Weather Whys; Frost

This is weather-wise. In many areas of the country, autumn is now firmly entrenched, and quite a few northern and mountainous locations have already seen frost. A frost glazed landscape can be picturesque, but when that frost occurs earlier than normal, it may also be deadly to plants and flowers. That means the timely forecasting of an oncoming frost is important, so farmers and gardeners can be prepared. Meteorologists know to look for frost on cool, clear nights, because that's when the Earth radiates away a lot of the heat it receives during the day. By giving off heat, the ground becomes cooler, and so the air next to the surface gets cooler too. Air at a given temperature can only hold a certain amount of moisture, and warm air can hold more of it than cooler air. So when air cools, it may suddenly become incapable of holding all the moisture it previously contained, and the excess is forced out.

In other words, it condenses on something like the grass, car windshields, or low slung spiderwebs. That moisture is due, and the temperature at which it forms is called the dew point. But if the dew point happens to be below freezing, the moisture will be deposited as tiny ice crystals. So frost isn't a form of precipitation. It doesn't fall from the sky, nor does it form out of the plants on the ground. It simply forms out of the air from the invisible water vapor that's part of the air. Whether wise is produced with the assistance of the Cooperative Institute for Mesoscale Meteorological Studies, the Oklahoma Climateological Survey, and the School of Meteorology, all at the University of Oklahoma. Our writer is Christine Harbor, our editor Brian Walkie, and our executive producer is Karen Hope. For whether wise, I'm Drew Barlow.