Welcome to You Ask Andy

Stephen Wowkowych, age 11, of Rochester, New York, for his question:

Why do we get snow instead of hail in winter?

When it comes to the weather, it is never safe to say that something is downright impossible. Summer spells may surprise us in mid winter and snow flurries have been known to arrive out of season. Once in a while, a raging hurricane blows way off course and sometimes after winter sets in, Indian summer brings a warm spell. Hail is unusual in the snowy, winter season, but not impossible. Naturally there are reasons why we are more likely to get snow instead of hail during the winter months.

Hailstones are quite complex little pellets, created by complex weather conditions. Snowflakes are delicate designs assembled by a very different set of weather conditions. As a rule, the snow making situation can and does occur at any season of year, day or night. Hailstorms occur quite frequently on winter nights over the oceans and also, of all places, in Iceland. In Northern Europe, a cold winter polar front may start off with a barrage of hail pellets and the snow falls later.

Snow flakes are built from minuscule crystals of ice, frozen directly from molecules of gaseous water vapor. The air must be at least 32 degrees Fahrenheit, the temperature at which liquid water normally freezes to solid ice. Often the air is super cooled below this temperature before the creation of the ice crystals begins. The intricate job of assembling the tiny crystals in lacy snowflakes also may require below freezing temperatures. Summer does not provide such chilly conditions, which is why snow is a wintry weather event.

Hail formation requires a weather confrontation between different air masses  ¬warm and cool, damp and dry. The basic conditions are caused by the heating and cooling of different levels of the atmosphere, from the surface of the globe up to perhaps seven miles. This creates a steep temperature gradient, similar to those that trigger summer thunderstorms, when the surface level is in the 80s and the air aloft is way below freezing.

A steep temperature gradient also occurs when the lower air level is near freezing and the upper level chills way below zero. This sort of weather condition is responsible for the winter hailstorms that fall on Iceland. A similar situation produces hail on winter nights over the oceans. After sunset the upper air chills

very fast. But the watery ocean tends to hold onto the heat it absorbed during the day, creating a steep temperature gradient likely to give birth to thunderstorms and pellets of hail.

Some people claim that the weather may become too cold to snow. This is not likely, though snow falls less frequently when temperatures drop way below zero. Cold air contains less of the vapor needed to form basic ice crystal for building snowflakes. As the air cools, it can hold less vapor and tends to become drier. However, some vapor is always present and weather conditions never really get too cold for snow.

 

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