Weather (2024)

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 gen

erally

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, sev

eral

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.

Temp

erature

is measured with a thermometer and refers to how hot or cold the

atmosphere

is.

Meteorologists

report

temp

erature

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

temp

erature

using the Celsius scale.

Temp

erature

is a relative measurement. An afternoon at 70 degrees Fahrenheit, for example, would seem cool after sev

eral

days of 95 degrees Fahrenheit, but it would seem warm after

temp

eratures

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

temp

eratures

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. Av

erage

atmospheric pressure

at sea level is about one

atmosphere

(about 1,013

millibars

, or 29.9 inches). An av

erage

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

temp

erature

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

temp

erature

. 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

temp

erature

,

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

temp

erature

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

temp

erature

, 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 gen

eral

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

temp

erature

, 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

temp

erature

,

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 gen

eral

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

temp

erature

,

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 Canav

eral

, Florida.

TIROS-1

was mostly an

orbiting

television cam

era

, 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-gen

erated

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

temp

erature

, 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 vuln

erable

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

temp

eratures

, 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

.

National Geographic Kids: Weather 101 Videos

The Weather Channel: Current Weather Maps

Citizen Weather Observer ProgramThe Weather Channel: When Weather Changed HistoryWorld Meteorological Organization: World Weather Information ServiceNOAA: Education—Weather Coloring BooksArizona State University and the World Meteorological Organization: Climate Extremes ArchiveNOAA: National Weather Service—U.S. Daily Weather Forecast Map