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-A volcano is an opening in the crust of a planet or moon that allows molten rock, heated gases, etc.

-As layers of rock and ash accumulate from numerous eruptions, volcanoes will take the form of a hill or mountain.

-Volcanic eruptions can cause health risks and natural forces such as wildfires, floods, mudslides, electricity outages, and contaminated drinking water.

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Get more information about volcanoes from National Geographic.

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-Drought is a time when the weather is drier than usual and there are issues with the water supply.

-Long-term issues with public health can also result from drought, such as water shortages and poor water quality.

-Impacts on food and nutrition, sanitation and hygiene, and air quality. More diseases, including the West Nile Virus, are spread by Droughts

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Drought is an extended period of unusually dry weather when there is not enough rain. The lack of precipitation can cause a variety of problems for local communities, including damage to crops and a shortage of drinking water. These effects can lead to devastating economic and social disasters, such as famine, forced migration away from drought-stricken areas, and conflict over remaining resources.

Because the full effects of a drought can develop slowly over time, impacts can be underestimated. However, drought can have drastic and long-term effects on vegetation, animals, and people. Since 1900, more than 11 million people have died and more than 2 billion people have been affected by drought. Drought is also one of the costliest weather-related disasters. Since 2014 California has lost at least 2 billion-dollars a year, due to drought.
Defining Drought
Drought is a complicated phenomenon, and can be hard to define. One difficulty is that drought means different things in different regions. A drought is defined depending on the average amount of precipitation that an area is accustomed to receiving.

Determining the start of a drought can be tricky. Unlike many natural hazards that bring about sudden and dramatic results-such as earthquakes, tornadoes, and hurricanes-the onset of a drought can be gradual and subtle. It can take weeks, months, or even years for the full effects of long-term inadequate rainfall to become apparent.

The end of a drought can also be difficult to determine. While a single rainstorm will provide short-term relief from a drought, it might take weeks or months before levels of precipitation return to normal. The start and end of a drought are often only clear in hindsight.
Causes of Drought
Most droughts occur when regular weather patterns are interrupted, causing disruption to the water cycle. Changes in atmospheric circulation patterns can cause storm tracks to be stalled for months or years. This disruption can dramatically impact amounts of precipitation that a region normally receives. Changes in wind patterns can also be disruptive to how moisture is absorbed in various regions.

La Niña is the counterpart to El Niño, when the surface water in the Pacific Ocean along the coast of South America decreases in temperature. The cooler waters affect storm patterns by contributing to drier-than-normal conditions in parts of North and South America. El Niño and La Niña both usually last about a year. The effects of La Niña on weather patterns are often more complex than El Niño. Two of the most devastating droughts in the history of the United States-the 1930s Dust Bowl and the 1988 drought in the Midwest-are associated with the effects of La Niña.
Impacts of Drought
Trees and other plants have adapted to withstand the effects of drought through various survival methods. Some plants (such as grasses) will slow their growth or turn brown to conserve water. Trees can drop their leaves earlier in the season to prevent losing water through the leaf surface. However, if drought conditions persist, much vegetation will die.

Certain plants have adapted so they can withstand long periods without water. Yuccas, for instance, have deep root systems that can seek out water with incredible efficiency. Cacti have spiny, hairy spines, spikes, or leaves that limit how much water they lose to evaporation. Mosses can withstand complete dehydration. Juniper trees can self-prune by steering water only toward the branches required for survival. Other plants only grow when there is enough water to support them. In periods of drought, their seeds can survive under the soil for years until conditions are favorable again.

However, many organisms cannot adapt to drought conditions, and the environmental effects of extended, unusual periods of low precipitation can be severe. Negative impacts include damage to habitats, loss of biodiversity, soil erosion, and an increased risk from wildfires. During the U.S. drought of 1988, rainfall in many states was 50 to 85 percent below normal. Summer thunderstorms produced lightning without rain and ignited fires in dry trees. In Yellowstone National Park 36 percent of the park was destroyed by fire.

Drought can also create significant economic and social problems. The lack of rain can result in crop loss, a decrease in land prices, and unemployment due to declines in production. As water levels in rivers and lakes fall, water-supply problems can develop. These can bring about other social problems. Many of these problems are health-related, such as lack of water, poor nutrition, and famine. Other problems include conflicts over water usage and food, and forced migration away from drought-stricken areas.

Traditional agricultural techniques allow land to "rest" by rotating crops each season and alternating areas where livestock graze. Now, with many areas in the world struggling with overpopulation and a shortage of farmland, there is often not enough arable land to support sustainable practices. Over-farming and overgrazing can lead to soil being compacted and unable to hold water. As the soil becomes drier, it is vulnerable to erosion. This process can lead to fertile land becoming desert-like, a process known as desertification. The desertification of the Sahel in North Africa is partly blamed on a prolonged drought whose effects were intensified by farming practices that result in overgrazing.
Historical Droughts
Scientists often study historical droughts to put modern-day droughts in perspective. Since our data from thermometers and rain gauges only goes back about 100 to 150 years, scientists must research paleoclimatology, the study of the atmosphere of prehistoric Earth. Scientists gather paleoclimatic data from tree rings, sediments found in lakes and oceans, ice cores, and archaeological features and artifacts. This allows scientists to extend their understanding of weather patterns for millions of years in the past.

In South America, massive migration out of the once-fertile Atacama Desert 9,500 years ago can be explained by the onset of extreme drought.

In Africa, the Sahel region experienced a dry period from 1400 to 1750 that radically altered the landscape. The water level in Lake Bosumtwi, Ghana, for instance, fell so low that an entire forest grew on the lake's edges. Today, visitors can still see the tops of trees growing out of the lake-where the water is now more than 15 meters (50 feet) deep.

What scientists have learned by looking at Earth's drought history is that periods of severe drought are a regular part of nature's cycle. As devastating as droughts in the last century have been, they are considered relatively minor compared to the severity of earlier droughts that have lasted more than a century.
Major Droughts in the Past Century
Mass migration was an indirect effect of the Dust Bowl. Farmers and their families were forced to migrate to other areas in search of work, and by 1940, 2.5 million people had fled the Great Plains. Of those, 200,000 moved to California. The influx of migrants into existing economies already strained by the Great Depression led to a rise in conflict, unemployment, and poverty.

In the 1950s, severe drought returned to the Great Plains and southwestern United States, affecting half of the continental U.S. Low rainfall and high temperatures caused the production of crops in some areas to drop nearly 50 percent. Hay became too expensive for ranchers, and they had to feed their cattle prickly pear cactus and molasses to keep them alive. By the end of the five-year drought in 1957, 244 of Texas' 254 counties had been declared federal drought disaster areas.

Droughts continue to affect the U.S. Texas has been suffering from drought since 2010, with 2011 ranking as the driest year in the state's history. A September 2012 National Geographic magazine article called Texas "The New Dust Bowl." By 2013, 99 percent of the state was dealing with drought.

Australia is also a frequent victim of drought. The last decade has been especially severe, earning it the name The Big Dry or The Millennium Drought. Much of the country was placed under water restrictions, wildfires spread in the dry weather, and the water level in some dams fell to 25 percent. In 2007, 65 percent of viable land in Australia was declared to be in a drought. The drought was officially declared over in April 2012.

Droughts that occur in the developing world can cause even greater devastation. The Sahel region in Africa, which includes eight countries, endured a series of droughts in the 1970s and 1980s where annual rainfall dropped by about 40 percent. In the early 1970s, more than 100,000 people died and millions of people were forced to migrate. Conditions continue to be critical in the area due to drought, overpopulation, failing crops, and high food prices. Drought emergencies for the region have been declared four times since 2000.

The Horn of Africa, which includes the countries of Ethiopia, Somalia, Eritrea, and Djibouti, is particularly vulnerable to droughts. Because almost 80 percent of the population is rural and depends on agriculture for food and income, famine often accompanies drought.

Struggles for the region's limited, remaining resources can lead to conflict and war. In 1984 and 1985, the Horn of Africa suffered one of the worst droughts of the 20th century. The U.N. estimates that in Ethiopia alone, 1 million people died, 1.5 million livestock died, and 8.7 million people were affected by the drought-including being hospitalized, forced to migrate, or forced to change professions. In Sudan, 1 million people died, at least 7 million livestock died, and 7.8 million people were affected.

The cycle of drought-famine-conflict has persisted in the region, with drought conditions returning every few years since 2000. In 2006, drought affected 11 million people across the Horn of Africa, and the resulting crisis killed between 50,000 and 100,000 people and affected more than 13 million.
Forecasting and Measuring Drought
Even though scientists are unable to predict how long a drought will last or how severe it will be, early warning systems and monitoring tools can minimize some of drought's damaging impacts. There are a number of tools used to monitor drought across the U.S. Due to the limitations of each system, data from different sources are often compiled to create a more comprehensive forecast.

The Palmer Drought Severity Index (PDSI), developed in 1965 by the National Weather Service, is the most commonly used drought monitor. It is a complex measurement system and an effective way to forecast long-term drought. Its limitations are that it does not provide early warnings for drought and is not as accurate for use in mountainous areas because it does not account for snow (only rain) as precipitation. The PDSI is often used by the U.S. Department of Agriculture to determine when to begin providing drought relief.

The U.S. Drought Monitor, started in 1999, is a joint effort between three U.S. government agencies-the Department of Agriculture, the Department of Commerce, and the National Oceanic and Atmospheric Administration (NOAA). The Monitor synthesizes data from academic and federal scientists into a weekly map indicating levels of dryness around the country. It is designed to be a blend of science and art that can be used as a general summary of drought conditions around the country. It is not meant to be used as a drought predictor or for detailed information about specific areas.

The Famine Early Warning System Network (FEWS NET) monitors satellite data of crops and rainfall across Africa and some parts of Central America, the Middle East, and Central Asia. Analysis of the data allows for early intervention to try to prevent drought-induced famine.
Preparing for Drought
People and governments need to adopt new practices and policies to prepare as much as possible for inevitable future droughts. Emergency spending once a crisis has begun is less effective than money spent in preparation. The Federal Emergency Management Agency (FEMA) estimates that every $1 spent in planning for a natural hazard will save $4 in the long term.

Many areas are extremely vulnerable to drought as people continue to be dependent on a steady supply of water. The U.S. Department of Agriculture recommends a series of conservation practices to help farmers prepare for drought. Some preventative measures include installing an efficient irrigation system that reduces the amount of water lost to evaporation, storing water in ditches along fields, regularly monitoring soil moisture, planting crops that are more drought-resistant, and rotating crops to allow water in the soil to increase.

In urban areas, many cities are promoting water conservation by addressing water usage habits. Some enforce water restrictions, such as limiting days when lawns and plants can be watered, and offering free high-efficiency toilets and kitchen faucets.

Some drought-ravaged cities are taking even more extreme measures to prepare for future droughts. In Australia, the city of Perth is planning for a massive wastewater-recycling program that will eventually provide up to a quarter of the city's water demands by 2060. Perth has been dealing with a decline in rainfall since the mid 1970s. The city, which is on the edge of a huge desert, is also struggling with its history of over-consumption of water. Water-hungry traditions such as planting large, lush lawns and parks will need to be addressed through conservation measures.

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Drought is an extended period of unusually dry weather when there is not enough rain.

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-A hurricane is a tropical storm that has sustained winds of 74 mph or greater and is referred to as a hurricane.

-A storm's eye typically measures 20 to 30 miles in width and can reach distances of 400 miles.

-Storm surge and powerful waves have the potential to destroy houses, cause road and bridge damage along the coast, eroding sand dunes, and cause fatalities

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Hurricanes are dangerous and can cause major damage from storm surge, wind damage, rip currents and flooding. They can happen along any U.S. coast or in any territory in the Atlantic or Pacific oceans. Storm surge historically is the leading cause of hurricane-related deaths in the United States.

Prepare for Hurricanes
Know Your Hurricane Risk Make an Emergency Plan Know your Evacuation Zone Recognize Warnings and Alerts Those with Disabilities Review Important Documents Strengthen your Home Get Tech Ready Help your Neighborhood Gather Supplies
Stay Safe During a Hurricane

Returning Home After a Hurricane

Additional Resources
Videos Graphics and Social Media More Information

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Learn how to prepare for a hurricane, stay safe during a hurricane, and what to do when returning home from a hurricane. Hurricanes are dangerous and can cause major damage because of storm surge, wind damage, and flooding. They can happen along any U.S. coast or in any territory in the Atlantic or Pacific oceans.

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-An earthquake is a sudden slip on a fault that causes the earth to tremble. The sides of the fault are forced together by stresses in the outer layer of the earth. When pressure builds up and the rocks suddenly slip, waves of energy are released.

-Around 81 percent of the strongest earthquakes on Earth occur along the circum-Pacific seismic belt, which is the largest earthquake belt in the world. It carries the nickname "Ring of Fire."

-If there is an earthquake cover your head, neck, and, if you can, your entire body, underneath a solid desk or table

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Why is an earthquake dangerous?

What are earthquake waves?

How is earthquake magnitude measured?

Where do earthquakes occur?

earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth's rocks. Seismic waves are produced when some form of energy stored in Earth's crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and "slip." Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth's crust. ( See the table of major earthquakes.)

Little was understood about earthquakes until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur.

About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. Very great earthquakes occur on average about once per year. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property.

The nature of earthquakes

Causes of earthquakes

Earth's major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. This has long been apparent from early catalogs of felt earthquakes and is even more readily discernible in modern seismicity maps, which show instrumentally determined epicentres. The most important earthquake belt is the Circum-Pacific Belt, which affects many populated coastal regions around the Pacific Ocean-for example, those of New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America. It is estimated that 80 percent of the energy presently released in earthquakes comes from those whose epicentres are in this belt. The seismic activity is by no means uniform throughout the belt, and there are a number of branches at various points. Because at many places the Circum-Pacific Belt is associated with volcanic activity, it has been popularly dubbed the "Pacific Ring of Fire."

A second belt, known as the Alpide Belt, passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies. The energy released in earthquakes from this belt is about 15 percent of the world total. There also are striking connected belts of seismic activity, mainly along oceanic ridges-including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean-and along the rift valleys of East Africa. This global seismicity distribution is best understood in terms of its plate tectonic setting.

Natural forces

Earthquakes are caused by the sudden release of energy within some limited region of the rocks of the Earth. The energy can be released by elastic strain, gravity, chemical reactions, or even the motion of massive bodies. Of all these the release of elastic strain is the most important cause, because this form of energy is the only kind that can be stored in sufficient quantity in the Earth to produce major disturbances. Earthquakes associated with this type of energy release are called tectonic earthquakes.

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Earthquake | Definition, Causes, Effects, & Facts

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Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth's rocks. Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. Learn more about the causes and effects of earthquakes in this article.

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-Snowfall storms produced a total of 15–38 centimeters (6–15 inches) in the Washington, D.C. area, and over 60 centimeters (24 inches) in neighboring Maryland, on top of the existing snow that was already on the ground.

-Known for being the most famous blizzard ever recorded, the Great Blizzard of 1888 struck between March 11 and March 14. This blizzard covered the entire region of Maine and Maryland in the United States, 55 inches.

-About 400 people died during the Great Blizzard of 1888.

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A blizzard is a dangerous weather event, bringing with it frigid temperatures, howling winds, and decreased visibility. Blizzards can be deadly, which is why it is important that meteorologists accurately measure atmospheric conditions and provide the public with timely warnings.

Subjects

Earth Science, Meteorology, Geography

Image

Car Stuck in Blizzard

Blizzards are not just bad snowstorms; they are specific types of snowstorms that consist of large amounts of snow lasting more than three hours, winds of at least 56 kilometers (35 miles) per hour, and visibility of less than 0.4 kilometers (0.25 miles)

Photograph by Rob Carr/Getty Images

The United States National Weather Service's winter weather advisory, watch, or warning system helps meteorologists determine whether atmospheric conditions should be classified as typical winter weather, a snowstorm, or a severe blizzard.

In order for meteorologists to classify a winter storm as a snowstorm, the air temperature high in the atmosphere and near the ground must be below 0°C (32°F). There also needs to be enough water vapor in the air to form snowflake crystals. While snowstorms do not typically last very long (less than a few hours), they can bring high snow accumulations, which can be hazardous.

For a snowstorm to be considered a blizzard, it must also meet specific, though more severe, conditions. To be categorized as a blizzard, the storm must last for at least three hours and produce a large amount of falling snow. Blizzards also have winds measuring over 56 kilometers (35 miles) per hour. These winds cause a large volume of snow to blow around in the air and near the ground, decreasing visibility. Meteorologists will declare blizzard conditions if the snow limits visibility to the point where it is difficult to see an object more than 0.4 kilometers (0.25 miles) away.

The most famous recorded blizzard is the Great Blizzard of 1888, which occurred between March 11 and 14. This blizzard extended all the way from the American states of Maine to Maryland. About 139 centimeters (55 inches) of snow brought newly growing cities in New England and New York City, New York, to a screeching halt. During this time, about one in four people living in the United States lived in one of the New England states, which made the blizzard all the more devastating. About 400 people died during this storm.

A more recent example of catastrophic blizzards would be when two 2010 blizzards formed back-to-back in the Washington, D.C., area on February 4-7 and February 9-11. The first storm brought snow to Pennsylvania and extended through North Carolina. The total amount of snow from this first storm varied from 25 to 76 centimeters (10 to 30 inches). The storm impacted over 45 million people in the northeastern part of the United States. Then, before the states could completely dig out from the storm, a second blizzard hit. The Washington, D.C., area reported snowfall totals of 15-38 centimeters (6-15 inches) from the new storm, and nearby Maryland reported over 60 centimeters (24 inches) on top of the snow that was already on the ground.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Director

Tyson Brown, National Geographic Society

Author

National Geographic Society

Production Managers

Gina Borgia, National Geographic Society

Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society, National Geographic Society

Margot Willis, National Geographic Society

other

Last Updated

October 19, 2023

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A blizzard is a dangerous weather event, bringing with it frigid temperatures, howling winds, and decreased visibility. Blizzards can be deadly, which is why it is important that meteorologists accurately measure atmospheric conditions and provide the public with timely warnings.