If you have lived in the South for any length of time, then you've probably heard the adage "mares' tails means the weather is about to change."
While other weather folklore isn't always true, the cool thing about this old adage is, that for the most part, it is actually correct!
Mares' tails are actually cirrus clouds. They form in the upper atmosphere when water vapor changes from a gas into a solid. They have a thin and wispy appearance due to the strong winds in the jet stream that give them their shape.
They usually appear a day or two in advance of weather systems such as cold fronts, due to the pressure and temperature changes that are usually associated with fronts. Fronts aren't the only system that they appear with.
Hurricanes and tropical storms usually have cirrus outer edges of the system as well. In that case, cirrus clouds are created by energy outflow or "exhaust" from the top of the storm.
For this reason, they are a cool forecasting tool that anyone can use!