What is a key characteristic of a tornado?

A tornado is defined as a rapidly rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. Its swirling wind pattern is one of its most distinguishing features. Tornadoes can exhibit a wide variety of shapes and sizes, but the characteristic funnel shape with a visible rotation is essentially what makes it a tornado.

The other options lack key aspects of what constitutes a tornado. Static weather conditions do not involve the dynamic and rapidly changing nature of a tornado. Tornadoes are not a type of precipitation like rain or snow, which fall from clouds rather than rotating within them. Lastly, while tornadoes can occur in various seasons, they are not limited to winter; in fact, they are most common in the spring and early summer months. Therefore, the characteristic of being a "swirly wind" perfectly encapsulates the essence of a tornado.