Nor'easter
A storm along the East Coast with winds blowing from the northeast. Nor'easters can bring heavy snow, rain, and coastal flooding, especially from Virginia to New England. The most powerful nor'easters rival hurricanes in intensity.
More Winter Weather Terms
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.
Black Ice
A thin, transparent layer of ice on road surfaces that is nearly invisible, making it extremely dangerous for drivers. Black ice forms when temperatures drop rapidly after rain or from refreezing meltwater, often at night or in shaded areas.
Lake Effect Snow
Intense, localized snowfall produced when cold air moves across relatively warm lake water, picking up moisture and heat. Lake effect snow can produce snowfall rates of 2-4 inches per hour and create dramatic differences in accumulation over short distances.
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.
Freezing Fog
Fog that forms when air temperature is below 32°F (0°C), depositing ice crystals on exposed surfaces. Freezing fog creates treacherous driving conditions and can coat everything with a layer of rime ice.
Ice Storm
A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.
Blizzard
A blizzard is a storm with sustained winds or frequent gusts of 35 mph or more and falling or blowing snow that frequently reduces visibility to below a quarter mile, lasting 3 hours or longer. The National Weather Service definition has no snowfall or temperature requirement: wind and visibility make a blizzard.
Bomb Cyclone
A bomb cyclone is a midlatitude storm that intensifies rapidly through a process called bombogenesis: its central pressure falls by at least 24 millibars in 24 hours, measured at 60° latitude. The threshold shrinks toward the equator, to about 17.8 millibars at the latitude of New York City.
Nor'easter: 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
Lake Effect Snow: Why One Town Gets Three Feet and the Next Gets Flurries
What lake effect snow is, how it forms, why Buffalo gets so much of it, why totals change within a few miles, and what a Lake Effect Snow Warning means today.
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.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Use Case
Weather App for Cyclists
Check wind, rain timing, and radar before a ride so conditions do not decide the route for you.
Tool
Wind Chill Calculator
Estimate cold-weather exposure risk from temperature and wind speed.
Guide
What Is Storm Surge?
How surge, storm tide, and inundation differ, and why a hurricane's category does not predict how high the water gets.
Glossary
Hurricane
A tropical cyclone with maximum one-minute sustained surface winds of 74 mph or greater in the Atlantic or eastern and central North Pacific.
Explore Other Terms
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.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
S-Wave
The secondary or shear wave, which arrives after the P-wave and moves rock perpendicular to its direction of travel. S-waves shake far more strongly than P-waves and cause much of the damage in an earthquake. They cannot travel through liquids, which is how scientists established that Earth's outer core is molten.
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
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