A tornado in the USA

A tornado is a violently 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. The most violent tornadoes are capable of tremendous destruction with wind speeds of up to 300 mph. They can destroy large buildings, uproot trees and hurl vehicles hundreds of yards. Damage paths can be in excess of one mile wide to 50 miles long. In an average year, 1,000 tornadoes are reported in the USA alone.

Tornadoes come in many shapes and sizes, but they are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most (less violent) tornadoes have wind speeds less than 110 miles per hour (177 km/h), are about 250 feet (76 m) across and travel a few miles (several kilometres) before dissipating.

Most tornadoes form from thunderstorms. In the USA, when warm, moist air from the Gulf of Mexico meets cool, dry air from Canada, instability is created in the atmosphere. A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

This very interesting postcard came from Cathy (beesknees) 3 October 2014 DS Postcrossing.