A tornado is not necessarily visible ; however , the extremely low pressure caused by the high wind speeds and rapid rotation usually causes water vapor in the air to condense into a visible condensation funnel . A tornado is not necessarily visible ; however , the intense low pressure causing the high wind speeds ( as described by Bernoulli 's principle ) and rapid rotation ( due to cyclostrophic balance ) usually causes water vapor in the air to become visible as a funnel cloud or condensation funnel .