Guide
Find practical explainers on snow, freezing rain, wind chill, and winter travel risk.
Feature
Severe Weather
Follow outlooks, watches, warnings, and storm signals in one severe-weather workflow.
Guide
How to Read Weather Radar
Learn the basics of radar interpretation so you can tell where storms are headed and how intense they may become.
Explore Other Terms
ECMWF
ECMWF is the European Centre for Medium-Range Weather Forecasts, an intergovernmental organization supported by 35 states, and the name commonly given to its global weather model, often called the European model or the Euro. Its physics-based Integrated Forecasting System (IFS) runs on a global grid of about 9 km and forecasts up to 15 days ahead.
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Tropical Depression
An organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph or less. Tropical depressions can strengthen into tropical storms and eventually hurricanes.
Frost
Frost is a thin layer of ice crystals that forms on surfaces when they cool to 32°F (0°C) or below and water vapor in the air freezes onto them. It is the surface that has to reach freezing, not the air: frost can form while the official air temperature is still in the mid 30s.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
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