An earthquake is an intense shaking of Earth's surface, and scientists use an instrument called a seismometer to measure the shaking waves. In 1935, Charles F. Richter of the California Institute of Technology introduced the idea of earthquake magnitude. Each whole number up in magnitude means a tenfold increase in measured wave size and about 32 times more energy released, and seismic moment equals the strength of the rock along the fault times the area of the fault that slipped times the distance the fault moved. Moment magnitude equals two-thirds of the base-10 logarithm of the seismic moment minus 9.1, and it is based on all the waveforms recorded from the shaking, and it is designed to be roughly equal to the Richter Scale where the two overlap. For most earthquakes the moment magnitude scale is a more accurate measure of size than the Richter scale, which is now used mainly for small local earthquakes.
Parts of the San Andreas Fault creep steadily in many tiny shocks, but where creep is not constant, strain can build up for hundreds of years and produce great earthquakes. Earth's outer layer is made of giant puzzle pieces called tectonic plates that slowly move, and an earthquake has a single magnitude, but the shaking it causes varies from place to place with distance, type of surface material and other factors. Aftershocks are smaller earthquakes that follow a larger earthquake in the same area, and as of November 2026, the USGS developed the ShakeAlert Earthquake Early Warning system together with many partner organizations, including universities and state agencies. If the ground shakes, drop down, cover your head and neck, and hold on until the shaking stops, and families can fasten bookcases, televisions and other heavy things so they stay put in an earthquake.
An earthquake under the ocean can push the water above it and start a tsunami, and a tsunami can also be started by a volcano in the ocean. The tsunami hazard is greatest near subduction zones, where large earthquakes and landslides can produce damaging waves. The most likely tsunami sources on the U.S. East and Gulf Coasts are underwater landslides and meteotsunamis, because those coasts are not near subduction zones. DART systems were developed by NOAA for early detection, measurement and real-time reporting of tsunamis in the open ocean. Once a tsunami is detected, warning centers use forecast models to forecast wave height and arrival times, the location and amount of flooding, and how long it will last. In deep water a tsunami moves as fast as a jet plane, but near land it slows to about the speed of a car, 20 to 30 miles per hour. For a tsunami close to a coast there may not be time for a detailed forecast, so people should recognize natural warnings and be ready to respond.
A tornado is a spinning column of air that reaches from a storm cloud down to the ground. Winds moving in different speeds and directions at different heights make the rising air in a storm start to spin. A supercell is a long-lived, highly organized storm with a tilted, rotating updraft, and a mesocyclone, the large rotating updraft in a supercell, is usually 2-6 miles in diameter, much larger than the tornado that may develop within it. Most tornadoes form in supercell thunderstorms, but not all supercells produce tornadoes. Scientists do not fully understand how tornadoes form. Studies of tornado formation suggest it can happen without temperature patterns at the edge of the downdraft; very little temperature variation was seen near some of the most destructive tornadoes, on 3 May 1999. NSSL researchers discovered the Tornado Vortex Signature, a Doppler radar velocity pattern that shows a region of intense concentrated rotation.
A Tornado Vortex Signature does not guarantee a tornado, but it strongly increases the probability of one. A hook echo is a pattern in radar images that looks like a hook on the right-rear part of a storm; it is often linked to a mesocyclone and favorable conditions for tornado formation. Dual-polarization radar can detect random shaped and sized targets like leaves and insulation, helping meteorologists confirm a damaging tornado is on the ground, especially at night. Phased array radar technology can scan an entire storm in less than one minute, letting forecasters see signs of developing tornadoes well ahead of current radar. As of November 2026, current tornado warnings are largely based on observations; the Warn-on-Forecast research aims to issue warnings based on a computer forecast. In ensemble forecasting a supercomputer runs several models at once; if the runs look alike, the weather will likely follow the rules, and if they differ, something is making the weather misbehave.
Experts rate a tornado by studying the damage it caused, and the Enhanced Fujita Scale has six ratings, EF0 to EF5. EF Scale wind speeds are estimates based on damage, not measurements. EF0 is the weakest tornado rating, with wind gusts up to 85 mph, and EF5 is the strongest, with gusts over 200 mph. Official tornado records only go back to 1950, so the true average number of tornadoes each year is not known. The widest tornado path on record, near El Reno, Oklahoma on 31 May 2013, was officially 2.6 miles wide. After the Tri-State tornado of 18 March 1925, checks and papers fell from the sky in Missouri, Illinois, Indiana and even Ohio, which the tornado did not reach. "Tornado Alley" is a nickname invented by the media for a broad area of the central United States where tornadoes are fairly common.
A tornado watch means tornadoes are possible in and near the area, and a tornado warning means a tornado is near and it is time to move to a safe place. A safe room built to FEMA criteria or a storm shelter built to ICC 500 standards is the best tornado protection; the next best is a small interior windowless room on the lowest level. In a tornado warning, go to a small room with no windows on the lowest floor of a sturdy building. A NOAA Weather Radio with tone alert is one way a family can hear when a warning has been issued. Opening windows to equalize pressure does not help in a tornado, because the tornado will blast open the windows anyway, and the "safe southwest corner" of a basement is a myth, because walls and furniture can fall into any corner.
A hurricane is a big storm that forms over warm ocean water. Scientists call hurricanes and their relatives tropical cyclones, and a tropical cyclone is a hurricane when its winds reach 74 miles per hour. Hurricane conditions occur in the tropics, typically between five and 20 degrees of latitude north and south of the equator. Because of the Coriolis effect, hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Hurricane names come from six lists that rotate, so each list is used again every six years. Few tropical storms grow into hurricanes, because all the needed conditions must occur at the same time and place. The Saffir-Simpson scale rates hurricanes from 1 to 5 by their wind speed, and hurricane categories are based on wind speed, not storm size, so hurricanes that look small on radar can have very high wind speeds.
Storm surge is sea water pushed onto land by the winds of a storm, and it is primarily caused by the strong onshore winds of a hurricane or tropical storm. Storm surge height depends on a storm's intensity, forward speed, size, angle of approach, and the shape of the coastline. A wide, gently sloping continental shelf allows a higher storm surge than a narrow, steeply sloping one. The Gulf Coast, especially Louisiana and Mississippi, is especially open to storm surge because the ocean floor deepens gradually offshore, while Florida's southeast coast has a shelf that drops off quickly. In 2011 Cyclone Yasi made a direct hit on the Queensland, Australia coast and produced seven meters (23 feet) of storm surge. The National Weather Service SLOSH model lets forecasters customize storm surge predictions for particular storms and coasts. When high tide happens at the same time as a storm surge, the combination is called a storm tide, and the water can be 20 feet or more above normal.
A flood is water that covers land that is usually dry, and floods can come from heavy rain, from fast-melting snow, or when ocean waves come on shore. Melting snow or warm rain can break up river ice, and chunks of ice can jam against bridges and make the water rise quickly behind the jam. Flash floods can happen within minutes of heavy rain, and where it rains it can flood, even if a place is not near a river or on a flood map. In 2005, flooding hit New Orleans days after Hurricane Katrina came onshore because the levees designed to protect the city failed. As of November 2026, homeowner's insurance policies do not cover flooding, and a flood insurance policy typically takes up to 30 days to go into effect. A family can make a plan together for what to do in a disaster, and each of these warnings and tools is there so that a plan can be used in time.
Words to know
- magnitude: earthquake size
- subduction: plate sinking
- mesocyclone: rotating updraft
- ensemble: group of runs
- latitude: distance from equator
- levees: flood walls
