Winter Weather Terminology
When old man winter threatens, the national weather service urges everyone to keep abreast of local forecasts and warnings and to become familiar with key winter weather terminology.
Winter storm outlooks are issued within the hazardous weather outlook (HWO) product. Winter weather hazards are mentioned when conditions are favorable for a significant hazardous winter weather event to develop over part or all of central Alabama in the 3 to 7 day forecast period. The main objective of the outlook is to inform users of the potential of an upcoming winter event, especially for those who need considerable lead time to prepare.
Winter storm watches are issued when potential winter events are expected in the 12 to 48 hour time frame. For snow, when more than 2 inches of snow are possible in a 12 hour time frame. For freezing rain or freezing drizzle, ice accumulations of 0.25 inch or more. For sleet, accumulations of ice pellets one inch or more. For wind chill, wind chills of -10 degrees or colder.
Winter storm warnings are issued when hazardous winter weather is occurring, is imminent or has a high probability of occurrence. These potential winter events are expected in the 0 to 36 hour time frame. For snow, when more than 2 inches of snow are possible in a 12 hour time frame. For freezing rain or freezing drizzle, ice accumulations of 0.25 inches or more. For sleet, accumulations of ice pellets 1 inch or more. For wind chill, wind chills of -10 degrees or colder. If forecasters confidence in a predominant precipitation type is high, the winter storm warnings can be event specific such as heavy snow warning, sleet warning and ice storm warning.
Winter weather advisories are issued for winter events that are of significance to the public but do not constitute a serious enough threat to life and property to warrant the issuance of a warning. These advisories are issued for lesser accumulations than warnings.
Winter weather advisories are issued for accumulations of snow, freezing rain, freezing drizzle or sleet which will cause significant inconvenience and moderately dangerous conditions. These potential winter events are expected in the 0 to 36 hour time frame. For snow, when snow of one quarter inch to two inches accumulating within 12 hours. For sleet, accumulation of ice pellets less than one inch. For freezing rain and freezing drizzle, ice accumulations less than one inch are expected. For wind chill, wind chills of zero to -10 degrees.
Wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold temperatures. As the wind increases, heat is carried away from the body at an accelerated rate, driving down the body temperature. Animals are also affected by wind chill. Inanimate objects such as pipes and car radiators are not affected. When the wind chill approaches minus 20 degrees, frostbite can occur in 15 minutes or less.
Blizzard warnings are issued for sustained wind speeds or frequent gusts of 35 mph or more and falling and/or blowing snow which reduces visibilities to less than one quarter of a mile for at least 3 hours.
Snow is frozen precipitation in the form of white or translucent hexagonal ice crystals that fall as soft, white flakes.
Snow grains are precipitation of very small, white, opaque grains of ice similar in structure to snow. These grains are fairly flat or elongated. The diameters are generally less than 1 millimeter.
Snow flurries are intermittent light snowfall of short duration. Generally light snow showers with no measurable accumulation.
Snow showers are brief periods of snowfall in which intensity can be varied and may change rapidly. Some accumulation is possible.
Snow squalls are brief, intense snow showers accompanied by strong, gusty wind. Accumulation may be significant. Snow squalls are best known in the Great Lakes region.
Heavy snow is snowfall accumulating to more than 2 inches in depth in 12 hours or less.
Sleet is rain drops that freeze into ice pellets. These are transparent or translucent pellets of ice of 5 millimeters or less in diameter before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. However, it can accumulate like snow and cause a hazard to motorists and pedestrians.
Freezing rain is rain that falls onto a surface with a temperature below freezing. This causes it to freeze on contact with these surfaces. An ice coating or glaze can form on trees, cars and roads. Even small accumulations of ice can become a significant hazard.
Freezing drizzle is drizzle that falls onto a surface with a temperature below freezing. This causes it to freeze on contact with these surfaces. An ice coating or glaze can form on trees, cars and roads. Even small accumulations of ice can become a significant hazard. Drizzle particles are smaller than aindrops and typically do not accumulate as much as rain.
Frostbite is damage to body tissue caused by tissue being frozen. Frostbite causes the loss of feeling and a white or pale appearance in extremities such as fingers, toes, ear lobes or the tip of the nose. If symptoms are detected, slowly warm the affected areas and seek medical help immediately.
Hypothermia is the loss of heat from the body. Warning signs are uncontrollable shivering, memory loss and distortion. Medical treatment should be sought immediately.