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.” 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.