Weather Whys; Fog Classification

This is WeatherWise. There are all sorts of interesting weather phenomena in the United States that can delight and fascinate the amateur weather watcher, unusual cloud formations, record breaking temperatures or precipitation, and snowstorms all have their own peculiar charm. Severe thunderstorms even command attention as they wind their way across the landscape. On the other hand, there is fog. For some weather watchers, that isn't the sort of atmospheric event that's liable to peak much interest. But some scientists have been interested enough in fog to bother to classify different kinds of it. Most fog arises due to natural processes, such as the cooling that occurs at night. But if you've ever been out on a cold winter morning, you may have even created a few small fog clouds yourself. When you exhale, if conditions are right, your breath forms a little puff of evaporation fog. That's because air at a given temperature can only hold so much water vapor, and while warmer air can hold more vapor, cooler air can hold less.

The air you exhale will be a lot warmer than the air outside on a winter morning, and it contains a lot more water vapor than the ambient air can handle. So the excess vapor condenses into tiny suspended droplets of water. Those droplets refract the light coming through them in such a way that the entire collection of suspended droplets takes on a whitish appearance. That's what makes the puff of breath take on the cloud-like appearance. Whether wise is produced with the assistance of the National Weather Service Forecast Office, and the National Severe Storms Laboratory, both in Norman, Oklahoma, and the Cooperative Institute for Mesoscale Meteorological Studies, the Oklahoma Climateological Survey, and the School of Meteorology, all at the University of Oklahoma. Whether wise is made possible by a grant from the National Science Foundation. For Whether wise, I'm Drew Barlow.