Dear Tom,
Could a tornado be invisible? Clean air is not visible.
— Jason White
Dear Jason,
It’s possible. Airborne dirt and debris carried aloft by a tornado’s winds make the funnel visible. Videos of tornadoes in action show that such material is sometimes raised only a few hundred feet, with the remaining mid and upper portion of the funnel invisible. This is especially true with just-formed tornadoes. In such cases the ground-based “dust collar” might not be apparent to an observer a mile or two away. However, most tornadoes are rendered visible by another phenomenon: condensation of water vapor into visible clouds. Most of a visible tornado is cloud formed from the condensation of moisture when air that is drawn into the tornado experiences the sharp pressure reduction that accompanies it.
