Original sentence: A tornado is a violent, dangerous, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.Although tornadoes have been observed on every continent except Antarctica, most occur in the United States.They also commonly occur in southern Canada, south-central and eastern Asia, the Philippines, east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand
Edited sentence: A tornado is a violent, dangerous, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud(otherwise known as a thundercloud) or, in rare cases, the base of a cumulus cloud.Although tornadoes have been observed on every continent except Antarctica, 75% of them occur in the United States.The remaining tornadoes occur within regions at mid-latitude where warm and cool air fronts collide.These regions include: southern Canada, south-central and eastern Asia, the Philippines, east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand