Name: ________________________________________________________ Date: ____________ Period: ____________
Inquiry 6.2 - Modeling the Effects of Air Pressure on Cloud Formation
Investigative Question: How does air pressure affect cloud formation?
1. What do you already know about cloud formation? _____________________________________________________
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2. What are the “ingredients” for cloud formation? (Hint: Read Hurricane formation and the water cycle on page 72).
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3. How could you create these conditions in a bottle? ______________________________________________________
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4. If you want to test how air pressure affects cloud formation, how could you create high pressure in the capped bottle?
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5. How could you create low pressure in the capped bottle? _________________________________________________
Table 6.2: How Air pressure in a Bottle Affects Cloud Formation
Bottle
Predictions
Observations
(What I Think Will Happen)
(What Did Happen)
When
Air
Pressure
is Low
Explanation
(Why Did You See These Results?)
When
Air
Pressure
is High
Reflecting on What You’ve Done:
1. Use your own words to describe how air pressure and cloud formation are related.
Clouds form under low pressure conditions. Under high pressure conditions the weather is clear and sunny.
2. Give two factors discussed in this lesson that are important for cloud formation and explain how they affect cloud
formation.
a. Air pressure – Clouds form under low pressure conditions
b. Humidity – Clouds form when there is a lot of water vapor in the air that can cool and condense to form
clouds.
Read: What Are Clouds And Why Does It Rain?
Almost all the air around us is moist. That means that it contains water in the form of vapor. You can't see it because
water vapor is a gas, but it is still water. Water can exist in three states; liquid (water), solid (ice) and gas (water vapor).
Obviously, you can see and touch water and ice, but water vapor has no smell, you can't pick it up, and it's invisible. That
doesn't mean that you can't feel it though.
Perhaps you can remember a hot and sticky day in summer, or a cold foggy day in winter, or even being in a hot shower
full of steam? In each of those situations you will have felt water vapor all around you. If you stopped and really looked
at that fog or steam you would have spotted millions and millions of tiny water droplets floating in the air. What you saw
was the same process that makes clouds - millions of tiny water droplets condensing out of the air to form liquid water.
We've all seen fog and steam, but why does water condense out of air and become visible? Well, warm air can hold
more water vapor than cool air, so if warm air starts to cool, it can no longer hold as much water vapor. The extra water
vapor has to go somewhere, so it condenses out as water. So... cooling the air reduces its ability to hold water vapor,
and triggers the formation of water droplets.
Go outside and stare up at a cloud (not one right next to the Sun though - you don't want to go blind!). Watch that cloud
for a while, especially its edges. As you watch it, the edges will change, either growing larger or getting smaller. What
you are seeing is cloud formation in action. As the cloud grows you are seeing more of those water droplets condensing
out of the air, and as it shrinks, you are witnessing the droplets evaporating - changing from visible liquid water into
invisible water vapor.
Now, it doesn't take a genius to point out that the clouds are usually a long way up, and not every cloud has rain pouring
out of it. So, how does the formation of a cloud lead to actual rain?
To get rain, the water condensing in the clouds has to become heavy enough to fall to Earth. The tiny droplets just aren't
heavy enough to fall. Just like fog or in the shower, they go whichever way the wind and eddy currents blow them, or
they just hang there, suspended in the air.
To become heavier, the droplets need to grow into drops. To do this they have to acquire more water and become
larger. Some will collide with other droplets and become larger, and others will grow as water condenses out the air
directly into the droplet. Others will grow by both methods. It's a bit like watching drops of rain water on a window small drops fall; they join with other small drops, become larger drops, and so on. In the right clouds, this process will be
happening to millions of tiny droplets, all growing at the same time, but at different speeds.
Eventually, if the droplets keep growing, they will reach a mass where they can't stay floating in the cloud because they
are too heavy - and will start to fall. Some may get caught in upward blowing winds and get blown back into the clouds
for a while, but once they are heavy enough to overcome the force of the wind, they will fall to earth - as rain! It will
keep raining as long as the conditions are right to make the clouds and let the water droplets grow heavy enough to fall.
So, there you go - now you know what a cloud is, and how it creates rain.
The next thing is to discover what causes the air to cool and start the process. Earlier on I mentioned (and told you to
remember it) that cooling the air reduces its ability to hold water vapor, and triggers the formation of water droplets.
To create clouds, and to have rain, the air has to be cooled. Air can be cooled in three different ways, leading to three
different types of rain:
1. Convectional Rain: On a warm day, sunshine heats up the ground and the air above that ground heats up too as
it heats it also absorbs more water vapor. The warm air begins to rise because the warm air is less dense than
the air around it. The atmosphere gets cooler as you go higher - by roughly one degree centigrade cooler for
every 100m of altitude in dry air - so as the warm air rises it becomes cooled again by the colder air around it.
Eventually the air reaches a height where the temperature forces the water vapor in the warm air to start
condensing. This is called the condensation point, and is where the clouds begin to form. The rain forming
process starts, and usually leads to very heavy rain, perhaps with thunder and lightning.
2. Frontal Rain: When rain forms in this way two bodies of air are involved - an area of relatively warm air and an
area of relatively cool air. The warmer air is less dense, so when it meets the cooler air it rises up over the cooler
air mass. The cooler air stays where it is, and lies underneath the warmer air. As the warmer air rises over the
cold air it starts to cool. As the air cools, water vapor is precipitated and the cloud forming process begins,
leading to rain.
3. Relief Rain: A physical object of some kind is needed, so that warm moist air is forced to rise up over it.
Mountain ranges, big hills and even cliffs along the coast be large enough to force the air to rise. As the warm air
rises over the object it cools and clouds form. Rain falls from the clouds, or if the droplets don't grow large
enough, fog may form over the hill tops. Fog is basically just clouds at ground level.
Questions:
1. Warm air holds more water vapor than cold air.
2. What happens as warm air cools? When warm air cools, the extra water vapor condenses out as liquid water.
3. What happens when a cloud grows? More water vapor is condensing out of the air.
4. How does cloud formation lead to rain?
Millions of tiny water droplets need to grow until they are heavy enough to become rain. Water droplets may
collide or condense on one another until the rain drop grows heavy enough to fall as precipitation.
5. Summarize: what are three ways air can be cooled, leading to rain?
The three ways air can be cooled, leading to rain are: the air rises high enough into the atmosphere that it cools
leading to convectional rain; a warm air mass and a cold air mass meet, the warm air mass is pushed up over the
cold air mass and as it rises it cools leading to frontal rain; A warm air mass is pushed up over a mountain or other
physical object causing the air to rise and cool leading to relief rain.
Read: “The Truth About Air” on Page 76 of your textbook, then answer the following questions:
1. What are some low clouds? Stratus Describe them. Dense and thick
2. What are some high clouds? Cirrus Describe high clouds: Thin and whispy
3. Air flows from High pressure to Low pressure.
4. Think back to Lesson 5 lab with the convection tubes. Why did the air from the cold moist convection tube rush into
the tube with the candle?
The air pressure was different between the two tubes, The cold air tube had higher pressure than the warm air
tube so the air flows from cold (high pressure) to hot (low pressure) .
5. When people climb Mt Everest or Mt Rainer, why do they need oxygen masks as they approach the summit?
Air molecules are more spread out (the air pressure is lower) so there is less oxygen to breathe.
6. Where is air pressure or air density greater? Sea level (closer to Earth’s surface) Why? All the weight of the air in
the atmosphere is weighing down compressing the air molecules and making the air more dense (higher pressure)
7.
What instrument is used to measure air pressure? Barometer
8. How does it work?
Air outside the barometer presses on the liquid in the reservoir and pushes liquid up a tube. The more the air
presses down on the liquid, the higher the liquid rises in the tube indicating high air pressure.
9. What does the fluid (mercury or water) in the height of the tube represent? Air pressure