Several common precipitation types that occur are rain, snow, sleet and freezing rain. These are the precipitation types this short essay will focus on. The surface temperature will not determine the precipitation type except by these rules of thumb: 1. If the ground surface temperature is at or below freezing and liquid precipitation is falling to the surface then the precipitation type will be freezing rain. 2. If the ground surface temperature is above freezing and liquid precipitation is falling to the surface then the precipitation type will be rain. 3. If the surface air temperature is above about 45 F then most likely the precipitation type will be rain. Sleet and snow can still occur in situations in which the surface air temperature is above 45 F when the air aloft cools very rapidly with height but this situation is not common and the precipitation will melt very quickly once it reaches the ground surface. 4. If the ground surface temperature is above freezing, sleet and snow will melt as it reaches the surface. An exception to this is when heavy sleet or snow falls and the accumulation rate exceeds the melting rate. If sleet or snow are seen falling, even if the precipitation melts when it reaches the surface, it will still be reported as sleet or snow. Besides the rules of thumb above, it is the temperature profile above the surface up to around 700 mb that determines the precipitation type that will reach the surface. For example, three locations can have a surface air temperature of 30 F, yet one location has freezing rain, another has sleet and yet another has snow. Going by the surface air temperature alone is not enough to determine which precipitation type will occur unless further information is known. That further information is what the temperatures are in the air aloft. Generally the temperatures at an elevation above 700 mb are always going to be below freezing in the cool season. It is the temperature profile between the surface and 700 mb that will determine the precipitation type. For information on forecasting the precipitation type refer to the webpage below. The page gives the most likely precipitation type as a function of different thickness values, the surface temperature and other characteristics. Before examining the website refer to the definitions and requirements below: Elevated Warm Layer (EWL), a.k.a. Melting Layer: This is a layer of above freezing air that is bounded above and below by subfreezing air. For example there may be a situation in which the temperature between the surface and 950 mb is below freezing, between 950 mb and 900 mb is above freezing, while it is below freezing above 900 mb. The EWL exists between 950 and 900 mb. Freezing rain: Precipitation melts aloft into liquid precipitation. It reaches the surface as a liquid but then freezes since the surface ground temperature is at or below freezing. Partial melting: This is snow that melts enough so that it is no longer identifiable as only ice crystals. After about 50% of the snowflake melts it begins to look more like rain. Partially melted snowflakes will turn to sleet if they fall through a subfreezing layer. Sleet: This precipitation starts as snow high aloft, then partially melts as it falls through EWL, then refreezes as it falls through subfreezing layer. This precipitation reaches surface as little frozen balls of ice. Snow: It requires a temperature profile at or below freezing. Portions of the profile can be at or above freezing if it is not warm enough to melt the snow too much before it reaches the surface. Thickness: This is the vertical distance in meters between two pressure surfaces. As thickness increases it indicates a layer of air is warming and as thickness decreases it indicates a layer of air is cooling. This occurs because colder air is more dense than warmer air. Thickness values can be used to help predict most likely precipitation type. http://www.theweatherprediction.com/winterwx/thicknesscriteria/ |