14.5: Tornadoes
- Page ID
- 46278
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Recall from the prior chapter that supercell storms can produce tornados. A supercell is a severe thunderstorm with rotation. Typically the rotation of a supercell thunderstorm comes from directional wind shear in the environment. Occasionally a supercell storm produces a tornado, a region of rapidly rotating air in cyclostrophic balance. Recall from Chapter 10 that cyclostrophic balance is a balance between the pressure gradient force and the centrifugal force. As the rotating air motion in a supercell is vertically stretched, it narrows and speeds up. To picture this, imagine a figure skater pulling their arms inward as they spin. Their diameter narrows and their rotation speed increases. The same physics—conservation of angular momentum—occurs as a tornado forms.
Tornados are difficult to forecast well in advance. Typically a Tornado Watch will be issued by a Storm Prediction Center when all of the ingredients for a tornado are present in the atmosphere. These ingredients are the same as for a severe thunderstorm: warm moist air; instability (quantified by large values of CAPE); potential for a trigger; and, of course, directional wind shear to provide rotation. A Tornado Warning is issued when a tornado is imminent or occurring. For example, one may be issued when a funnel cloud is spotted or when a “hook echo” is seen on radar. The issue of forecasting tornados is called “nowcasting”. Although very little advance notice can be given to the public, forecasters do their best to provide as much notice as possible.
The above image shows a hook echo from radar. The key feature is the bottom left region of the storm where a region of rain can be seen rapping around a region of dry inflow air. Right at the cusp of the hook is where a tornado would form if it were occurring. The strong rotation in a tornado creates this radar signature.
Tornado strength is categorized after they occur. A scale called the Enhanced-Fujita scale is used to classify tornado strength based on the amount of damage that was caused. While the wind speed for each step in the EF scale is given, it is not measured. The wind speed is determined afterwards by the scale of damage and the winds required to cause the damage.
| Scale | Wind Speed (3 sec. gust, mph) | Damage |
|---|---|---|
| EF0 | 65-85 | Minor |
| EF1 | 86-110 | Moderate |
| EF2 | 111-135 | Considerable |
| EF3 | 136-165 | Severe |
| EF4 | 166-200 | Extreme |
| EF5 | >200 | Massive/Incredible |
The central US is the world’s hot-spot for tornados, however, tornados and tornado-like storm features can occur elsewhere. A dust devil and a waterspout are tornado-like features that are associated with cumuliform and sometimes cumulonimbus clouds. However, dust devils and waterspouts typically are not associated with a supercell. Waterspouts look like tornados but form over water surfaces. Waterspouts are often narrow, relatively short lived, and formed by the same physics as a tornado—a rotating updraft with condensation in the funnel cloud. Below is an image of a waterspout observed in Hawaii off the coast of Oahu.
