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.
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.
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.
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.
Hail: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Weather Fronts Explained: Cold Front vs Warm Front, Occluded Fronts, and Map Symbols
What cold, warm, stationary, and occluded fronts are, how each is drawn on a weather map, and the clouds, rain, wind shift, temperature, and pressure changes to expect before, during, and after one passes.
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Understanding Flash Floods: Causes, Warnings, and Safety
Learn what causes flash floods, how to spot the risk on radar, the difference between watches, warnings, and emergencies, and critical safety steps that save lives.
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How to Read Satellite Imagery: A Complete Guide to Weather From Space
Master visible, infrared, and water vapor satellite imagery. Learn the science behind each channel and how meteorologists use them to track storms.
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.
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Severe Weather
Follow outlooks, watches, warnings, and storm signals in one severe-weather workflow.
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Weather Radar Colors Explained
Understand what green, yellow, red, and purple radar returns usually mean in real-world storms.
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What Causes Thunder and Lightning?
How lightning forms through charge separation, what causes thunder, and how to stay safe.
Explore Other Terms
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
Reflectivity
A measure of how much radar energy is reflected back by precipitation, measured in dBZ (decibels of Z). Higher values indicate heavier precipitation or larger particles. Values above 50 dBZ typically indicate heavy rain or hail.
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.
Alberta Clipper
A fast-moving low-pressure system that originates near Alberta, Canada, and tracks southeast across the northern United States. Clippers typically bring light snow (1-4 inches) followed by bitter cold and gusty winds.
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