What makes a thunderstorm “severe”

July 9

June 4, 2012 – Simms, Montana. By Chris Streeks.

What exactly distinguishes a “severe” thunderstorm from any other thunderstorm?  The official definition of a “severe” storm may strike you as  arbitrary. According to the National Weather Service, a severe thunderstorm must include winds of 58 MPH or greater, hail one inch in diameter or larger, or a tornado. But why are these the criteria and how did they come about?

One of the criteria – a tornado – is obvious, so that leaves in question the reasoning behind the minimum requirements set on wind and hail. The parameters a thunderstorm must meet to be considered severe mostly came about through research by engineers.

Let’s start with hail. As with all science, the answer changes with new discoveries. This is why the NWS increased the size criteria for severe hail from the size of a penny (0.75 inches) to the size of a quarter (one inch) in January 2010. Meteorologists consider hail the size of quarters to be “severe” because it is the minimum size required to produce damage relevant to most property in the United States. The caveat “relevant to most property” is important because hail as small as peas (0.25 inches) can cause damage to crops and other vegetation. Imagine the number of severe thunderstorm warnings we would see if hail that small was considered severe.

The hail size criteria increased in 2010 for two reasons, according to a document released by the National Weather Service. The first reason is that, as stated above, researchers found that property damage did not become significant enough until hail reached an inch in diameter. The report relies extensively on roof damage, which found that no shingle damage occurred when researchers pelted the material with hail the size of pennies. Damage to older shingles only began at quarter-size hail, and widespread damage began with hail around 1.25 inches in diameter.

The second reason behind the change in hail criteria is less scientific and more sociological in nature. The “crying wolf” effect is a well-known pest in the weather world. If too many tornado warnings are issued in storms that do not end up producing tornadoes, people will begin to ignore them as false alarms. The same goes for all warnings, and this holds true for severe thunderstorms.

The NWS took into account the fact that raising the minimum threshold for hail size in a severe thunderstorm would cause a marked reduction in the number of severe thunderstorm warnings issued, thereby curtailing the “crying wolf” effect. According to the NWS, it worked. The reduction in warnings made them more reliable and less commonplace in news reports, and emergency managers reported a “stronger confidence” in the potential for damage as the result of a storm.

The wind damage threshold, on the other hand, is a legacy that is less about scientific research and more about preventing airplane crashes. According to the National Severe Storms Laboratory, there were three types of severe weather watches in the early days of forecasting: tornado watches, public severe thunderstorm watches, and aviation severe thunderstorm watches.

The public severe thunderstorm watch set the damaging wind criteria at 75 mph while the aviation watches were set at 50 knots, or 58 mph. The difference between the speeds makes sense because old man Jenkins’ homestead is less susceptible to 58 mph winds than, say, a Boeing 727 on downwind in Atlanta.

When the public and aviation severe thunderstorm watches were consolidated in 1970 to the severe thunderstorm watch we know and love today, the wind criteria dropped down to the aviation minimum of 58 MPH (or 60 MPH, as it is often rounded-up for the public).

Now, that’s not to say that 58 mph winds are harmless to surface dwellers. Winds that gusty can certainly cause tree and power line damage, as well as blow around loose objects outdoors and prove dangerous to some high-profile vehicles. Even lighter winds can bring down large trees if the soil is soggy.

Only the three aforementioned severe weather events – tornado, hail, and high winds – qualify as “severe weather” according to the National Weather Service. Severe is a subjective term and the definition exists to lend it some objectivity. Though it seems reasonable that the amount of heavy rain or lightning would factor into the labeling of a thunderstorm as “severe,” it doesn’t.  A storm could drop seven inches of rain and produce a thousand flashes of lightning in thirty minutes, but if it doesn’t have quarter-size (or larger) hail, 58 mph winds, or a tornado, it is not considered “severe.”


Hail and wind damage near Tallahassee, Florida, after storms in March 2013. (NWS Tallahassee)

Meteorologists use special guidance to help determine if and when a heavy rainstorm produces enough precipitation to merit a flood or flash flood warning. This accounts for thunderstorms with heavy rain.

Lightning is a different beast altogether. Meteorologists are actively trying to develop systems to better predict when a storm could produce lightning, but until they can accurately predict how much lightning a storm might produce, it would be hard to objectively say much more than “this storm is producing frequent lightning” while the storm is already ongoing.

Will the severe thunderstorm criteria change in the future? It’s possible. After decades of using the Fujita Scale for quantifying tornado strength via damage, experts re-evaluated the scale and developed the Enhanced Fujita Scale in 2007 to take into account changes in building practices. As we saw in 2010, the criteria for severe hail bumped up a quarter of an inch after research proved that the larger hail size is the minimum at which damage occurs. Over time, building materials will change and our priorities will shift, and there may come a time when experts find that severe winds or severe hail mean something different to the public in the future.  That’s the beauty of science.

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