It takes the right combination of wind,temperature,pressure and humidity to create even a weak tornado.

VOA: special.2010.05.11

Tornado seasons are the result of wind and weather patterns.

VOA: special.2010.05.11

The NRC determined that the design-basis tornado wind speeds for new reactors should correspond to the exceedance frequency of one-in-ten-million per year.

FORBES: Tornados and Nukes -- How Big Can They Get?

The Fujita scale is used to estimate the wind speed of a tornado by the damage the tornado causes.

CNN: Tornadoes Fast Facts

"Through embedded sensors, the probes can measure a tornado's wind speed and direction, barometric pressure, humidity, and temperature, " the magazine wrote.

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