The Weather &
Snow Glossary

A National Weather Service alert issued for weather conditions that are inconvenient but not severe enough to warrant a warning. Common examples include winter weather advisories and wind advisories.

A thin, transparent layer of ice that forms on roads and sidewalks, usually when temperatures hover near freezing and moisture refreezes. It is nearly invisible against dark pavement, making it one of the most dangerous winter driving hazards.

A severe winter storm officially defined by sustained winds over 35 mph, considerable falling or blowing snow, and visibility under a quarter-mile for at least three hours. Snow accumulation is not part of the criteria.

Snow lifted from the surface by wind, typically reducing visibility to under seven miles. Blowing snow is a primary driver of school and road closures even when no new snow is falling.

The process by which moving air strips heat from a warm surface, including human skin. Convective heat loss is why a windy 40°F day feels far colder than a calm 40°F day.

The temperature at which the air becomes saturated and water vapor condenses into liquid. A high dew point on a hot day produces oppressive humidity; a low dew point makes the air feel dry.

A radar system that detects precipitation and measures its movement using the Doppler effect. It powers nearly all live storm tracking, including the radar used in modern snow day predictions.

A periodic warming of the central and eastern Pacific Ocean that disrupts global weather patterns, often producing wetter winters in the southern United States and milder winters in the north.

A rapid, dramatic temperature drop that turns wet roads and surfaces into solid ice within minutes. Flash freezes typically follow rain showers as a strong cold front moves through.

Rain that falls as liquid but freezes on contact with cold surfaces. It coats roads, trees, and power lines with a glaze of ice and is one of the most disruptive winter precipitation types.

A measure of how hot the air actually feels when humidity is factored in with the air temperature. A 90°F day with 70% humidity has a heat index near 105°F.

An area where atmospheric pressure is higher than its surroundings, characterized by sinking air, clear skies, and stable weather. High-pressure systems usually mean dry, calm conditions.

A storm that produces freezing rain heavy enough to coat surfaces with at least a quarter-inch of ice. Ice storms are infamous for crippling power grids by snapping tree limbs and power lines.

Lines on a weather map connecting points of equal atmospheric pressure. Tightly-packed isobars indicate strong winds and rapidly-changing weather.

A narrow band of fast-moving air high in the atmosphere that steers weather systems across continents. Dips in the jet stream pull frigid Arctic air south, producing North America's most severe winter outbreaks.

A cooling of the central and eastern Pacific Ocean — the opposite phase of El Niño. La Niña often produces colder, snowier winters in the northern United States and drier conditions in the south.

Heavy, localized snowfall produced when cold air passes over warmer lake water, picking up moisture that falls as snow downwind. Cities like Buffalo, NY can receive several feet of snow from a single lake-effect band while areas just miles away see nothing.

An area where atmospheric pressure is lower than its surroundings, characterized by rising air, cloud formation, and unstable weather. Low-pressure systems are responsible for most rain and snow events.

A powerful storm system that strikes the U.S. East Coast with strong northeasterly winds. Nor'easters are notorious for producing blizzards in winter and damaging coastal flooding year-round.

A weather front formed when a fast-moving cold front overtakes a slower warm front. Occluded fronts typically produce extended periods of precipitation as the system winds down.

Any form of water — liquid or solid — that falls from clouds to the ground, including rain, snow, sleet, hail, and freezing rain.

An electronic system that bounces radio waves off precipitation to map storm location, intensity, and motion. Modern Doppler radar is the foundation of nearly all real-time storm tracking.

Frozen pellets of ice formed when raindrops refreeze before hitting the ground. Sleet bounces on impact, unlike freezing rain, which coats surfaces.

A day on which schools or workplaces close due to heavy snow, ice, or dangerous wind chill. Closure decisions depend on accumulation rate, road conditions, and the timing of the storm relative to the morning commute.

A brief, intense burst of heavy snow accompanied by gusty winds that drops visibility to near zero. Snow squalls are notoriously dangerous on highways and trigger emergency advisories from the National Weather Service.

The total depth of snow on the ground after a storm, typically measured in inches. The rate of accumulation often matters more than the total: 4 inches in an hour is far more disruptive than 8 inches over a full day.

The layer of accumulated snow that builds up over a winter season, especially in mountain regions. Snowpack is critical for spring water supply and is closely monitored by water resource agencies.

A flat, gray, layered cloud that blankets the entire sky. Stratus clouds typically produce drizzle or light, steady snow and signal stable but dreary weather.

Air or wind chill temperatures below 0°F. Subzero conditions can cause frostbite within minutes of exposure to bare skin and are the threshold at which schools commonly close for cold alone.

A storm produced by cumulonimbus clouds, accompanied by thunder, lightning, heavy rain, and often hail or strong winds. Severe thunderstorms can spawn tornadoes.

A violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can produce winds over 300 mph and remain the most destructive small-scale weather phenomenon on Earth.

The distance at which an object can be clearly seen. Snowstorms and fog can drop visibility to near zero, which is the legal trigger for many winter driving advisories and school closures.

A change in wind speed or direction over a short distance. Strong wind shear in the atmosphere is a key ingredient in severe thunderstorms and tornado formation.

A National Weather Service alert issued when severe winter weather is occurring, imminent, or highly likely. Warnings are more serious than watches and indicate that travel will become dangerous.

A National Weather Service alert issued when severe winter weather is possible within 48 hours. A watch means conditions are favorable; a warning means it is happening.