The weather chart for the day indicated that as the temperature rose from 10 to 30 degrees F the humidity dropped down to around 60 percent. This was puzzling. Maybe there was a way the air could get drier without it snowing. Or maybe the humidity figures had a more complicated meaning.
"It's not the heat, it's the humidity," goes the old saying. That one is usually put away with the barbecue and lawn mower come winter. The heat is gone but the humidity remains, sometimes 99 percent of it. We tend to think of winter air as dry, but it's really just cold. And those widely varying humidity values prevail year-round. This is because those percentage figures, called "relative humidity," refer to the amount of water the air can hold at a particular temperature. If the relative humidity is 99 percent, the air has to cool just a little before it starts to precipitate. If it's 60 percent, the air has to cool quite a bit more before the rain or snow begins.
As air is warmed, it becomes capable of absorbing more water, and given the opportunity, it does. Air, after all, has no choice. If air passes over the ocean or a large lake, like Lake Erie, water will evaporate into the air, increasing the relative humidity, or at least keeping it the same even as the air warms further. If that air should cool as it moves east across the Buffalo area, the water has to go because the cooled air can no longer hold it - hence "lake-effect snow."
Most snow or rain happens not from a lake effect, but from a clash of the titans. The titans are air masses, each with different temperature, pressure and water content. When two of these vaporous monsters collide, the water is squeezed out of the air from the warmer mass as it is cooled by contact with the colder mass. Then down comes the rain, or snow.
Meteorologists often talk about "moving fronts." When one air mass moves in on another things get chaotic at the boundary between the two masses. The terms "warm front" and "cold front" refer to the higher or lower temperature of the faster moving air mass. This pushier air mass shoves the slower air mass as if to say, "Move along, buddy." The "front," or zone of confrontation between the masses, usually moves at a pace between the speeds of the two colliding masses, but it can stand still relative to the ground, becoming a "stationary front," prolonging the inclement weather.
Typically, the faster mass slides either over or under the slower mass, tending to expand the zone of chaos, stirring up stormy weather as energy is released in various ways. But the lower mass may be either hotter or colder than the upper mass, and this is part of what determines if the temperature at ground level goes up or down. Ground heat, which can be trapped in the lower mass, is another determinant. Clouds, which are mostly suspended water droplets (not water vapor, which is a clear gas), tend to trap ambient heat, warming the air at night, and to block sunlight, cooling the air by day.
The "dew point" given on weather tables is the temperature at which the air at a given relative humidity becomes too cold to hold all the water in it. That's the temperature to watch for on your thermometer. When the mercury hits that magic number water will likely fall down in some form. However, you may have noticed that occasionally it rains when the air temperature is below freezing, or snows when the temperature is above freezing. This is because the precipitation forms at some altitude above the ground where the cooling air reaches 100 percent humidity, at its dew point. The water (in the form of droplets, snowflakes or ice pellets) may melt or freeze on the way down. Stranger still, when water vapor condenses as ice (like frost) on falling ice pellets that cool the air they contact, the result is hail. Raining frogs is another story, one we will leave for another day. ++