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Glossary
Slush

Wet snow with a water content in excess of 30%. It has no compressive strength but can refreeze, creating a hazardous condition. #WeatherTerms


Glossary
Relative Humidity

The amount of water vapor in the air relative to what it could hold at a given temperature. #WeatherTerms


Glossary
Snow

Precipitation in the form of ice crystals. It originates in clouds when temperatures are below the freezing point, when water vapor in the atmosphere condenses directly into ice without going through the liquid stage. Once an ice crystal has formed, it absorbs and freezes additional water vapor...


Glossary
Sleet

Frozen raindrops. Sleet occurs when rain falls through a deep enough layer of the atmosphere to freeze before reaching the ground surface. It can occur when surface temperatures are either above or below freezing; and unlike freezing rain, sleet hits the ground as frozen ice pellets that can...


Glossary
Temperature

A measure of the coldness or warmth at a specific time and location expressed in either Fahrenheit (F) or Celsius (C) degrees. #WeatherTerms


Glossary
Surface Temperature

The temperature of a ground surface (e.g. black top, concrete, grass) at a specific time and location. #WeatherTerms


Glossary
Road Weather Information Systems (RWIS)

A system that comprises automatic weather stations (referred to as environmental sensor stations) in the field; a communication system for data transfer; and central systems to collect field data from numerous stations. These stations measure real-time atmospheric and pavement temperatures,...


Glossary
Snow Adhesiveness

The attraction of dissimilar materials (e.g., snow and pavement). New, dry snow falling on cold, dry pavement will not adhere (bond) strongly. As the temperature rises above about 20º F (-7º C), the amount of liquid surrounding the snow grains increases. This increases the adhesion of the snow...


Glossary
Snow Hardness

Refers to the snow’s resistance to collapse or penetration. Snow hardness depends on the grain structure and temperature. The grain structure depends on the snow density and extent of bonding between adjacent grains. Snow hardness can be determined by measuring the resistance to penetration. ...


Glossary
Temperature Instability

Temperature Instability affects the amount of liquid coating the snow particles, which in turn influences compressibility , hardness , density and cohesiveness . This characteristic is probably the most important factor affecting its properties, and thus the most important factor affecting...