Encyclopedic Entry

Thunderbolts and lightning, very very frightening.

Photograph by Richard Hardman, My Shot

Make Some Noise—Thunder
Do you know what makes the noise you hear with thunder? Lightning. Lightning is very, very hot because it has so much energy. When lightning strikes, the molecules in the air expand very rapidly. The heated air creates a huge sound wave, which is thunder.

To make your own thunder, you will need a paper lunch bag. Blow into the bag until it's filled with air. Quickly twist the top closed with one hand, and with the other hand, hit the bag. When you hit the bag, the air pressure increases very quickly. In fact, the air pressure breaks the bag. The air rushes outside the bag, creating a sound wave. When it reaches your ear, you hear a loud boom.

Red Sky in Morning . . .
Before meteorologists were able to accurately predict weather, people noticed different characteristics associated with different weather patterns. They used these characteristics to predict the weather.

Red sky in morning, sailors take warning/Red sky at night, sailors delight is a popular rhyme that attempts to predict the weather. In fact, its pretty accurate. Weather patterns usually move from west to east. Red sky in the morning (in the east, where the sun rises) indicates that the sun is reflecting off rainclouds, meaning the day will likely have rain (difficult weather for sailing.) Red sky at night (in the west, where the sun sets) indicates clear, calm weather (perfect weather for sailing).

When windows won't open and salt clogs the shaker/Weather will favor the umbrella maker is another weather-predicting rhyme. When the air is humid, or full of water vapor, wood swells. It absorbs the water in the air. Salt also absorbs water in the surrounding air, forming large lumps. Wood-framed windows and salt in shakers become difficult to use. As the humid air reaches its saturation point, rain (and umbrellas) are likely to be seen.

One of the first things you probably do every morning is look out the window to see what the weather is like. Looking outside and listening to the day’s forecast helps you decide what clothes you will wear and maybe even what you will do throughout the day. If you don’t have school and the weather looks sunny, you might visit the zoo or go on a picnic. A rainy day might make you think about visiting a museum or staying home to read.

The weather affects us in many ways. Day-to-day changes in weather can influence how we feel and the way we look at the world. Severe weather, such as tornadoes, hurricanes, and blizzards, can disrupt many people’s lives because of the destruction they cause.

The term “weather” refers to the temporary conditions of the atmosphere, the layer of air that surrounds the Earth. We usually think of weather in terms of the state of the atmosphere in our own part of the world. But weather works like dropping a pebble in water—the ripples eventually affect water far away from where the pebble was dropped. The same happens with weather around the globe. Weather in your region will eventually affect the weather hundreds or thousands of kilometers away. For example, a snowstorm around Winnipeg, Manitoba, Canada, might eventually reach Chicago, Illinois, as it moves southeast through the U.S.

Weather doesn’t just stay in one place. It moves, and changes from hour to hour or day to day. Over many years, certain conditions become familiar weather in an area. The average weather in a specific region, as well as its variations and extremes over many years, is called climate. For example, the city of Las Vegas in the U.S. state of Nevada is generally dry and hot. Honolulu, the capital of the U.S. state of Hawaii, is also hot, but much more humid and rainy.

Climate changes, just like weather. However, climate change can take hundreds or even thousands of years. Today, the Sahara Desert in northern Africa is the largest desert in the world. However, several thousand years ago, the climate in the Sahara was quite different. This “Green Sahara” experienced frequent rainy weather.  

What Makes Weather

There are six main components, or parts, of weather. They are temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness. Together, these components describe the weather at any given time. These changing components, along with the knowledge of atmospheric processes, help meteorologists—scientists who study weather—forecast what the weather will be in the near future.

Temperature is measured with a thermometer and refers to how hot or cold the atmosphere is. Meteorologists report temperature two ways: in Celsius (C) and Fahrenheit (F). The United States uses the Fahrenheit system; in other parts of the world, Celsius is used. Almost all scientists measure temperature using the Celsius scale.

Temperature is a relative measurement. An afternoon at 70 degrees Fahrenheit, for example, would seem cool after several days of 95 degrees Fahrenheit, but it would seem warm after temperatures around 32 degrees Fahrenheit. The coldest weather usually happens near the poles, while the warmest weather usually happens near the Equator.

Atmospheric pressure is the weight of the atmosphere overhead. Changes in atmospheric pressure signal shifts in the weather. A high-pressure system usually brings cool temperatures and clear skies. A low-pressure system can bring warmer weather, storms, and rain.

Meteorologists express atmospheric pressure in a unit of measurement called an atmosphere. Atmospheres are measured in millibars or inches of mercury. Average atmospheric pressure at sea level is about one atmosphere (about 1,013 millibars, or 29.9 inches). An average low-pressure system, or cyclone, measures about 995 millibars (29.4 inches). A typical high-pressure system, or anticyclone, usually reaches 1,030 millibars (30.4 inches). The word “cyclone” refers to air that rotates in a circle, like a wheel.

Atmospheric pressure changes with <altitude. The atmospheric pressure is much lower at high altitudes. The air pressure on top of Mount Kilimanjaro, Tanzania—which is 5,895 meters (19,344 feet) tall—is 40 percent of the air pressure at sea level. The weather is much colder. The weather at the base of Mount Kilimanjaro is tropical, but the top of the mountain has ice and snow.

Wind is the movement of air. Wind forms because of differences in temperature and atmospheric pressure between nearby regions. Winds tend to blow from areas of high pressure, where it’s colder, to areas of low pressure, where it’s warmer.

In the upper atmosphere, strong, fast winds called jet streams occur at altitudes of 8 to 15 kilometers (5 to 9 miles) above the Earth. They usually blow from about 129 to 225 kilometers per hour (80 to 140 miles per hour), but they can reach more than 443 kilometers per hour (275 miles per hour). These upper-atmosphere winds help push weather systems around the globe.

Wind can be influenced by human activity. Chicago, Illinois, is nicknamed the “Windy City.” Chicago was built on the shores of Lake Michigan, where the weather is naturally more breezy than inland areas. After the Great Chicago Fire of 1871 destroyed the city, city planners rebuilt it using a grid system. This created wind tunnels. Winds are forced into narrow channels, picking up speed and strength. The Windy City is a result of natural and manmade winds.

Humidity refers to the amount of water vapor in the air. Water vapor is a gas in the atmosphere that helps make clouds, rain, or snow. Humidity is usually expressed as relative humidity, or the percentage of the maximum amount of water air can hold at a given temperature. Cool air holds less water than warm air. At a relative humidity of 100 percent, air is said to be saturated, meaning the air cannot hold any more water vapor. Excess water vapor will fall as precipitation. Clouds and precipitation occur when air cools below its saturation point. This usually happens when warm, humid air cools as it rises.

The most humid places on Earth are islands near the Equator. Singapore, for instance, is humid year-round. The warm air is continually saturated with water from the Indian Ocean.

Clouds come in a variety of forms. Not all of them produce precipitation. Wispy cirrus clouds, for example, usually signal mild weather. Other kinds of clouds can bring rain or snow. A blanketlike cover of nimbostratus clouds produces steady, extended precipitation. Enormous cumulonimbus clouds, or thunderheads, release heavy downpours. Cumulonimbus clouds can produce thunderstorms and tornadoes as well.

Clouds can affect the amount of sunlight reaching the Earth’s surface. Cloudy days are cooler than clear ones because clouds prevent more of the sun’s radiation from reaching the Earth’s surface. The opposite is true at night—then, clouds act as a blanket, keeping the Earth warm.

Weather Systems

Cloud patterns indicate the presence of weather systems, which produce most of the weather we are familiar with: rain, heat waves, cold snaps, humidity, and cloudiness. Weather systems are simply the movement of warm and cold air across the globe. These movements are known as low-pressure systems and high-pressure systems.

High-pressure systems are rotating masses of cool, dry air. High-pressure systems keep moisture from rising into the atmosphere and forming clouds. Therefore, they are usually associated with clear skies. On the other hand, low-pressure systems are rotating masses of warm, moist air. They usually bring storms and high winds.

High-pressure and low-pressure systems continually pass through the mid-latitudes, or areas of the Earth about halfway between the Equator and the poles, so weather there is constantly changing.

A weather map is filled with symbols indicating different types of weather systems. Spirals, for instance, are cyclones or hurricanes, and thick lines are fronts. Cyclones have a spiral shape because they are composed of air that swirls in a circular pattern.

A front is a narrow zone across which temperature, humidity, and wind change abruptly. A front exists at the boundary between two air masses. An air mass is a large volume of air that is mostly the same temperature and has mostly the same humidity.

When a warm air mass moves into the place of a cold air mass, the boundary between them is called a warm front. On a weather map, a warm front is shown as a red band with half-circles pointing in the direction the air is moving.

When a cold air mass takes the place of a warm air mass, the boundary between them is called a cold front. On a weather map, a cold front is shown as a blue band with triangles pointing in the direction the air is moving.

A stationary front develops when warm air and cold air meet and the boundary between the two does not move. On a weather map, a stationary front is shown as alternating red half-circles and blue triangles, pointing in opposite directions.

When a cold front overtakes a warm front, the new front is called an occluded front. On a weather map, an occluded front is shown as a purple band with half-circles and triangles pointing in the direction the air is moving. Cold fronts are able to overtake warm fronts because they move faster.

History of Weather Forecasting

Meteorology is the science of forecasting weather. Weather forecasting has been important to civilizations for thousands of years. Agriculture relies on accurate weather forecasting: when to plant, when to irrigate, when to harvest. Ancient cultures—from the Aztecs of Mesoamerica to the Egyptians in Africa and Indians in Asia—became expert astronomers and predictors of seasonal weather patterns.

In all of these cultures, weather forecasting became associated with religion and spirituality. Weather such as rain, drought, wind, and cloudiness were associated with a deity, or god. These deities were worshipped in order to ensure good weather. Rain gods and goddesses were particularly important, because rain influenced agriculture and construction projects. Tlaloc (Aztec), Set (Egyptian), and Indra (India), as well as Thor (Norse), Zeus (Greek), and Shango (Yoruba), are only some gods associated with rain, thunder, and lightning.

Developments in the 17th and 18th centuries made weather forecasting more accurate. The 17th century saw the invention of the thermometer, which measures temperature, and the barometer, which measures air pressure. In the 18th century, Sir Isaac Newton was able to explain the complex physics of gravity, motion, and thermodynamics. These principles guided the science of meteorology into the modern age. Scientists were able to predict the impact of high-pressure systems and low-pressure systems, as well as such weather events as storm surges, floods, and tornadoes.

Since the late 1930s, one of the main tools for observing general conditions of the atmosphere has been the radiosonde balloon, which sends information needed for forecasting back to Earth. Twice each day, radiosondes are released into the atmosphere from about a thousand locations around the world. The U.S. National Weather Service sends up radiosondes from more than 90 weather stations across the country.

A weather station is simply a facility with tools and technology used to forecast the weather. Different types of thermometers, barometers, and anemometers, which measure wind speed, are found at weather stations. Weather stations may also have computer equipment that allows meteorologists to create detailed maps of weather patterns, and technology that allows them to launch weather balloons.

Many weather stations are part of networks. These networks allow meteorologists from different regions and countries to share information on weather patterns and predictions. In the United States, the Citizen Weather Observer Program depends on amateur meteorologists with homemade weather stations and internet connections to provide forecasts across the United States.

The Aircraft Meteorological Data Relay (AMDAR) also assists in gathering weather data directly from the atmosphere. AMDAR uses commercial aircraft to transmit information about the atmosphere as the planes fly through it.

Weather balloons and AMDAR instruments gather information about temperature, pressure, humidity, and wind from very high levels in the atmosphere. Meteorologists input the data to computers and use it to map atmospheric winds and jet streams. They often combine this with data about temperature, humidity, and wind recorded at ground level. These complex weather maps using geographic information system (GIS) technology can calculate how weather systems are moving and predict how they might change.

This type of forecasting is called synoptic forecasting. Synoptic forecasting is getting a general idea of the weather over a large area. It relies on the fact that in certain atmospheric conditions, particular weather conditions are usually produced. For example, meteorologists know that a low-pressure system over the U.S. state of Arizona in winter will bring warm, moist air from the Gulf of Mexico toward Colorado. The high-pressure weather system of the Rocky Mountains drains the water vapor out of the air, resulting in rain. Meteorologists know that heavy snow may result when that warm air mass heads toward Colorado. Businesses, such as ski resorts, rely on such information. Transportation networks also rely on synoptic forecasting.

If meteorologists knew more about how the atmosphere functions, they would be able to make more accurate forecasts from day to day or even from week to week. Making such forecasts, however, would require knowing the temperature, atmospheric pressure, wind speed and direction, humidity, precipitation, and cloudiness at every point on the Earth.

It is impossible for meteorologists to know all this, but they do have some tools that help them accurately forecast weather for a day or two in advance. But because the atmosphere is constantly changing, detailed forecasts for more than a week or two will never be possible. Weather is just too unpredictable.

Weather Satellites

A new era in weather forecasting began on April 1, 1960, when the first weather satellite, TIROS-1, went into orbit. TIROS-1, which stands for Television Infrared Observation Satellite, was launched by NASA from Cape Canaveral, Florida. TIROS-1 was mostly an orbiting television camera, recording and sending back images. It gave meteorologists their first detailed look at clouds from above. With images from TIROS-1, they could track hurricanes and other cyclones moving across the globe.

Since then, meteorologists have depended on weather satellites for the most up-to-date and reliable information on weather patterns. Some satellites have geostationary orbits, meaning they stay in the same spot and move at the speed the Earth rotates. Geostationary satellites track the weather over one region. Other satellites orbit the Earth every 12 hours. These satellites can trace weather patterns, such as hurricanes, over the entire part of the globe they orbit.

Weather satellites can give more than just information about clouds and wind speeds. Satellites can see fires, volcanoes, city lights, dust storms, the effects of pollution, boundaries of ocean currents, and other environmental information.

In 2010, the volcano Eyjafjallajokull, in Iceland, erupted. It sent millions of tons of gases and ash into the atmosphere. Weather satellites in orbit above Iceland tracked the ash cloud as it moved across western Europe. Meteorologists were able to warn airlines about the toxic cloud, which darkened the sky and would have made flying dangerous. Hundreds of flights were canceled.

Radiosonde instruments are still more accurate than weather satellites. Satellites, however, can cover a larger area of the Earth. They also cover areas where there are no weather stations, like over the ocean. Satellite data have helped weather forecasts become more accurate, especially in the remote areas of the world that don’t have other ways to get information about the weather.


Radar is another major tool of weather observation and forecasting. It is used primarily to observe clouds and rain locally. One type of radar, called Doppler radar, is used at weather stations throughout the world. Doppler radar measures changes in wind speed and direction. It provides information within a radius of about 230 kilometers (143 miles). Conventional radar can only show existing clouds and precipitation. With Doppler radar, meteorologists are able to forecast when and where severe thunderstorms and tornadoes are developing.

Doppler radar has made air travel safer. It lets air traffic controllers detect severe local conditions, such as microbursts. Microbursts are powerful winds that originate in thunderstorms. They are among the most dangerous weather phenomena a pilot can encounter. If an aircraft attempts to land or take off through a microburst, the suddenly changing wind conditions can cause the craft to lose lift and crash. In the United States alone, airline crashes because of microbursts have caused more than 600 deaths since 1964.

Radar allowed meteorologists in the U.S. to track Hurricane Katrina in 2005, and predict the power of the storm with great accuracy. The National Weather Service and the National Hurricane Center created sophisticated GIS maps using radar, satellite, and balloon data. They were able to predict the site of the storm’s landing, and the strength of the storm over a period of days. A full day before the storm made landfall near Buras, Louisiana, the National Hurricane Center released a public warning: “Some levees in greater New Orleans area could be overtopped.” The National Weather Service warned that the area around New Orleans, Louisiana, “would be uninhabitable for weeks, if not longer. Human suffering incredible by modern standards.”

In fact, both of those forecasts were true. Levees in New Orleans were overtopped by the Mississippi River. Hundreds of homes, schools, hospitals, and businesses were destroyed. Many areas between New Orleans and Biloxi, Mississippi, were uninhabitable for weeks or months, and rebuilding efforts took years. More than a thousand people died.

Making a Weather Forecast

To produce a weather forecast for a particular area, meteorologists use a computer-generated forecast as a guide. They combine it with additional data from current satellite and radar images. They also rely on their own knowledge of weather processes.

If you follow the weather closely, you, too, can make a reasonable forecast. Radar and satellite images showing precipitation and cloud cover are now common on television, online, and in the daily newspaper.

In addition, you will probably see weather maps showing high- and low-pressure systems and fronts. In addition to bars representing different fronts, weather maps usually show isotherms and isobars. Isotherms are lines connecting areas of the same temperature, and isobars connect regions of the same atmospheric pressure. Weather maps also include information about cloudiness, precipitation, and wind speed and direction.

More Accurate Forecasts

Although weather forecasts have become more reliable, there is still a need for greater accuracy. Better forecasts could save industries across the world many billions of dollars each year. Farmers and engineers, in particular, would benefit.

Better frost predictions, for example, could save U.S. citrus growers millions of dollars each year. Citrus fruits such as oranges are very vulnerable to frost—they die in cold, wet weather. With more accurate frost forecasts, citrus farmers could plant when they know the new, tender seedlings wouldn’t be killed by frost. More accurate rain forecasts would enable farmers to plan timely irrigation schedules and avoid floods.

Imperfect weather forecasts cause construction companies to lose both time and money. A construction foreman might call his crew in to work only to have it rain, when the crew can’t work. An unexpected cold spell could ruin a freshly poured concrete foundation.

Outdoor activities, such as concerts or sporting events, could be planned with greater accuracy. Sports teams and musicians would not have to reschedule, and fans would not be inconvenienced.

Power companies would also benefit from accurate forecasts. They adjust their systems when they expect extreme temperatures, because people will use their furnaces and air conditioning more on these days. If the forecast predicts a hot, humid day and it turns out to be mild, the power company loses money. The extra electricity or gas it bought doesn’t get used.

Small businesses, too, would benefit from a better forecast. An ice cream store owner, for example, could save her advertising funds for some time in the future if she knew the coming weekend was going to be cool and rainy.

Responding to such needs, meteorologists are working to develop new tools and new methods that will improve their ability to forecast the weather.


Term Part of Speech Definition Encyclopedic Entry






the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

Encyclopedic Entry: agriculture

air conditioning


system that cools the air.



vehicle able to travel and operate above the ground.

Aircraft Meteorological Data Relay (AMDAR)


international program that collects weather information using instruments on commercial aircraft.



system or business that provides air transportation.

air mass


a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

Encyclopedic Entry: air mass

air traffic controller


person who monitors the position, speed, and direction of different aircraft to ensure safe and efficient air travel.



the distance above sea level.

Encyclopedic Entry: altitude



person who studies and works at an activity or interest without financial benefit or being formally trained in it.



a device that measures wind speed.

Encyclopedic Entry: anemometer



large weather system where air spins around a center of high pressure. Anticyclones spin clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.



person who studies space and the universe beyond Earth's atmosphere.



(atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.



layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

atmospheric pressure


force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.

Encyclopedic Entry: atmospheric pressure



an instrument that measures atmospheric pressure.

Encyclopedic Entry: barometer



storm with high winds, intense cold, heavy snow, and little rain.



line separating geographical areas.

Encyclopedic Entry: boundary



to reach a conclusion by mathematical or logical methods.



city where a region's government is located.

Encyclopedic Entry: capital

Celsius scale


scale for measuring surface temperature, used by most of the world, in which the boiling point of water is 100 degrees.



thin, high-altitude cloud.

Citizen Weather Observer Program


network that allows amateur meteorologists with computerized weather stations to contribute to forecasts across the United States.



type of fruit tree, including lemon and orange.

city planner


person who plans the physical design and zoning of an urban center.



complex way of life that developed as humans began to develop urban settlements.

Encyclopedic Entry: civilization



all weather conditions for a given location over a period of time.

Encyclopedic Entry: climate



visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

Encyclopedic Entry: cloud

cold snap


short period of intense, cold weather.



having to do with the buying and selling of goods and services.






hard building material made from mixing cement with rock and water.



arrangement of different parts.



low-level cloud that produces rain, thunder, and lightning. Also called a thunderhead.



steady, predictable flow of fluid within a larger body of that fluid.

Encyclopedic Entry: current



weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.


Plural Noun

(singular: datum) information collected during a scientific study.



very holy or spiritual being.



area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

Encyclopedic Entry: desert






distracting or preventing an orderly or planned flow of events.

Doppler radar


weather tracking system that reads the direction and speed of moving objects, such as drops of precipitation.



heavy rain.



period of greatly reduced precipitation.

Encyclopedic Entry: drought

dust storm


weather pattern of wind blowing dust over large regions of land.



set of physical phenomena associated with the presence and flow of electric charge.



person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).



imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Encyclopedic Entry: equator



time period.



extra or surplus.



volcano that erupted in Iceland in March 2010.



a building or room that serves a specific function.

Fahrenheit scale


scale for measuring surface temperature used by Belize, Liberia, Myanmar, and the United States.



person who cultivates land and raises crops.



overflow of a body of water onto land.

Encyclopedic Entry: flood



to predict, especially the weather.



person who oversees a project, especially construction.



structure on which a building is constructed.



boundary between air masses of different temperatures and humidities.

Encyclopedic Entry: front



thin coat of ice covering objects when the dew point is below freezing.

Encyclopedic Entry: frost



device used for heating by burning a fuel, such as wood or coal.

geographic information system (GIS)


any system for capturing, storing, checking, and displaying data related to positions on the Earth's surface.

Encyclopedic Entry: GIS (geographic information system)

geostationary orbit


orbit around the Earth directly above the Equator.



physical force by which objects attract, or pull toward, each other.

Great Chicago Fire


(1871) urban disaster that killed hundreds and destroyed almost all of downtown Chicago, Illinois.

Green Sahara


(7000-3000 BCE) moist, temperate climate of the present-day Sahara Desert during the Neolithic Subpluvial period. Also called the Wet Sahara.



horizontal and vertical lines used to locate objects in relation to one another on a map.



the gathering and collection of crops, including both plants and animals.

heat wave


period of unusually hot weather.

high-pressure system


weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.



amount of water vapor in the air.

Encyclopedic Entry: humidity



tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.

Hurricane Katrina


2005 storm that was one of the deadliest in U.S. history.

inches of mercury


method of measuring atmospheric pressure, based on the pressure exerted on a 1-inch column of mercury at 0 degrees Celsius (32 degrees Fahrenheit). Abbreviated inHg.



to disturb or bother.



Hindu deity of rain and thunder.

Industrial Revolution


change in economic and social activities, beginning in the 18th century, brought by the replacement of hand tools with machinery and mass production.



vast, worldwide system of linked computers and computer networks.



watering land, usually for agriculture, by artificial means.

Encyclopedic Entry: irrigation

Isaac Newton


(1642-1727) English physicist, mathematician, and philosopher.



body of land surrounded by water.

Encyclopedic Entry: island



lines on a weather map connecting areas of equal atmospheric pressure.



line that unites points of equal temperature.

jet stream


winds speeding through the upper atmosphere.

Encyclopedic Entry: jet stream



bank of a river, raised either naturally or constructed by people.

Encyclopedic Entry: levee



sudden electrical discharge from clouds.

Encyclopedic Entry: lightning

low-pressure system


weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.



area extending from central Mexico through southern Nicaragua.



person who studies patterns and changes in Earth's atmosphere.



study of weather and atmosphere.

Encyclopedic Entry: meteorology



powerful winds that originate in rain clouds, hit the ground, and fan out.



geographic regions between the Equator and the poles.



(mbar) unit of pressure equal to .001 bar of atmospheric pressure.



space where valuable works of art, history, or science are kept for public view.



(acronym for National Aeronautics and Space Administration) U.S. agency responsible for space research and systems.

National Hurricane Center


branch of the National Weather Service responsible for tracking and predicting tropical storms.

National Weather Service


branch of the National Oceanic and Atmospheric Association (NOAA) whose mission is to provide "weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy."



low-level cloud that produces continuous precipitation.

Norse mythology


stories, traditions, and beliefs of ancient Scandinavia.

occluded front


weather pattern in which a cold front overtakes a warm front. Occluded fronts are associated with the formation of cyclones.



path of one object around a more massive object.



to begin or start.


Plural Noun

(singular: phenomenon) any observable occurrence or feature.



study of the physical processes of the universe, especially the interaction of matter and energy.



person who steers a ship or aircraft.



to place in the earth or water for growth.



extreme north or south point of the Earth's axis.



introduction of harmful materials into the environment.

Encyclopedic Entry: pollution



all forms in which water falls to Earth from the atmosphere.

Encyclopedic Entry: precipitation



first or most important.



(RAdio Detection And Ranging) method of determining the presence and location of an object using radio waves.



energy, emitted as waves or particles, radiating outward from a source.



instrument attached to a balloon that measures temperature, humidity, pressure, and other aspects of the atmosphere.

relative humidity


ratio between the amount of water vapor in the air and the air's saturation point. Relative humidity is expressed as a percentage.



period of great development in science, art, and economy in Western Europe from the 14th to the 17th centuries.



to fill one substance with as much of another substance as it can take.

saturation point


temperature at which the air can hold no more water vapor and clouds or precipitation forms. Also called dew point.

sea level


base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

Encyclopedic Entry: sea level



young tree or other plant.



Egyptian god of deserts, storms, and darkness. Also called Seth.






a leader of the Yoruba gods, and god of thunder and lightning.




ski resort


facility where people can ski for recreation or sport.



knowledgeable or complex.



belief in supernatural powers.

stationary front


weather pattern that develops when warm air and cold air meet and the boundary between the two does not move.

storm surge


abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

Encyclopedic Entry: storm surge

synoptic forecasting


method of predicting weather patterns over a large area.



the science of using tools and complex machines to make human life easier or more profitable.



degree of hotness or coldness measured by a thermometer with a numerical scale.

Encyclopedic Entry: temperature



not lasting or permanent.



the study of the relationship between heat and mechanical energy, or work.



device that measures temperature.

Encyclopedic Entry: thermometer



Norse god of thunder.



to make a loud, deep noise.



low-level cloud that produces rain, thunder, and lightning. Also called cumulonimbus.



(1960) first weather satellite, launched by NASA.



Aztec god of rain.



a violently rotating column of air that forms at the bottom of a cloud and touches the ground.






movement of people or goods from one place to another.



existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.



place where no people make a permanent home.



unexpected or uncertain.



visible liquid suspended in the air, such as fog.

volcanic ash


fragments of lava less than 2 millimeters across.

Encyclopedic Entry: volcanic ash



an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

warm front


mass of warm air that replaces a mass of cold air.



state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Encyclopedic Entry: weather

weather balloon


hydrogen-filled balloon equipped with tools to measure temperature, humidity, pressure, and other aspects of the atmosphere.

weather map


representation of data on the condition of a specific area's atmosphere.

weather satellite


instrument that orbits the Earth to track weather and patterns in the atmosphere.

weather station


area with tools and equipment for measuring changes in the atmosphere.

weather system


movement of warm or cold air.



movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

wind tunnel


narrow passage or chamber where air flows.

Windy City


nickname of Chicago, Ill.



people and culture native to western Africa.



leader of the Greek gods, and god of thunder and lightning.


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.


Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt


Tim Gunther
Mary Crooks, National Geographic Society


Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne


Caryl-Sue, National Geographic Society


Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

User Permissions

For information on user permissions, please read our Terms of Service.

If you have questions about licensing content on this page, please contact natgeocreative@ngs.org for more information and to obtain a license.

If you have questions about how to cite anything on our website in your project or classroom presentation, please visit our FAQ page.


Some media assets (videos, photos, audio recordings and PDFs) can be downloaded and used outside the National Geographic website according to the Terms of Service. If a media asset is downloadable, a download button appears in the lower right hand corner (download) of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.