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Earth Science · Unit 7 · Grades 6–8

Weather and the Atmosphere

You live at the bottom of an ocean of air.

A thin blanket of air wraps around Earth. It lets us breathe, keeps us warm, and shields us from the Sun’s harshest rays. Heated by the Sun and stirred by Earth’s spin, that air never sits still. Its constant motion is what we call weather: sunshine, wind, clouds, rain, snow, and storms.

In this unit you will learn how the atmosphere is layered, why air pushes on you, how clouds and rain form, what fronts are, why wind blows, how to stay safe in severe weather, and how weather differs from climate. Then you can take a shift at the Forecast Desk.

Play Forecast Desk Take the self-guided quiz

Sunshine, fair-weather clouds, and a thunderstorm can all share one sky.

Layers of the atmosphere

The atmosphere is the layer of gases around Earth. Dry air is about 78 percent nitrogen and 21 percent oxygen, with a little argon (almost 1 percent) and carbon dioxide (about 0.04 percent). Real air also holds water vapor, from almost none up to about 4 percent. Water vapor is a small part of the air, but it makes clouds, rain, and snow.

Scientists divide the atmosphere into five layers, based on whether the temperature rises or falls as you go up.

The five layers of the atmosphere, with rough heights of the top of each layer. Not drawn to scale: the thermosphere and exosphere are far thicker than they look here.

TroposphereThe lowest layer, where we live. It reaches up about 12 kilometers (7.5 miles) on average, less near the poles and more near the equator. It holds about 99 percent of the atmosphere’s water vapor, so almost all weather happens here. It gets colder as you climb.
StratosphereUp to about 50 kilometers. It holds the ozone layer, which absorbs much of the Sun’s harmful ultraviolet light. Because ozone soaks up that energy, the stratosphere gets warmer as you go higher. The highest-flying jets reach its lower part.
MesosphereUp to about 80 kilometers. The top of the mesosphere is the coldest place in the atmosphere, about −85°C (−120°F). Most meteors burn up here.
ThermosphereUp to about 700 kilometers. The air is extremely thin, but the little there is gets very hot from the Sun. Auroras (the northern and southern lights) glow here, and the International Space Station orbits here.
ExosphereThe outermost layer, where the last traces of air fade into space. Many satellites orbit here.

Did you know?

There is no sharp edge where the air ends and space begins. Many scientists use the Kármán line, 100 kilometers (62 miles) up, as the boundary of space. That is about as far as an hour’s drive on a highway, but straight up.

Air pressure

Air has weight. Air pressure is the force of all the air above you pressing down. At sea level it averages about 1013 millibars (29.92 inches of mercury), or about 14.7 pounds on every square inch. You don’t feel squashed because the air inside your body pushes back out just as hard.

The higher you go, the less air there is above you, so the pressure drops. Half of all the air in the atmosphere is below about 5.6 kilometers (18,000 feet). Weather forecasters measure air pressure with a barometer.

Air is crowded near the ground and thin up high. In a high, air sinks; in a low, air rises.

High-pressure system (H)An area where air sinks toward the ground and spreads outward. Sinking air makes it hard for clouds to form, so highs usually bring clear skies and calm weather.
Low-pressure system (L)An area where air flows in and rises. Rising air cools and its water vapor can form clouds, so lows often bring clouds, rain, and storms.

Try it

Drink part of a bottle of water at the top of a tall hill or on a plane, then screw the cap on tight. Back down at the bottom, the bottle will look crushed. The higher air pressure at the bottom squeezed it.

Water in the air

You met the water cycle in earlier lessons. Here is how it makes weather. The Sun warms oceans, lakes, and wet ground, and water evaporates into invisible water vapor. Warm, moist air rises. As it rises, it expands and cools. When it cools enough, the vapor condenses onto tiny specks of dust, smoke, and sea salt, making droplets or ice crystals. Billions of them together make a cloud. When the droplets or crystals grow too heavy to stay up, they fall as precipitation: rain, snow, sleet, or hail.

Evaporation, condensation, wind, and rain move water from the ocean to the land and back again.

HumidityHow much water vapor is in the air. Relative humidity compares that to the most the air could hold at its temperature, as a percent.
Dew pointThe temperature the air would have to cool to for its relative humidity to reach 100 percent. When air cools to its dew point, water condenses: you see dew on grass, fog, or clouds. A high dew point means the air feels muggy.

Kinds of clouds

Clouds are named with a few Latin words. Cirrus means a curl of hair, cumulus means a heap, stratus means a layer, and nimbus means rain. Scientists group the ten main cloud types by how high they form.

Common clouds at high, middle, and low levels. A cumulonimbus can stretch through all three.

High cloudsCirrus, cirrocumulus, and cirrostratus. They are so high and cold that they are made of ice crystals. Thin, wispy cirrus often show up a day or so before a warm front arrives.
Middle cloudsAltocumulus (puffy patches), altostratus (a gray sheet the Sun shows through dimly), and nimbostratus (a thick, dark layer that brings steady rain or snow).
Low cloudsCumulus (fair-weather puffs with flat bottoms), stratocumulus (lumpy gray rolls), and stratus (a flat, gray blanket; fog is a stratus cloud at the ground).
CumulonimbusThe thunderstorm cloud. It starts low but can tower up through the whole troposphere and spread out in an anvil shape at the top. It brings lightning, heavy rain, hail, and sometimes tornadoes.

Air masses and fronts

An air mass is a large body of air with about the same temperature and humidity all through it. Air masses take on the character of the place they form. Air that forms over northern Canada is cold and dry. Air from over the Gulf of Mexico is warm and moist.

A front is the boundary where two different air masses meet. Warm air is lighter than cold air, so at a front the warm air is pushed up. As it rises, it cools and forms clouds. That is why fronts bring changes in the weather. Fronts often stretch out from the center of a low-pressure system.

Side views of a cold front and a warm front, and the symbols forecasters draw for four kinds of fronts.

Cold frontCold air pushes in and shoves warm air up steeply. That makes a narrow band of tall clouds and showers or thunderstorms, followed by cooler, drier air. On a map it is a blue line with triangles.
Warm frontWarm air slides gently up and over cold air. The gentle slope spreads clouds out far ahead of the front, from high cirrus down to low layers, with wide areas of steady rain. On a map it is a red line with half circles.
Stationary frontA boundary that is hardly moving. It can bring days of cloudy, wet weather. Its map symbol alternates blue triangles and red half circles on opposite sides.
Occluded frontForms when a fast cold front catches up with a warm front. It is drawn in purple with both triangles and half circles.

On a weather map, the triangles and half circles sit on the side of the line the front is moving toward. On a stationary front they sit on opposite sides, because the front is hardly moving.

Wind and the Coriolis effect

The Sun heats Earth unevenly. Warm air rises and leaves lower pressure behind, while cool air sinks and makes higher pressure. Wind is air moving from high pressure toward low pressure. The bigger the difference in pressure over a short distance, the stronger the wind.

You can feel this at the beach. On a sunny day, land heats up faster than water. Air over the land rises, and cooler air from over the ocean flows in to replace it. That is a sea breeze.

Over long distances, something else steers the wind: Earth’s spin. Earth rotates from west to east. Because of this, air moving across the planet bends to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect.

Air moving toward the equator is bent to the right in the north and to the left in the south. That is why hurricanes spin opposite ways in the two hemispheres.

Did you know?

The Coriolis effect is named after Gaspard-Gustave de Coriolis, a French mathematician who lived from 1792 to 1843. He studied how energy moves in spinning machines like waterwheels. The same effect also steers ocean currents.

Severe weather and safety

Every thunderstorm needs three ingredients: moisture, unstable air (warm air that keeps rising once it starts), and something to lift the air, such as a front, a sea breeze, or a mountain. The United States gets about 100,000 thunderstorms a year, and about 10 percent of them become severe, meaning hail 1 inch across or bigger, winds of 58 miles per hour or more, or a tornado.

LightningA giant spark of electricity in a storm. It heats the air around it to about 50,000°F, about five times hotter than the surface of the Sun. That heated air expands with a boom: thunder. When thunder roars, go indoors. Only a building or a hard-topped vehicle is safe; trees, tents, picnic shelters, and dugouts are not. Wait 30 minutes after the last thunder before going back outside.
TornadoesViolently spinning columns of air that reach from a thunderstorm down to the ground. About 1,200 tornadoes hit the United States each year, and they have been reported in all 50 states. They are most likely between 4 and 9 p.m. Their strength is rated on the Enhanced Fujita (EF) scale from the damage they do.
HurricanesHuge spinning storms that form over warm tropical oceans, where the water is at least 80°F (26.5°C). A storm becomes a hurricane when its winds reach 74 miles per hour. The Saffir-Simpson scale rates hurricanes from Category 1 (74 to 95 mph) to Category 5 (157 mph or more), using wind speed only. Storm surge, flooding rain, and tornadoes can be just as dangerous.
FloodsFloods kill more people in the United States each year than tornadoes, hurricanes, or lightning. Just 6 inches of fast-moving water can knock an adult down, 12 inches can carry away most cars, and 2 feet can sweep away SUVs and trucks. Never walk, swim, or drive into flood water. Turn around and find another way.

Watch or warning?
Alert	What it means	What to do
Tornado watch	Conditions are right for tornadoes to form nearby.	Be prepared. Review your plan and know where your safe place is.
Tornado warning	A tornado has been seen or shows up on weather radar.	Act now. Go to a basement, safe room, or inside room on the lowest floor, away from windows.

Real example

On September 8, 1900, a Category 4 hurricane struck Galveston, Texas. Storm tides 8 to 15 feet high covered the whole of Galveston Island. About 8,000 people died (estimates range from 6,000 to 12,000). It is still the deadliest weather disaster in United States history. Today, satellites, radar, and forecasters at the National Hurricane Center give people days of warning so they can evacuate.

Weather versus climate

Weather is what the atmosphere is doing at a certain place and time: hot or cold, wet or dry, calm or windy, right now or over the next few days. Climate is the usual pattern of weather in a place over many years. A simple way to remember it: weather is your mood today, and climate is your personality.

To describe climate, scientists average 30 years of weather records. These averages are called climate normals; the current set covers 1991 to 2020.

Daily temperatures (weather) jump up and down. The long-term average (climate) is a smooth curve.

Try the interactive version at jugami.com/earth-science/weather-atmosphere/

Game: Forecast Desk

You are the forecaster on duty. Six weather reports come in. Read the sky, the rain, the temperature, and the weather map. When you think you know what is causing the weather, choose it. The answer is explained after you choose.

This game needs JavaScript. With JavaScript turned off, the six weather reports and their clues are listed below.

The six weather reports and their clues:

A spring afternoon in Kansas: cold front. Sky: Towering cumulonimbus clouds in a narrow line, with lightning. Rain: A short burst of heavy rain and thunder, then the sky clears. Temperature: Warm and humid before the storms. After they pass, it turns much cooler and drier. Weather map: A blue line with triangles is moving toward you.
A fall day in Chicago: cold front. Sky: A band of tall, lumpy clouds rolls in quickly. Rain: Gusty showers and a few rumbles of thunder for about an hour. Temperature: Mild at lunch, then the temperature drops sharply by evening. Weather map: A blue line with triangles stretches south from a low-pressure center.
A winter morning in Ohio: warm front. Sky: First high, wispy cirrus, then the clouds thicken and lower into a gray sheet. Rain: Light, steady rain that lasts many hours. Temperature: Cold at first, then slowly getting milder. Weather map: A red line with half circles is moving toward you.
A spring day in Pennsylvania: warm front. Sky: A low, flat, gray layer of cloud covers the whole sky. Rain: Drizzle and light rain on and off all day. Temperature: Cool now, but tomorrow is forecast to be much warmer and more humid. Weather map: A red line with half circles lies just to your south and is moving north.
An autumn week in Arizona: high pressure. Sky: Clear blue sky, with hardly a cloud. Rain: None at all. Temperature: Pleasant, sunny days. Weather map: A big blue H sits right over your area.
Late summer on the Florida coast: hurricane. Sky: Bands of dark clouds and squalls spiral in from the ocean. Rain: Bursts of very heavy rain that keep getting worse. Temperature: Warm and humid. The ocean water is warmer than 80°F (26.5°C). Weather map: A huge swirl of clouds spinning counterclockwise, with an eye in the middle and winds of 74 mph or more.

Self-guided quiz

Nine questions. Choose one answer for each, then press Check my answers.

1. In which layer of the atmosphere does almost all of our weather happen? Stratosphere Troposphere Mesosphere

2. Why is air pressure lower on a mountaintop than at the beach? There is less air above pushing down The mountain blocks the wind from blowing Rock on the mountain soaks up the air around it

3. Which way does wind blow? From low toward high pressure Straight up from warm ground into space From high toward low pressure

4. In the Northern Hemisphere, air moving over long distances is bent by the Coriolis effect. Which way? To its left To its right Nowhere, because Earth’s spin has no effect on air

5. On a weather map, what does a blue line with triangles show? A cold front A warm front A stationary front that is not moving

6. Wispy cirrus clouds appear. The next day, clouds thicken and lower, and light rain falls for hours. What is most likely coming? A strong high-pressure system A fast-moving line of thunderstorms A warm front

7. A tornado WARNING is issued for your town. What should you do? Keep playing outside, but keep a close eye on the sky Get to the lowest floor, in an inside room Open the windows so the air pressure can even out

8. You hear thunder during a soccer game. What is the safest choice? Get inside a building or a hard-topped car Stand under a tall tree until the storm passes Wait in an open picnic shelter or the dugout

9. Which sentence is about climate, not weather? It is snowing hard in Denver right now Phoenix usually has hot, dry summers Tomorrow afternoon will be windy and rainy

Answer key: 1. Troposphere. 2. There is less air above pushing down. 3. From high toward low pressure. 4. To its right. 5. A cold front. 6. A warm front. 7. Get to the lowest floor, in an inside room. 8. Get inside a building or a hard-topped car. 9. Phoenix usually has hot, dry summers.

Talk about it

Where is the safest place in your home and your school to go during a tornado warning? Where would you go if you were outside?
Look at the sky today. Which clouds can you see, and what do you think they say about the weather coming next?
Why do you think so many more people die from floods than from tornadoes, even though tornadoes seem scarier?
One cold week doesn’t prove that a place’s climate is getting colder. Why not?

Sources

NASA Science, Earth’s atmosphere: a multi-layered cake (layer heights, the coldest mesosphere, meteors, auroras, the space station, satellites); UCAR Center for Science Education, Layers of Earth’s atmosphere.
NOAA JetStream, The atmosphere (makeup of dry air and water vapor) and Air pressure (1013.25 millibars, 29.92 inches, 14.7 pounds per square inch; half the air below 18,000 feet).
NOAA NESDIS, What are high and low pressure systems?
NOAA JetStream, How clouds form and Ten basic clouds; National Weather Service (NWS) La Crosse, Dew point vs. humidity.
NOAA JetStream, Air masses and Weather maps; NWS Weather Prediction Center, Surface front symbols.
NOAA JetStream, Origin of wind and The sea breeze; NOAA Ocean Service, The Coriolis effect (right in the north, left in the south; Gaspard-Gustave de Coriolis).
NOAA JetStream, Ingredients for a thunderstorm; NOAA National Severe Storms Laboratory, Severe weather 101: thunderstorms, tornadoes, and floods.
NWS, How hot is lightning?, Lightning safety outdoors, Tornado watch vs. warning, During a tornado, and Flood safety.
NOAA National Hurricane Center, Saffir-Simpson Hurricane Wind Scale; NOAA Ocean Service, How do hurricanes form?; NOAA AOML, Tropical cyclone FAQ.
NOAA, Hurricane history (the 1900 Galveston hurricane: Category 4, storm tides of 8 to 15 feet, about 8,000 deaths).
NOAA National Centers for Environmental Information, U.S. climate normals (30-year averages, 1991–2020).

Credits

Written by Mary, Jugami’s Earth science teacher. The sky, atmosphere, cloud, front, and wind drawings are original, hand-coded SVG made for Jugami. They are simplified sketches, not drawn to scale.