Weather: Air Pressure & Evaporation

• Weather Experiments
• Skies of Blue
• Fabulous Facts
• Science Links

Weather Experiments

The science of weather affects all of us every day! Convection, high-pressure and low-pressure systems, evaporation - these things help determine if our game will be rained out or if we will have a sunny day for sledding. Learn more about the way weather works by doing these hands-on experiments.

Experiment 1: Convection Current
Have you ever heard that hot air rises? That's true! As air heats up, its molecules expand and spread out, making the air less dense than it was before. It floats up through the denser cooler air. As the warm air rises, it starts to cool off and its molecules move closer together, causing it to sink again. This circulation is called convection, and the rising and falling of the air are called currents. Convection currents are part of what causes different kinds of weather. (You'll find out how in Experiment 2.)

We can't see convection in the air; do you think water might act the same way? Do this experiment to find out! You should have an adult help you with the hot water and the knife.

Materials

• Large glass jar or beaker
• Small cup or beaker (it needs to fit inside the jar)
• Food coloring
• Knife
• Plastic wrap
• Rubber band
• Water

Procedure:

1. Fill the small cup or beaker with very hot (almost boiling) water and add several drops of food coloring. Stretch the plastic wrap smoothly over the cup and seal it with the rubber band. (The plastic wrap will puff up--this is because the hot air on the surface of the water is expanding!)
2. Fill the jar almost full with cold water from the tap.
3. Use a pair of tongs to set the cup of hot water in the bottom of the jar.
4. Slice open the plastic wrap with the knife and watch what happens! (One long gash should do it.)

What happened? The hot water was less dense than the cold water surrounding it, so it rose to the top in a convection current. What happens as the colored water gets to the top? Does it stay there? Why or why not?

• Click here to continue your weather experiments - make a sea breeze and set up an evaporation station!

Skies of Blue

One of the most beautiful things about our world is a blue sky on a clear, sunny day.   Maybe you have seen pictures of the Apollo astronauts on the moon - even in bright sun the sky above them was black as night.   What makes the difference?   Why is Earth's sky blue?  

Unlike the moon, the earth is surrounded by an atmosphere. The atmosphere is a mixture of gasses, mostly nitrogen and oxygen.   The way the atmosphere interacts with the sun's light is what determines the color of the sky.

Light is made up of several different colors, like you see in a rainbow.   Each of these colors travels in a wave, but the wavelength (distance between the peaks of a wave) varies.   Red light has a long wavelength, while blue light has a much shorter wavelength.   When light from the sun enters our atmosphere, the waves collide with the gas molecules.   The longer wavelengths, like red and yellow, pass straight through and appear to us as "regular" sunlight.   Shorter wavelengths, like blue, are scattered in different directions by the gas molecules.   The blue light is scattered by the gas molecules, so our sky looks blue.  

You can see some light scattering by mixing half a teaspoon of milk with a jar of water.   Shine a flashlight through the jar and look at the water.   It should have a bluish tint, because the milk particles are scattering the blue light from the flashlight just like the gas molecules in our atmosphere do.  

Blue sky isn't the only beautiful thing produced by the atmosphere and light. Colorful sunsets happen because of light scattering, too!   When the sun is on the horizon, its light has to travel through a lot more atmosphere to reach you.   The blue light is scattered so much in the extra atmosphere that it doesn't reach your eye, leaving you to see the beautiful reds and oranges instead.   Sometimes clouds or air pollution can make a sunset even more red because the particles in the cloud help scatter away the shorter wavelengths.

Add a little more milk to your jar - do the extra milk particles allow you to see an orange tint?   Try looking in the side of the jar directly opposite where the flashlight is.   This is like looking at the sun on the horizon.  

Fabulous Facts

Heavy Air.   Air feels light, but in reality it is weighing down heavily on your body.   At sea level, air pushes against you with a pressure of 14.7 pounds per square inch.   This means that the force on a square foot equals about one ton!   Why aren't we smashed by the weight of air?   Well, there's air and fluid inside our bodies, too, pushing out, which balances the air pushing in.  

Pop! Have you ever felt your ears plug when you take off in an airplane or drive up into the mountains?   This happens because air pressure decreases at higher altitudes, so the air pressure outside your ear is less than the air pressure inside your ear.   When your ear "pops," it has adjusted the pressure inside so that it balances with the outside pressure again.  

Science Links

Track the weather in your area with the help of the National Weather Service.

Find fun weather activities and learn more about weather from a meteorologist at Weather Wiz Kids.

Look at the weather from space! See satellite images at the Geostationary Satellite Server.

Visualize how air pressure affects weather and build your own barometer at NASA's Earth Science site.  

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