LESSON 4 - ACTIVITY 2 - AIR MASSES

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Objectives
Materials
Background Information
Key Questions
Vocabulary
Procedure
Evaluation

Objectives

  1. Explain what a barometer is.
  2. Explain how wind is created.
  3. Explain why the wind changes directions at the shore.

Materials

  • small coffee can
  • plastic wrap
  • scissors
  • straw
  • index card
  • rubberband
  • 2 small margarine containers per group
  • 2 thermometers per group
  • water
  • potting soil
  • watch or clock

Background Information

Weather is influenced and created by temperature, atmospheric pressure, humidity, wind speed, cloud cover, and elevation.

Wind is air in motion which is caused by the uneven heating of the earth’s surface by the sun. Since the earth’s surface is made up of land, water, desert and forest, the surface absorbs the sun’s heat differently.

During daylight, air above land heats up more quickly than air above water. Warm air above land expands & rises, heavier, colder air moves in to take its place, creating winds. In the same way, large atmospheric winds that circle the earth are created because land near the earth’s equator is heated more by the sun than land near the North and South Poles.

Two major influences on atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis Effect). This results in atmospheric wind patterns known as prevailing winds. Prevailing winds are patterns in wind speed and direction over a specific loacation on Earth's surface. An area's prevailing wind is a pattern of air movement in the atmosphere. The prevailing westerlies are found between 30 & 60 degrees north and south, and blow from the SW in the northern hemisphere and from the NW in the southern hemisphere. These are the prevailing winds that interact with North America.

Coriolis Effect image NASA
Prevailing Westerlies image NASA
Trade Winds image NASA
Polar Easterlies image NASA

The unequal heating of earth's atmosphere also creates the jet stream which forms at areas where air masses meet which have significant temperature differences. Because of the rotation of the Earth, the jet stream flows from west to east in a wave-like manner. A jet stream is a fast moving, air current that is located about 7.5 miles high in our atmosphere. The wind speeds in a jet stream are dependent on the temperature differences between the air masses and average 35 mph in summer and 75 mph in winter, but speeds of over 250 mph are known. To be termed a "jet stream", the winds have to be over 55 mph.

Jest Stream image NASA

An air masses is categorized according to temperature & moisture. Arctic, Polar, & Tropical describe the air mass' temperature, arctic is cold & tropical is the warm. Maritime describes an air mass as being moist and continental describes dry air (as compared to Maritime). Often more than one description is combined to provide a more accurate description of the air mass. For example, polar continental would be a cold dry air mass. The name of the air mass explains how an air mass gains the attributes of the area it is over. Cold polar air masses will be found in polar regions & maritime (or marine) air masses form over areas of water. Sometimes air masses move, so a continental air mass traveling over water will gain moisture & become maritime, and tropical air masses can move north and cool becoming polar.

Air Masses image NASA

Air masses are not defined by borders, but there is a small area where air masses mix. This area of mixing is termed a weather front, and is where weather & weather changes happen. Air masses travel around & take over one another, which causes the fronts to move from one terrain type to another creating changes in the weather. The name of a weather front comes from the air mass it is taking over. A warm front is created when a warmer air mass displaces a cooler one.

Cold and warm front image NASA

Key Questions

  1. How do air pressure and temperature create wind?
  2. Why does the wind change direction at the shore?
  3. What are air masses?
  4. Where do air masses form?
  5. How are air masses different with regards to moisture and temperature?
  6. What happens when air masses collide?

Vocabulary

  • air mass
  • jet stream
  • barometer
  • air pressure

1. Students will construct a barometer to observe changes in air pressure.

Materials

  • small coffee can
  • plastic wrap
  • scissors
  • straw
  • index card
  • rubberband

Procedure

Cover the coffee can with the plastic wrap and secure it in place with the rubber band. The plastic wrap should create an air tight seal. Place the straw on the plastic wrap with approximately one-third of the straw hanging off of the edge of the can. Tape the straw to the plastic wrap near the middle of the can. Tape the index card to the coffee can behind the straw. Record the initial location of the straw on the index card. After about 15 minutes, record the new location of the straw on the index card. Recheck the straw every 15 minutes or so and keep recording the location of the straw as often as desired. Make sure to keep the coffee can away from windows and other sources of heat because your barometer is sensitive to heat.

Here's what's happening. High pressure will make the plastic wrap sink in, which will cause the straw to go up. Low pressure will cause the plastic wrap to raise up, which will cause the plastic straw to go down.

Observe your barometer when a thunderstorm is approaching. Can you use your barometer to predict a storm?

2. Students will break into groups of four or five to conduct an experiment that will demonstrate the heat retention and release properties of soil. You can use this experiment to explain why the air flow at the shore reverses its self at night. Differential heating causes land breezes & sea breezes, which are also called on-shore and off-shore winds. Water is a rapid absorber and radiator of heat, but land absorbs heat more slowly and takes longer to releases it. In areas where sea & land meet, heat that is absorbed over the day is released more quickly by water. Air that comes in contact with water cools. Over land, heat is still being released into the air, causing the warm air to rise. This convective motion pulls cool sea air towards land which replaces the warm rising air, which results in a sea breeze. During the day, the pattern is reversed. Land, cooled down from a night of radiation, will continue to absorb heat even though the water has lost its capacity to absorb heat. Warm air over water rises, which pulls cool air from land to replace it.

Materials

  • 2 small margarine containers per group
  • 2 thermometers per group
  • water
  • potting soil
  • watch or clock
  • weather maps from newspapers

Procedure

Keep the water and soil at room temperature.

  1. Distribute to each group 2 bowls, water, potting soil, and two thermometers.
  2. Assign one student in each group to be timekeeper.
  3. Put water in one bowl and potting soil in the other. Make sure that both bowls are filled to the same depth.
  4. Have your groups place one thermometer in each bowl. Wait for the mercury to stop moving and record the initial temperature of the soil and the water. Place both bowls in direct sunlight
  5. Instruct your students to create a chart of the temperatures for both the soil and water. The temperatures should be read every 4 minutes.
  6. At the 16 minute mark, have your students remove both bowls from the sunlight and record the temperatures every four minutes for 16 minutes.

Ask your students to discuss the reasons for the temperature differences. Use the background information above to explain why winds change directions at the shore.

3. Explain to your students that air masses are classified according to their temperature and moisture content. The terms Arctic, Polar, and Tropical define the temperature of an air mass with arctic being the coldest and tropical being the warmest. Maritime air is a moist air mass, whereas continental air is relatively dry. The four main air mass categories found in the United States are:

  • Polar latitudes - Found near both poles
  • Continental - Found over large areas of land
  • Maritime - Found over oceans
  • Tropical latitudes - Found near the equator

Hand out the outline map of North America.

Have your students locate the 4 air masses for North America using this image as a guide.

  1. Polar latitudes - Found near both poles
  2. Continental - Found over large areas of land
  3. Maritime - Found over oceans
  4. Tropical latitudes - Found near the equator

Have your students construct two charts. The first will be for weather predictions, the second for real-time data. Students may use the following headings:

  • Location (1-4)
  • Temperature (is the air mass hot or cold)
  • Moisture (is the air mass wet or dry)
  • Wind direction

Discuss the data collected by your students and ask them what weather predictions they made for each area. Students should be able to determine temperature and moisture content of the different air masses.

Evaluation

  1. Have your students examine a weather map from the newspaper. Ask them to identify one of the air masses, describe it in terms of temperature and moisture and indicate which direction it is traveling.
  2. Students will be able to write a brief description of air masses that affect weather in North America.

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