Torrence L. Goodman, ag;10, Williamsport, PA,for his question:

Is it ever too cold to snow?

Snow forms from vapor in the air and there is always a certain amount of vapor, even in the driest air. This vapor is the gaseous form of water, It mingles with the other invisible gases of the  air. Cold air, as a rule, tends to be much drier than warm air. It has less vapor to turn into snow. However., it always has some vapor and if conditions are right this vapor can become snow.

In A1aska, snow has fallen when the temperature is 50 degrees below zero. This below zero snow, however, is rare and it usually falls in the form of fine, powdery grains. Ice needles may also fall at this low temperature. These are fine splinters of ice measuring perhaps 20 to 40 to an inch. Sometimes they form a ground. fog;. They after form a haze in the cold air high above our heads. This is what causes the ghostly halo around. the moon.

There is, of course, a reason why snow is not so likely to fall in really cold weather. Air loses its moisture as it chills. Usually, cooling air has already lost most of its vapor below before it gets to be zero cold. Like most things in nature, this is a vary orderly process.

The air can hold so much vapor and no more, depending upon its temperature. When it holds its full quote of vapor we say it is saturated. The relative humidity is 100%. The relative humidity varies with the temperature.  One cubic meter of air at 4.3 degrees if is saturated when it holds 7.27 grams of vapor. At 32 degrees it is saturated when it holds only 4.82 grams of vapor.

When saturated air chills, it is suddenly loaded with a surplus of vapor. If the temperature is above, freezing, this surplus vapor becomes liquid. cloud mist. If it is below freezing, it may become ice and snow.

Snow is not frozen rain. It does not form from the liquid droplets of cloud mist. It forms when the vapor particles in the air change directly from gas to from gas to solid ice. These icy particles are very small indeed.

Countless numbers of them are necessary to form a single snowflake, a snowflake is a crystal. When a chemical, such as ice, crystallizes it follows a certain pattern. The crystal pattern depends upon the shape of the particles from which the chemical is made. Particles of ice form six‑sided crystals. To form a snowflake, they group together with tiny pockets of air to make a lacy six‑sided design. The particles of ice with the pockets of air play tricks with the light to make the snowflake snow white.

All this happens when air which contains its full quota of vapor is chilled below freezing. Most of the vapor has turned to cloud mist or ice fragments long before the temperature drops way below zero. There is not much left to make snow. But there is some and, if conditions are right, it is never too cold to snow.