To Rain, or to Snow?

What determines if it will rain or snow? Sometimes the air is 32 degrees F and we get rain, while other times it is 33 degrees F and it snows. Why?

Much depends not only on the temperature at the ground (where you are standing), but also the temperature of the air above you. Sometimes – especially March and April – there can be a layer of very cold air several thousand feet overhead. As the atmosphere warms, this cold layer of air becomes increasingly unstable, which can cause precipitation. If the higher altitude is below freezing, the precipitation falls as snow. Near the ground there may be a very narrow band of warmer temperatures.

As the snowflakes descend through the warmer air, they don’t have enough time to melt. Instead they turn into very large snowflakes – sometimes the size of a silver dollar – OK, maybe quarters, but they are big.

This is reversed when there is a very shallow layer of below-freezing air near the ground, while the air several thousand feet above you is unusually mild and warm. Precipitation that falls is liquid (rain).

Upon falling into the shallow, cold air near the ground, either freezes upon contact with roads, forming a glaze (freezing rain) or the raindrops freeze before hitting the ground (sleet).

We see this frequently in Southern cities like Atlanta around January 1st. Ice storms, as we had in January 1998 in the Northeast, is a very unusual combination of contrasting air masts right about your head. For drivers, ice is one of the most dreaded possibilities.

March and April are months where this occurs. You will also see it as we ease into winter.

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