Winter Weather 101 – Precipitation Types

Winter weather is certainly not a stranger to the east-central Ohio region from late fall through early spring.  More often than not, the main precipitation type that we experience is snow.  However, sleet and occasionally freezing rain and graupel also affect the region.

What causes these different types of precipitation?  The vertical temperature profile in our atmosphere above is the primary control in determining whether snow, sleet, or freezing rain occurs.  Just as important, in order for ice crystals (snow) to form, there needs to be sufficient saturation (relative humidity values of 70% higher) present within a layer known as the dendritic growth zone, or snow growth zone.  This is a layer in the atmosphere with temperatures between 10.4 and 0.4° F (-12° and -18°C) where snow forms.  If there is not enough saturation present, snow and sleet will not form.

When winter arrives most of us think snow. Snow is by far the most common winter precipitation type we see in east-central Ohio during the cold season. Snow will fall when the temperature at the surface upwards to the clouds is at freezing (32 degrees F or colder).

Occasionally during the fall and early winter snow will fall even if temperatures are warmer than freezing at the surface. This will happen when there is a short distance between freezing temperatures overhead and warmer air at the surface. Once snowflakes reach the warmer surface, they melt.

People often confuse sleet – a winter precipitation type – with hail. Hail forms in thunderstorms during the warm season. Sleet, however, forms during cold weather an under a completely different process.

Sleet actually starts its life as snow. As the snowflake passes through a warm layer, it melts into rain. This raindrop then continues into another cold layer and re-freezes into an ice pellet. Sleet is easily recognizable as a clear ice pellet.

Graupel is very similar to sleet and forms much in the same way. It looks much different, though. Graupel falls as a pellet but it isn’t clear like a sleet pellet – it’s white, like snow. Graupel is a mix of snow and sleet.

Like sleet, graupel starts out as a snowflake. On it way to the surface, the snowflake encounters a layer of supercooled water vapor. In a process known as accretion, ice crystals form and build over the snowflake until it no longer resembles anything like a snowflake. It becomes a pellet of white or cloudy frozen water. Some folks call them snow pellets.

The difference between graupel and sleet is easy to recognize. Sleet pellets are hard and clear. Grauple on the other hand, are white or cloudy and easily break apart.

Freezing rain is the most hazardous and destructive winter precipitation type. It can make travel and even walking a dangerous situation. Freezing rain can also bring down tree limbs that can cause injury and power lines resulting in widespread interruptions in electric service.

Freezing rain forms when rain falls to a surface – be it a road, tree limb or power line – and instantly freezes forming a layer of ice on that surface. As the rain continues, layers of ice can form on these surfaces. In the case of power lines and tree limbs, the added weight stresses can cause them to break. Even thin layers of ice from freezing rain on roads and walkways will make them treacherous for travel. Black ice – a thin layer of ice on roads and walkways – is particularly dangerous.




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