With winter just around the corner, many keep a close eye on the forecast, watching for the chance of snow. However, below-freezing surface temperatures do not always guarantee we will see snowflakes.

All precipitation begins as snow in the upper levels in the atmosphere.

Snow growth occurs in clouds where the air temperatures are below freezing.

When snow falls from a cloud, it passes through the levels of the atmosphere before reaching the surface.

The difference in air temperature at those various levels of the atmosphere determines what type of precipitation will ultimately reach the surface.

Falling snow must survive the journey from the cloud it forms in down to the surface. As the snowflake falls, the temperature of the air it falls through will act upon it.

If the entire column of air from cloud level where the snowflake forms reaching down to the surface is below freezing, then snow is likely.

Therefore, we primarily see snow during the winter when temperatures are lower on average. However, it is not uncommon to see snow in the fall or spring when occasionally this temperature criteria is met.

When the seasons begin to change and average temperatures rise, the chance for snow lessens.

Although all precipitation begins as snow, when temperatures are above freezing through much of the atmosphere, falling snow melts on its journey to the surface.

That melting snow then reaches the surface as rain.

Meteorologists use various forecast models and tools to identify temperatures at various levels in the atmosphere to help predict the chance for rain or snow.

Often, the vertical atmospheric temperature profile is not entirely above or below freezing.

Those vertical temperatures differences can lead to different types of precipitation as well.

Sleet occurs when a portion of the atmosphere is above freezing, leading to melting as snowflakes travel through that layer.

At a point in the lower levels of the atmosphere, temperatures drop below freezing, leading to the attempted re-freezing of the melted snow flake.

If the snowflake can re-freeze entirely as it passes through the sub-freezing layer of the atmosphere before reaching the surface, sleet will be produced.

Because melting and re-freezing occurs through this process, sleet often looks like small ice pellets rather than true snowflakes, though both are forms of frozen precipitation.

Freezing rain occurs when much of the atmosphere is above freezing and just a small layer at the surface is below freezing.

The initial snowflake melts and becomes a raindrop on its journey through the large above-freezing atmospheric layer.

The rain drop reaches the surface where the temperature is below freezing and re-freezes on contact.

Although liquid precipitation reaches the surface, rapid freezing occurs, leading to a coating of ice on roads, cars and more.

Monitoring the temperature at all levels of the atmosphere becomes critical through the winter months when forecasting the chance for snow, rain, sleet and freezing rain.

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