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

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.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

accurate

Adjective

exact.

agriculture

Noun

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

Encyclopedic Entry: agriculture

air conditioning

Noun

system that cools the air.

aircraft

Noun

vehicle able to travel and operate above the ground.

Aircraft Meteorological Data Relay (AMDAR)

Noun

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

airline

Noun

system or business that provides air transportation.

air mass

Noun

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

Encyclopedic Entry: air mass

air traffic controller

Noun

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

altitude

Noun

the distance above sea level.

Encyclopedic Entry: altitude

amateur

Adjective

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

anemometer

Noun

a device that measures wind speed.

Encyclopedic Entry: anemometer

anticyclone

Noun

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.

astronomer

Noun

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

atmosphere

Noun

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

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

atmospheric pressure

Noun

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

Encyclopedic Entry: atmospheric pressure

barometer

Noun

an instrument that measures atmospheric pressure.

Encyclopedic Entry: barometer

blizzard

Noun

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

boundary

Noun

line separating geographical areas.

Encyclopedic Entry: boundary

calculate

Verb

to reach a conclusion by mathematical or logical methods.

capital

Noun

city where a region's government is located.

Encyclopedic Entry: capital

Celsius scale

Noun

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

cirrus

Noun

thin, high-altitude cloud.

Citizen Weather Observer Program

Noun

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

citrus

Noun

type of fruit tree, including lemon and orange.

city planner

Noun

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

civilization

Noun

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

Encyclopedic Entry: civilization

climate

Noun

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

Encyclopedic Entry: climate

cloud

Noun

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

Encyclopedic Entry: cloud

cold snap

Noun

short period of intense, cold weather.

commercial

Adjective

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

component

Noun

part.

concrete

Noun

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

construction

Noun

arrangement of different parts.

cumulonimbus

Noun

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

current

Noun

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

Encyclopedic Entry: current

cyclone

Noun

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.

data

Plural Noun

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

deity

Noun

very holy or spiritual being.

desert

Noun

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

Encyclopedic Entry: desert

destruction

Noun

ruin.

disruptive

Adjective

distracting or preventing an orderly or planned flow of events.

Doppler radar

Noun

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

downpour

Noun

heavy rain.

drought

Noun

period of greatly reduced precipitation.

Encyclopedic Entry: drought

dust storm

Noun

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

electricity

Noun

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

engineer

Noun

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

Equator

Noun

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

Encyclopedic Entry: equator

era

Noun

time period.

excess

Noun

extra or surplus.

Eyjafjallajokull

Noun

volcano that erupted in Iceland in March 2010.

facility

Noun

a building or room that serves a specific function.

Fahrenheit scale

Noun

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

farmer

Noun

person who cultivates land and raises crops.

flood

Noun

overflow of a body of water onto land.

Encyclopedic Entry: flood

forecast

Verb

to predict, especially the weather.

foreman

Noun

person who oversees a project, especially construction.

foundation

Noun

structure on which a building is constructed.

front

Noun

boundary between air masses of different temperatures and humidities.

Encyclopedic Entry: front

frost

Noun

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

Encyclopedic Entry: frost

furnace

Noun

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

geographic information system (GIS)

Noun

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

Noun

orbit around the Earth directly above the Equator.

gravity

Noun

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

Great Chicago Fire

Noun

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

Green Sahara

Noun

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

grid

Noun

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

harvest

Noun

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

heat wave

Noun

period of unusually hot weather.

high-pressure system

Noun

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

humidity

Noun

amount of water vapor in the air.

Encyclopedic Entry: humidity

hurricane

Noun

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

Noun

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

inches of mercury

Noun

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.

inconvenience

Verb

to disturb or bother.

Indra

Noun

Hindu deity of rain and thunder.

Industrial Revolution

Noun

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

Internet

Noun

vast, worldwide system of linked computers and computer networks.

irrigation

Noun

watering land, usually for agriculture, by artificial means.

Encyclopedic Entry: irrigation

Isaac Newton

Noun

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

island

Noun

body of land surrounded by water.

Encyclopedic Entry: island

isobar

Noun

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

isotherm

Noun

line that unites points of equal temperature.

jet stream

Noun

winds speeding through the upper atmosphere.

Encyclopedic Entry: jet stream

levee

Noun

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

Encyclopedic Entry: levee

lightning

Noun

sudden electrical discharge from clouds.

Encyclopedic Entry: lightning

low-pressure system

Noun

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

Mesoamerica

Noun

area extending from central Mexico through southern Nicaragua.

meteorologist

Noun

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

meteorology

Noun

study of weather and atmosphere.

Encyclopedic Entry: meteorology

microburst

Noun

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

midlatitudes

Noun

geographic regions between the Equator and the poles.

millibar

Noun

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

museum

Noun

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

NASA

Noun

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

National Hurricane Center

Noun

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

National Weather Service

Noun

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."

nimbostratus

Noun

low-level cloud that produces continuous precipitation.

Norse mythology

Noun

stories, traditions, and beliefs of ancient Scandinavia.

occluded front

Noun

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

orbit

Noun

path of one object around a more massive object.

originate

Verb

to begin or start.

phenomena

Plural Noun

(singular: phenomenon) any observable occurrence or feature.

physics

Noun

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

pilot

Noun

person who steers a ship or aircraft.

plant

Verb

to place in the earth or water for growth.

pole

Noun

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

pollution

Noun

introduction of harmful materials into the environment.

Encyclopedic Entry: pollution

precipitation

Noun

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

Encyclopedic Entry: precipitation

primarily

Adverb

first or most important.

radar

Noun

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

radiation

Noun

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

radiosonde

Noun

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

relative humidity

Noun

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

Renaissance

Noun

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

saturate

Verb

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

saturation point

Noun

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

sea level

Noun

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

Encyclopedic Entry: sea level

seedling

Noun

young tree or other plant.

Set

Noun

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

severe

Adjective

harsh.

Shango

Noun

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

shore

Noun

coast.

ski resort

Noun

facility where people can ski for recreation or sport.

sophisticated

Adjective

knowledgeable or complex.

spirituality

Noun

belief in supernatural powers.

stationary front

Noun

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

storm surge

Noun

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

Encyclopedic Entry: storm surge

synoptic forecasting

Noun

method of predicting weather patterns over a large area.

technology

Noun

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

temperature

Noun

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

Encyclopedic Entry: temperature

temporary

Adjective

not lasting or permanent.

thermodynamics

Noun

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

thermometer

Noun

device that measures temperature.

Encyclopedic Entry: thermometer

Thor

Noun

Norse god of thunder.

thunder

Verb

to make a loud, deep noise.

thunderhead

Noun

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

TIROS-1

Noun

(1960) first weather satellite, launched by NASA.

Tlaloc

Noun

Aztec god of rain.

tornado

Noun

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

toxic

Adjective

poisonous.

transportation

Noun

movement of people or goods from one place to another.

tropical

Adjective

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

uninhabited

Adjective

place where no people make a permanent home.

unpredictable

Adjective

unexpected or uncertain.

vapor

Noun

visible liquid suspended in the air, such as fog.

volcanic ash

Noun

fragments of lava less than 2 millimeters across.

Encyclopedic Entry: volcanic ash

volcano

Noun

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

warm front

Noun

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

weather

Noun

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

Encyclopedic Entry: weather

weather balloon

Noun

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

weather map

Noun

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

weather satellite

Noun

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

weather station

Noun

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

weather system

Noun

movement of warm or cold air.

wind

Noun

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

Noun

narrow passage or chamber where air flows.

Windy City

Noun

nickname of Chicago, Ill.

Yoruba

Noun

people and culture native to western Africa.

Zeus

Noun

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

Credits

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.

Writer

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

Illustrator

Tim Gunther
Mary Crooks, National Geographic Society

Editor

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

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

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