While New England has been the target of three major nor’easters, it is important to look into what specifically in the atmosphere determines the type of precipitation that falls from the sky. Temperatures above the surface can have a significant impact on the precipitation type that falls to the ground. The temperature of the surface itself can also play a role, however it is not as significant.
First, let’s talk about snow. Snow is ice crystals in a variety of forms. These forms could either be plates, columns, or flakes. Snow can also be a conglomerate of ice crystals. Snow is likely to develop when the cloud layer, this is the layer of the atmosphere above the surface where clouds form, is less than -10°C. However, the optimal snow growth region is when the temperature of the cloud layer is from -12°C to -16°C. Anything that comes out of this layer will make dendrites. Dendrites are the best for snow growth, since these are bigger and will join with other snowflakes.
Next, let’s talk about sleet. Sleet are ice pellets that are composed of somewhat melted snowflakes, not frozen rain. Sleet bounces on contact with the surface and can have different appearances. Sleet can look clear, or it could look milky white. This all depends on how the snowflake melts. In order for sleet to occur, there has to be an area of warmer temperatures above the surface.
Freezing rain is an entirely melted snowflake. The melted snowflake is super-cooled. This freezes on contact with a surface. For freezing rain to occur, snowflakes fall through a layer of that atmosphere that is warm enough to melt them into liquid. Then the melted snowflakes will encounter near freezing temperatures above the surface. The surface itself is at or below freezing.
In conclusion, the story of an air parcel can have many different endings such as: rain, snow, freezing rain, or sleet. This all depends on the temperature of the atmosphere at a given time and height.
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© Weather Forecaster Jennifer Naillon