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.
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 the next experiment.)
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.
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?
Air seems like the lightest thing in the world, but it actually pushes down on you and the ground with a great deal of force. This force is called air pressure. Air pressure doesn't always stay the same; meteorologists measure its changes with a barometer. In the last experiment we saw that when air heats up it begins to rise. When it rises, it doesn't push on the ground with as much pressure. An area full of light, warm air is called a low-pressure zone. Areas with cool, denser air are called high-pressure zones. What happens when a low-pressure zone and a high-pressure zone are right next to each other? Do this experiment to find out! Have an adult help you with the oven and matches.
What happened? When you lit the candle the first time you did it in an area where the air pressure was constant, so the smoke flowed straight up. When you set the pans side by side, the ice cooled the air around it, creating a mini high-pressure zone, and the sand warmed the air around it to create a mini low-pressure zone. Air always flows from a high-pressure zone to a low-pressure zone to even up the pressure - this is what causes wind. You made a tiny breeze between the pan of ice and the pan of sand, and the smoke floated sideways in the breeze. The same thing happens between cold ocean water and hot beach sand, which is why there is almost always a breeze at the beach!
Air pressure changes cause wind, but they are responsible for other types of weather too. A low-pressure zone usually causes clouds and rain, because as the hot air rises it carries with it evaporated moisture that can condense into clouds. A high-pressure zone usually results in clear skies and sunny days because sinking currents prevent moisture from rising up and forming clouds.
Low-pressure zones create clouds because the rising hot air carries moisture with it. The moisture is in the form of a gas called water vapor. When the water vapor cools, it forms water droplets that join together to form clouds. How does the water vapor get into the air in the first place? Most of it comes from evaporation. Evaporation happens when water molecules warm up - they gain enough energy to change from a liquid into a gas, and then they rise up into the air to be carried on rising convection currents.. You have seen this happen in your kitchen when steam rises from boiling water.
Are there factors that can change how fast water evaporates? You can find out by setting up an experiment to test the effect of wind, temperature and surface area on the rate of evaporation. The following procedure will give you the basics, but feel free to come up with your own methods of testing and measuring the results. This kind of experiment would make a great science fair project. (Be patient: some of these tests can take more than one day!)
Question & Hypothesis: How do factors such as wind, temperature, and surface area affect the rate of evaporation? Will wind or heat cause water to evaporate faster? Will a greater surface area speed up or slow down evaporation? Write down your predictions.
Conclusions: Were your predictions correct? Which speeds up evaporation more, wind or heat? Based on your results, do you think the temperature or speed of wind would affect the evaporation rate? Can you think of other factors to test or a more precise way to test these factors again?