Clouds
and Storms
Intro
Preview
Body, Clouds
Body, Lightning
Body, Thunder
Experiment
Summary
Closure
Intro:
In our last lesson we talked about how water evaporates and becomes water
vapor, and the vapor condenses into clouds.
Today we're going to learn more about clouds and their role in storms, thunder, and lightning.
Later on, we'll be creating a little thunder and lightning of our own here in the classroom.
First, we'll learn about the five basic cloud types and the weather associated with each of them.
Then we'll learn about lightning: What it is, what causes it, and what to do during a lightning storm.
The three main types of clouds are cumulus, stratus and cirrus, but their are many subtypes. Clouds are classified by shape and altitude. We're concerned with just five kinds. Click on each name to see an enlarged photo.
Cirrus are high wispy clouds made up of ice crystals. Cirrostatus and cirrocumulus are types of cirrus clouds.
Cumulus clouds are bright white, fluffy and round. They indicate fair weather. Over time they may develop into storm clouds. Altocumulus and stratocumulus are just cumulus clouds at higher altitudes with a greater potential to bring bad weather.
Cumulonimbus clouds are thunderclouds. They are sometimes called thunderheads and are known by their anvil like shapes. These clouds are often accompanied by high winds, thunder and lightning. Think of them as "super" cumulus clouds.
Stratus clouds are thick, gray, low-altitude layers that usually bring light rain or fog. They like to spread outward in sheets and sometimes cover higher ground.
Nimbostratus clouds are low altitude versions of stratus clouds. They often mean bad weather such as fog, snow, or rain.
In our lesson on the water cycle we learned that water is constantly evaporating from oceans, lakes and rivers. In the air the water vapor condenses into very tiny droplets that continue to float.
As air and water molecules in the cloud bump into each other they pick up positive or negative charges, and behave like a tiny magnets. These charges help hold the cloud together. We'll perform some experiments that involve static electricity later in the lesson.
WHY CLOUDS GATHER
Most of the time clouds are moved primarily by the wind. Remember from our air mass lesson that wind happens when air moves from an area of high pressure into an area of low pressure. For the most part, clouds are simply carried along by the wind.
If there is no wind, clouds may still be attracted to each other due to their positive and negative static electric charges.
When clouds are carried on the wind they frequently rub and bump against each other and this causes friction. The friction results in more static electricity building up in the clouds.
Positive static electric charges gather near the tops and bottoms of clouds, while negative charges gather in the middle. The more clouds rub up against each other, the more static electricity they build up inside.
A cloud can hold a lot of static electricity, but eventually it can't hold any more and the electricity explodes out in the form of lightning.
The positively charged areas of clouds send lightning bolts to negatively charged areas, and negatively charged areas send lightning bolts to positive areas, or to the ground.
Enlarge View
Thunder is caused by lightning. When a lightning discharge occurs the energy of the lightning bolt heats the air around it to 54,000°F (30,000°C). This super heating causes the air to expand suddenly with explosive force, resulting in a loud sonic boom we call thunder.
Lightning and thunder occur together, yet it often seems the thunder occurs several seconds later. This is because we are usually miles from the lightning we see. Light travels so much faster than sound that it reaches our eyes before the sound of thunder reaches our ears. You can use this fact to estimate the distance you are from the lightning.
The class will now divide into small groups of at least three students. Each group will perform three experiments: Static Attraction, Making Lightning, and Making Thunder.
To save time the hypothesis for each experiment has been stated on the instruction sheet.
The students will help each other make predictions about the experiments, record the results and compare the results to the hypothesis.
Next, the students will record their conclusions about what has been proven or disproven.
Instructor guides the discussion by the students and confirms or corrects each conclusion.
Next week's lesson: tornados and hurricanes.