ACTIVITY #11: How Do Clouds Form from Rising Air?

We often look into the day sky to observe the clouds for their beauty and ability to help forecast the weather. Some clouds are puffy and vertical in appearance, while others are layered and flat in appearance. Some clouds are found at high elevations and are made of ice crystals and other clouds are found at lower elevations and are made of water droplets condensed onto condensation nuclei.

·  Why do clouds form at different heights?

·  What is fog and how does it form?

·  What mechanisms can cause air to rise and cool?

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GOALS: In this lab activity, you will …

·  Observe, measure, and predict changes in weather using atmospheric properties (wind speed and direction, cloud cover and type, temperature, dew point, air pressure, and relative humidity). Describe how air pressure and temperature change with increasing altitude and/or latitude.

·  Describe how the formation of clouds is influenced by the dew point, environmental temperature and amount of particles in the air. Explain how various lifting mechanisms affect cloud formation.

Activity Overview: A synopsis of this lesson is as follows…

The purpose of this activity is to determine why clouds form at different heights. The previous topics of dew point and relative humidity are used in this activity. Students will first be asked to identify what would happen to a parcel of moist air if it was cooled to the dew point. If this same parcel of air were pushed outside of the house up into the atmosphere, what would happen to the parcel of moist air in these conditions? Students will investigate the influence of altitude on air pressure and temperature of the air and how this influences clouds formation at different elevations. In Part 2 of the activity, students look at several web sites and learn how radiation and advection fog form. The formation of fog and other types of cloud formations are also discussed in Part 2 of this activity.

CONNECTIONS

Scientific Content -

·  For a cloud to form, the following conditions must be met. There must be condensation nuclei for the water droplet to condense upon. There must be warm moist air that is forced upward, expands and is cooled to the dew point temperature for condensation to occur.

·  The height of cloud formation is determined by where the warm moist air meets the dew point temperature.

·  When the air is forced upward and cools, the amount of water vapor needed for saturation decreases and the relative humidity will increase. When the relative humidity reaches 100 %, the air is saturated and water vapor will soon begin to condense into tiny water droplets around small particles in the air, such as dust and salt. Billions of these water droplets form a cloud.

·  The shape and height of any given cloud varies with pressure, temperature, and the amount of water vapor present in the atmosphere.

·  When air is cooled to its dew point near the ground, it forms fog (a low stratus cloud).

·  Air can be uplifted by many mechanisms to cause cooling of this parcel of air to the dew point temperature. These mechanisms that cause the air to be uplifted include convection, convergence, topography and frontal activity.

Scientific Process –

·  Students will be observing, measuring and predicting changes in weather using cloud types and how they are formed.

Math/Graphing –

·  Students will use mathematics in this exercise by using differences in air temperature as elevation increases to calculate the height at which a particular cloud forms. The math includes simple addition and subtraction.

MAKING SENSE OF HOW CLOUDS FORM…

Clouds are formed when warm, moist air rises and cools to the dew point temperature and condenses onto condensation nuclei. When air particles separate as a parcel of air rises, the particles collide less often and generate less heat. This cooling effect creates clouds as the dew point temperature is met.

The upward motion that generates clouds and leads to precipitation is can be produced by convection, convergence of air near the base of clouds, lifting of air by fronts and lifting of air over elevated topography such as mountains.

High clouds form at elevations of 20,000 feet and higher. The air freezes at these elevations and forms ice crystals. Warm air is often pushed ahead of storms, and this warm, less dense air is forced to move over the colder, denser, surface air. This warmer air may extend a thousand miles from a storm center and rise to great heights, cooling, condensing and forming high clouds. In a warm front, cirrus clouds are first seen about a day prior to rain developing. Cirrostratus clouds are seen next and often form a halo or ring around the Moon or Sun. Altostratus clouds are seen next, followed by nimbostratus (rain clouds). Altostratus clouds are middle level clouds which form at elevations between 6500-20,000 feet. They may be made of water droplets or ice crystals.

Low clouds form at elevations less than 6500 feet. Stratus clouds which include fog clouds fall into this category. Fog is formed when the dew point temperature is reached just above ground level. Stratocumulus clouds are stratus clouds that form rounded, puffy masses and often cause the sky to appear very dark and ominous.

Observe on the Web, both visible and infrared satellite pictures to determine cloud cover, thickness, and height of clouds. In the visible satellite picture, thicker clouds reflect light better and therefore appear brighter. In the infrared picture, high clouds are colder and appear brighter while low, warmer clouds appear darker. Go to www.wunderground.com and observe infrared and visible satellite images.

Let’s Investigate …

Part 1– Bubbles of Air

1. What would happen if a bubble of moist air in a house were air conditioned and cooled to the dew point?

2. What would happen if this same air parcel, instead of being cooled by air conditioning, were forced through the roof and pushed up into the atmosphere?

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3. Why does the air parcel appear to expand with altitude?

4. How does a decrease in atmospheric pressure lower the temperature of rising air?

Your Teacher will discuss mechanisms that could cause air to rise

5. What would happen if a bubble of moist air rose to an altitude whereby it cooled to the dew point?

6. If air temperature (because it spreads out), will cool 10 degrees C per 1000 meters as it rises in altitude and the dew point cools at a rate of 2 degrees C per 1000 meters, at what height will a cloud form if the air temperature at ground level is 32 degrees C and the dew point is 8 degrees C?

7. What is the air temperature today?

8. What is the dew point today?

9. At what height would a convective cloud form under this day’s temperature and dew point conditions? Show how you arrived at your answer. Assume air temperature, because it spreads out, will cool 10° C per 1000 meters as it rises in altitude. Also assume the dew point will cool at a rate of 2° C per 1000 meters.

Have students look at the following table:

Height Above
Sea Level (m) / Average Air Pressure (mb) / Average Air Temperature (° C)
0 / 1013.2 / 15.0
2,000 / 795.0 / 2.0
4,000 / 616.6 / -11.0
6,000 / 472.2 / -24.0
8,000 / 356.5 / -36.9
10,000 / 265.0 / -49.9

10. How is air pressure related to temperature as the altitude increases?

11. What affect does a decrease in pressure have on a parcel of air that rises in the atmosphere?

12. What would happen to an air parcel if its temperature at 2,000 ft were 5° C?

Part 2– Fog

Look at web sites #3 & 4 below to learn how radiation and advection fog form.

http://www.usatoday.com/weather/tg/wfallfog/wfallfog.htm

http://www.usatoday.com/weather/tg/wadvfog/wadvfog.htm

1. Identify and describe the formation of the fog shown below.

2. How is the formation of fog alike and different from the formation of clouds?

Investigating Further …

Have you ever felt light-headed after blowing up a balloon? Imagine having that feeling all day long, while trying to climb a hundred flights of stairs, wearing 35 pounds of gear, breathing only through a straw. This may give you an idea of the tremendous challenges that high altitude climbers face with every single step they take.

Very high altitudes can have serious and deadly, effects on the human body. As you may know, the higher your climb, the thinner the air becomes. At the top of the Mount Everest, the percentage of Oxygen in the air is the same as that at sea level (about 20%). However, atmospheric pressure at the summit of Everest is 33% that of sea level. Therefore, there is 66% less oxygen than there is at sea level. Helicopters cannot fly. Yet, your body continues to chemically change your food into energy!

Altitude sickness occurs most often when there is a rapid ascent, and the body does not have sufficient rest time at intermediate elevations to acclimatize. (If a person were flown to the top of Mt. Everest and released at the summit without oxygen, he or she would die within an hour.) Individual susceptibility to altitude sickness varies greatly, and is independent of a person’s state of physical fitness. The most effective and urgent treatment is descent.

The average person will notice a decrease in athletic performance at 5,000 feet (Denver, CO is 5,280 ft) The point at which altitude sickness will typically onset is at 8,000 feet. Altitudes from 8,000-14,000 ft can be encountered by climbers and tourists in the continental United States. Rocky Mountain National Park has 100 peaks over 11,000 ft. and Mt. Rainier is 14,410 ft.

Only experienced climbers will usually attempt climbing to altitudes of 14,000-18,000 ft. Acclimatization prior to reaching this range is essential if illness is to be prevented. At 18,000 ft you might feel dizzy and have the fear of passing out. You would have to rest between putting on your boot and tying your shoelace. In your tent, you might not be able to remember which sleeping bag is yours. From 18,000 to 29,000 ft, only the most acclimatized climbers can function. Those who are susceptible to altitude sickness will have to turn back. Research stories about people climbing great heights around the world and experiences they have had due to altitude. Are there any stories about problems with Olympic athletes competing at elevations different than where they live? If so, share these with the class.

Summary of Activity …

In your journal or notebook, write a concise summary of this activity.

Be sure to address the following questions and use your data to support your responses.

ü  Why do clouds form at different heights?

ü  What is fog and how does it form?

ü  What mechanisms can cause air to rise and cool?

Applying what you have learned …

1. At what height would a cloud form if the temperature is 21 degrees C and the dew point is 17.8 degrees Celsius at ground level?

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@ 2007 Delaware Science Coalition/Delaware Department of Education