How Is a Natural Disaster Classified?

According to the U.S. Department of Health and Human Services, natural disasters are naturally occurring events that can cause severe threats to the public health. Tornadoes, hurricanes, volcanoes and earthquakes each have developed classification systems that gauge the intensity of an event by assessing a number of set factors. These classification methods help researchers study disasters patterns, and help educate the public to plan and prepare for potential disasters.

Hurricanes

• The Saffir-Simpson Hurricane Wind Scale, revised in 2010, categorizes hurricanes from 1 to 5 based on the hurricane's strength at the indicated time. The scale provides examples of the type of damage associated with winds of the indicated intensity, with wind damage increasing by a factor of four from each category increase, according to the National Hurricane Center. Category One hurricanes feature "dangerous winds that produce some damage," with sustained wind speeds from 75 to 95 mph; Category Five hurricanes indicate that "catastrophic damage will occur," and feature sustained wind speeds of over 155 mph.

Tornadoes

• The Fujita Scale, also known as the Fujita-Pearson Scale, is the official classification system used to rate tornado intensity, from F0 to F5. According to NOAA, it works by measuring how much destruction is caused after a tornado has passed over a man-made structure, correlating damage to wind speed. Professor Tetsuya "Ted" Fujita and Allen Pearson (director of the National Severe Storm Forecast Center) invented the method in 1971. The F-scale is very subjective, largely depending on the judgment call of the surveyor.

  Weather researchers now use the "Enhanced F-scale," which classifies the original F0-F5 damage by using 28 different types of damage indicators, set by engineers and meteorologists. A customized approach is employed to assess the damage of buildings and other structures, depending on materials and foundations. Just like the original F-scale, the enhanced version rates the tornado as a whole based on most severe damage within its path, and is subjective. The enhanced F-scale took effect February 2007.

Volcanoes

• The Volcanic Explosivity Index is a scale designed to measure the intensity of volcanic eruptions, using a VEI index of 0 (non-explosive) to 8 (mega-colossal). In the 0-8 scale, each category represents an increase in intensity by a factor of ten. According to the U.S. Geological Survey, the VEI measures the volume of ash produced, the height of the eruption cloud over the vent, and the duration of the eruption.

Earthquakes

• The Richter Magnitude Scale is a mathematical method that compares the intensity of earthquakes. Seismic waves are vibrations within the earth that are measured on instruments called seismographs. Charles F. Richter invented the scale in 1935, which employs a logarithm to measure the amplitude of seismic waves recorded on seismographs, according to the U.S. Geological Survey. The magnitude of an earthquake is quantified by whole numbers and decimal fractions. A magnitude of 2.0 or less is called a microearthquake, 4.5 is strong enough to be recorded on seismographs globally. A magnitude of 8.0 or higher is called a great earthquake; and the USGS reports that on average, at least one of these occurs somewhere in the world each year. The Richter Magnitude Scale is used only to measure seismic activity, not damage.

Typhoons

• According to the National Institute of Informatics, the strength of a typhoon is measured by the maximum sustained wind speed, as reported by the World Meteorological Organization. The international standard measures a Class 2 cyclone, or Tropical Depression, as wind less than 33 knots. Class 3, a Tropical Storm, has wind speeds of from 34 to 47 knots; while Class 4, a Severe Tropical Storm, features winds of 48 to 63 knots. A Class 5 tropical cyclone is called a typhoon, and sports winds from 64 to 105 knots and beyond.

  U.S. meteorological organizations classify the intensity of typhoons in concert with hurricanes, utilizing the Saffir-Simpson Scale's categories 1 through 5 (see hurricanes, above). A typhoon refers to storms that begin in the Western Pacific (Japan, Philippines, Guam), which are usually stronger than hurricanes, but generally involve less land mass.