A tornado is a violently rotating column of air extending from the base of a thunderstorm to the ground surface. These phenomena develop when specific conditions create a rotating updraft, forming a vortex that varies dramatically in size and strength. Tornadoes range from those capable of leveling entire communities to those causing only minor, localized damage. The weakest tornadoes represent the lowest end of this intensity spectrum and are the most common.
Measuring Tornado Intensity
Meteorologists use the Enhanced Fujita (EF) Scale to classify tornado strength based on a post-event survey of the damage left behind. The scale ranges from EF0 (weakest) to EF5 (highest intensity). This rating is not determined by real-time wind speed measurements, but rather by correlating observed destruction with the estimated wind speeds necessary to cause that damage.
The classification process involves assessing damage against 28 different indicators, including various building types, structures, and vegetation. For each indicator, meteorologists evaluate the degree of damage to estimate the 3-second wind gusts that occurred at that location. This methodology provides a standardized and consistent way to rate a tornado’s severity.
Defining the Weakest Tornado
The weakest classification is the EF0 tornado, categorized by estimated wind speeds ranging from 65 to 85 miles per hour. While this speed range is low for a tornado, it is still equivalent to the wind speed found in a strong tropical storm or a Category 1 hurricane. The National Weather Service classifies EF0 and EF1 tornadoes together as “weak” events.
EF0 tornadoes account for the majority of all reported events, making up around 75% of all tornadoes observed in the United States. An EF0 is frequently short-lived and narrow, sometimes lasting only a few minutes. Visually, a weak tornado may appear as a thin, twisting column, often referred to as a “rope” funnel cloud that has made contact with the earth.
Typical Damage Profile
The damage caused by an EF0 tornado is generally minor, though it is still capable of causing localized destruction. The estimated wind speeds (65 to 85 mph) are strong enough to peel surface layers off roofs, particularly by removing shingles or tiles. While often superficial, this damage can create vulnerabilities in the building envelope that lead to further water intrusion.
In outdoor areas, common signs of an EF0 include damage to trees and minor structures. Weak tornadoes can snap small tree limbs and branches, especially those that are decayed or dead. Shallow-rooted trees may be pushed over, and fences, sheds, or detached garages can sustain significant damage or be completely knocked down. Well-constructed homes and commercial buildings usually remain intact, though they may experience broken windows or displaced signs.
Distinguishing Weak Tornadoes from Other Phenomena
The term “weak tornado” can be confused with other rotating atmospheric events that are not classified as true tornadoes. A funnel cloud is a rotating column of air that extends from a storm cloud but fails to make contact with the ground. If the funnel cloud touches the ground, it is immediately classified as a tornado, regardless of its strength.
Another distinct type of weak vortex is a landspout. While a type of tornado, it forms through a different process than the classic supercell tornado. Landspouts are often weak and form from the ground up, not requiring a rotating storm structure in the cloud above. In contrast, a dust devil is not a tornado, as it forms on clear, sunny days due to intense surface heating and is not associated with a thunderstorm cloud.