Storm glossary
Each glossary page opens with a plain-language answer to one question: what a warning, watch, or advisory means, or what a Kp number means.
Blizzard warningA Blizzard Warning is issued by the National Weather Service when wind gusts of 35 mph or more combine with falling or blowing snow to cut visibility to under a quarter mile for three hours or longer.Updated
Dense fog advisoryA Dense Fog Advisory is issued by the National Weather Service when widespread fog lowers visibility to 1/4 mile or less, making travel difficult.Updated
Flash Flood WarningA Flash Flood Warning means flash flooding is happening or about to happen.Updated
Flood watchA Flood Watch means flooding is possible because weather conditions are favorable, but it does not mean flooding will definitely happen.Updated
Foggiest cities in the USElkins, West Virginia leads this ranking of the foggiest cities, averaging 95 heavy-fog days a year.Updated
Freeze warningA freeze warning from the National Weather Service means air temperatures are forecast to fall below 32F for a long stretch during the growing season.Updated
Hail size chartThis chart shows the National Weather Service reference objects for estimating hail diameter, from pea to grapefruit.Updated
Heavy snow warningA heavy snow warning is what the National Weather Service now calls a Winter Storm Warning that lists heavy snow as a hazard.Updated
The best time to see the northern lightsThe best time of night is roughly 10pm to 2am local, when the aurora is usually strongest, and the best months are around the March and September equinoxes, when geomagnetic activity runs about double the solstice rate.Updated
Ice storm warningAn Ice Storm Warning is issued by the National Weather Service when freezing rain is expected to produce ice accumulation of at least 1/4 inch, and some offices set that line at 1/2 inch.Updated
Kp index, explainedThe Kp index is a 0 to 9 scale that measures global geomagnetic disturbance.Updated
Lake effect snowLake effect snow is snow that forms when cold, dry air passes over a warm, open lake and gains enough heat and moisture to produce narrow bands of heavy snowfall.Updated
Largest hail ever recordedThe largest hailstone ever recorded in the United States fell in Vivian, South Dakota, on July 23, 2010.Updated
Red flag warning: what it means and what to doWhen critical fire weather conditions are happening or forecast, the local National Weather Service office issues a Red Flag Warning: dry fuels, low relative humidity, and strong wind, sometimes dry lightning.Updated
Severe thunderstorm warningA severe thunderstorm warning is issued by your local National Weather Service office when a thunderstorm is already producing or is expected to produce hail at least 1 inch in diameter or wind gusts of 58 mph or stronger.Updated
Severe thunderstorm watchSevere thunderstorms could develop in or near the area covered by a Severe Thunderstorm Watch.Updated
Snow squallA snow squall is a short but intense burst of heavy snow and strong gusty wind.Updated
Watch vs warning: what's the difference?A Watch means conditions are favorable for a hazard in your area within the next several hours to two days, so you should be prepared.Updated
What causes the northern lightsThe northern lights happen when charged particles from the Sun, carried by the solar wind, are guided by Earth's magnetic field toward the poles.Updated
Winter storm warningA Winter Storm Warning means a significant winter event with snow, ice, sleet, or blowing snow is happening or about to happen, and the local National Weather Service forecast office issues it.Updated
Winter storm watchA Winter Storm Watch is issued by the National Weather Service when a significant winter storm could develop, but the timing and track are not yet locked in.Updated
Winter weather advisoryA Winter Weather Advisory means snow, sleet, ice, or blowing snow is expected, but amounts will stay below warning criteria.Updated
White Christmas odds for US citiesAcross 119 cities, snow covered the ground on Christmas in at least half the years since 1950 in 17 of them.Updated