Colorado State University

We polled CMMAP (Center for Multiscale Modelling of Atmospheric Processes) scientists as to what they thought were the 10 most important things that everyone should know about the atmosphere. We took their answers and condensed them into a list of 10 items, with explanations and activities you can use to teach these concepts to your students.

Each of our activities is titled with an intriguing question, followed by some introduction and theory, a parts list, and helpful suggestions for implementing these activities in your classroom! Also, it is important to note that the “10 Things” list is in no particular order!

1. The sky is falling. (Or a least the clouds.)

A cloud consists of droplets of water suspended in air. As you might expect, adding the droplets of water increases the density. A cloud at the same temperature as surrounding air will be more dense than the surrounding air. So why don’t clouds fall? In fact, in some sense, they do! The big, puffy cumulus clouds that you see form atop columns of rising air. The air is moving upward. So the bottom of the cloud is—relative to the air—moving downward. The clouds are falling! Whenever you see cumulus clouds in the sky, that tells you that the air is moving vertically. Clouds mean rising air (and thus falling clouds!) and clear sky means falling air.

Activity	Description
Does air weigh anything?	Indeed it does! But most simple experiments that purport to show this don’t work. This one does.
Why do raindrops sometimes land gently, and sometimes land with a splat?	An inexpensive investigation you can do with just water, food coloring, and mineral oil!

2. Most of the water isn’t in clouds.

Clouds are the most obvious form in which water is present in the air. We’ve all seen diagrams of the water cycle that show water evaporating over the ocean, forming clouds, clouds blowing over the land and raining water down.  This certainly makes the presence of water visible, but most of the water in the atmosphere isn’t visible. It’s in the form of water vapor, not the droplets of water or crystals of ice that make up clouds.

Activity	Description
Do cities affect the weather?	Cold water vapor won’t necessarily condense to form water droplets… nucleation sites are needed as well.
Why can warm air “hold” more moisture than cold air?	It all has to do with vapor pressure!

3. The earth gets more energy from the sky than the Sun.

Everything radiates energy.  The sun radiates energy to the earth, the earth radiates energy to space, and the atmosphere radiates energy as well.  If you look at the energy the earth receives, there is light energy from the sun but this isn’t enough to explain how warm our planet is. So, what keeps the earth so warm? The earth also receives energy coming down from the sky in the form of thermal energy. The sun shines during the day, but the sky shines all the time. And, when you add it up, the earth gets more radiant energy from the sky than from the sun.

Activity	Description
Can you “see” thermal radiation?	Sort of. You can certainly feel it, probably better than you expected!
How does the atmosphere keep the earth warmer?	The earth cools by infrared. And the atmosphere is somewhat opaque to the infrared, so it keeps the earth warmer. A simple investigation.
Why does it get colder on clear nights than on cloudy nights?	The earth gives up energy to space by radiation. If it’s cold up above, it cools off more at night.
What is the “greenhouse effect”?	The earth is warmer than the moon, and it’s because of our atmosphere. An investigation with an infrared thermometer.
What makes a gas a “greenhouse gas”?	Carbon dioxide and water vapor are both greenhouse gases. Why?

4. The sky is purple and clouds are white.

As light passes through the atmosphere, the molecules of air continually absorb and reemit the light. This changes the direction of the incoming light—which we call scattering—and this scattering effect favors the shorter wavelengths of light. If we judge by the most prominently scattered color, the sky is violet. But the sky appears blue— which is also scattered in large quantities—because our eyes are more sensitive to blue than violet.

Clouds, on the other hand, are composed of millions of tiny water droplets.The average size of cloud droplets are much bigger than the wavelengths of visible light.  Since the cloud droplets are much bigger than any wavelength of light, all the colors of light behave the same when they hit a droplet—they scatter. All the colors scatter equally, and the net result: clouds appear white!

Activity	Description
Why is the sky purple?	Perhaps you thought it was blue…
Why are clouds white?	Clouds are always white! Don’t believe it? Try these activities!

5. The atmosphere is thin and has a fuzzy edge.

The thickness of the Earth’s atmosphere is not a definite number because there is no set boundary where the atmosphere ends. As the altitude of air increases there is a decrease in atmosphere gases in the air—all the water and weather in the atmosphere are contained to the lower 30 km of atmosphere, 99% of the atmospheric gases are below 50 km in altitude, but trace atmospheric gases can trail out thousands of kilometers.

Activity	Description
How far away is space?	If you were to drive to space, how long would it take you?

6. The wind blows from high pressure to low, except when it doesn’t.

In the atmosphere, air pressure isn’t constant. This is primarily because the sun heats the earth’s surface unevenly. The resulting differences in pressure will push air from high pressure to low pressure. On a small scale, the wind does blow from high to low pressure. If the ground warms, air rises, and other air blows in to the resulting lower pressure spot. But  you’ve probably seen regions of high pressure and low pressure on weather maps. Why don’t these regions quickly mix to reach equilibrium? And why doesn’t the wind point from the H to the L? Because there is another “force” at action— the Coriolis effect causes air to curve, rather than traveling in straight lines from high pressure to low pressure areas.

Activity	Description
Why does the wind blow?	It has to do with pressure…
Why do hurricanes spin counterclockwise in the northern hemisphere?	It’s the Coriolis force. The earth rotates, which leads to certain ficticious forces…

7. The atmosphere warms from the bottom, cools from the top.

The earth sits in the vacuum of space, so the only way it can gain or lose energy is by radiation. The incoming radiation from the sun is mostly visible light, and able to go right through the atmosphere. The energy returning to space has to come out from the top of the atmosphere. But the outgoing radiation is mostly longer wavelength infrared, and doesn’t go through the atmosphere easily. Much of the infrared is absorbed by water vapor and carbon dioxide in the atmosphere. The vertical motion of the energy radiated from the earth to space moves the warm air near the surface of the earth to the cool air above, creating convection cells in the atmosphere, which is the driving force of weather.  It also makes for a warm, insulating layer between earth and space, a very nice thing indeed.

Activity	Description
How can clouds help keep the air warmer?	When water vapor condenses, it gives up heat. This is an important process of energy transfer in the atmosphere.
How does the earth cool itself off?	The earth can only cool itself off by radiating energy to space. A simple exercise with a placemat and an infrared thermometer.

8. The atmosphere warms at the equator, cools at the poles.

The earth isn’t heated evenly. At the equator, the sun hits the earth straight on.  At the poles, sunlight shines at an angle and is spread out over a large area. So there is much more heating at the equator than at the poles, as you know. This differential heating leads to motion of the air over large scales. Warm air rises at the equator and travels to the poles. The air cools as it reaches the polar regions, causing it to sink, and return back to the equator as a surface wind.

Activity	Description
What is a convection cell?	When hot air rises in the atmosphere, it may produce convection cells, where air rises at one spot and falls at another. Here’s a great way to show this.

9. The ocean and atmosphere work together.

Due to uneven heating of the earth’s surface, the atmosphere and oceans get the job of spreading the energy around. And they work together to do so. For instance, the wind can push the warm, top layer of the ocean, causing the warm water to pile up in some regions, and cold water to plume in other regions. This turn-over of the ocean surfaces colder water below, which is then heated by the atmosphere, continuing the cycle.

10. The most important part of the atmosphere is at the top.

The ozone in the upper atmosphere absorbs much of the incoming ultraviolet radiation from the sun. This is a good thing, considering some photons of UV radiation have enough energy to kill microorganisms, making this type of radiation good for sterilizers, but pretty tough on animals, especially our eyes. No life was present on the earth’s surface until life in the oceans added enough oxygen to the atmosphere that the ozone layer formed, blocking the dangerous shortwave ultraviolet radiation from the surface.

Activity	Description
Would you get a sunburn on Mars?	Indeed you would. A lethal one.

Additional Activities for Teaching about Weather and Climate

Activity	Description
If hot air rises, why is it cold in the mountains?	When air expands, it cools. And when it rises, it expands. A nice exercise on adiabatic heating and cooling.
Can it really rain fish and frogs?	A bit about extreme weather. A very zesty demonstration.
How can I turn a solar oven into a refrigerator?	This is quite surprising, and it really works!
How can freezing make something warmer?	A cool experiment with heat packs.
What’s beyond the rainbow?	A series of investigations on the electromagnetic spectrum.
What’s the difference between blue light and red light?	Photon energy! A cool investigation that glows in the dark.
Can energy be created or destroyed?	It can’t, of course, but here’s a nice quantitative investigation using solar garden lights.
What is energy?	A simple investigation into forms of energy using toys.
How can gravity make something go up?	This is a recipe for a simple solar hot air balloon. The cooler denser air around it falls, so the balloon rises!
What causes pressure?	The classic “molecule mosh pit” class exercise.
Can you drink through a 30 foot straw?	No. But the reason you can’t makes a nice point about air pressure.
What is a model?	How can we model the climate? This activity gives a taste of what a model actually does.
What’s the difference between weather and climate?	An important question that we answer with candy.
What’s the difference between weather and climate?	A more advanced version of the above investigation… it’s an exploration of chaos.