What is a tornado?
A tornado is a violently rotating column of air that extends from the base of a thunderstorm to the ground. On December 15, 2021, Iowa saw a record number of tornadoes for a single day with 63 tornadoes confirmed.
How does a tornado form?
A tornado forms when weaker surface winds and stronger upper-level winds create a horizontally rotating column of air. Because this is rotating horizontally instead of vertically, it is not picking up debris, so we usually can’t see anything happening at this stage. When this process happens in the vicinity of a rotating updraft within a thunderstorm, the rotating column of air will tilt and twist it vertically. This is when debris can be picked up and tossed. Tornadoes can form from the cloud and down, or start from the ground and develop upwards.
Iowa Tornado Statistics 1980-2019
Between 1980 and 2019 Iowa saw an average of 48 tornadoes. The most recorded in a single year during that time was 120 in 2004 and the least tornadoes recorded in a single year during those 39 years was 16 in 2012. Out of the 365 days, there were an average of 15 days with tornadoes per year. The peak month was June and the peak hours were 3-7 pm. There were 1,928 confirmed tornadoes during this period, but only 3.2% were of EF-3 strength or higher.
The Enhanced Fujita Scale
The Enhanced Fujita (or EF) Scale became operational in 2007 and is used to assign tornadoes with a rating based on estimated wind speeds and related damage. When tornadoes cause damage the National Weather Service will assemble a team of people to survey the damage. Assessors compare the damage to a list of damage indicators and degrees of damage which help estimate the range of speeds the tornado likely produced. A list of damage indicators can be found here. The original Fujita scale was developed by meteorologist and severe weather research scientist, Tetsuya Theodore (Ted) Fujita.