Atmospheric Moisture as a Tool for Locating the Jet Stream

Submitted by Michael Calhoun

Objectives

• Students will be able to read a psychrometer and interpret the readings.
• Students will understand how to interpret water vapor content measurements measured by infrared sensors on satellites.
• Students will gain an understanding of the relationship of water vapor high in the atmosphere to the movement of Jet Streams.

Introduction to Moisture and the Jet Stream

Moisture or water in the atmosphere causes clouds to form, fog to cover the landscape, and humidity to thicken the air. It also causes rain, snow, hail and sleet to form. The amount of water vapor in the air is referred to as relative humidity.

Relative humidity can be measured with a device called a psychrometer, which consists of a dry and wet bulb thermometer placed side-by-side as seen in the figure. A wet cloth, cotton or similar material, is placed around one of the thermometer bulbs. The relative humidity is determined by the difference between the dry and the wet thermometer. In order to accurately determine the relative humidity, you will need to use a relative humidity table or you can use an online weather calculator. (If using the online calculators, it is the fifth one down.)

The jet stream is a narrow current of air that moves around the Northern and Southern Hemispheres of the Earth in wavelike patterns. The jet stream varies from about 161 to 644 kilometers wide and 1.6 to 4.8 kilometers thick. Its strongest winds are generally encountered at about 9,144 meters in altitude.

Jet-stream winds usually have a speed of 241 to 482 kilometers per hour. Its general motion is from west to east. However, there also is a tropical easterly jet stream, which occurs during summer. This jet stream originates in the upper troposphere near Burma and extends to the west of Africa, some 10,000 kilometers to the west.

The jet stream shifts position frequently and actually migrates with the seasons. Sometimes two streams flow across the United States, one along the northern border and other toward the south.

Meteorologists use infrared sensors on satellites to measure water vapor in a layer of the atmosphere high above the Earth's surface where Jet streams are usually found. Since most weather systems in the United States are influenced by the movement of the Jet stream knowing the location and direction of movement can be an aid in forecasting the movement of weather system across the country.

Materials

Psychrometer

Internet accessible computers with network printing capabilities

LCD projector (Optional)

Teaching Procedures

1. Discuss atmospheric moisture and relative humidity as introduced above with the class. Have students predict what the current humidity reading might be (high or low) based on the weather outside then introduce the psychrometer.
2. Allow the students to measure the actual relative humidity. If a psychrometer is unavailable and/or to reinforce the science behind its use, students can also access an interactive computer simulation. This site’s Java Applet allows for the manipulation of temperature and wet bulb temperatures to see resulting precipitation patterns that will reach the ground. Among the possible precipitation types that can be produced are snowflakes, supercooled droplets, raindrops, sleet, freezing rain, and snow grains.
3. As an alternative to the use of the simulation program is an online interactive calculator for atmospheric moisture. Students can enter actual data that they recorded using their psychrometers and receive real-time humidity readings immediately.
4. Discuss with the class the influence that the Jet streams have on the movement of weather systems across the United States. You may also want to integrate a historical perspective on the lesson by discussing the influence of the jet stream in causing the Dust Bowl of the 1930s (the link explains the topic in full). Also ask the class - could knowledge of atmospheric moisture be used as a tool for locating the Jet Stream?
5. Have the students go this website to observe animated satellite images of water vapor and its relationship to the Jet stream in real time. This web resource hosts a Flash animation that points out water vapor content 6-10 km above the Earth’s surface measured by infrared sensors on satellites. Lighter areas represent high moisture content, darker areas little water vapor. Jet streams are viewed as elongated dark regions bordered by lighter sections.

Modification and Adaptations

If there are not enough computers available for an entire class use an LCD projector or a large screen computer monitor and conduct this activity as an interactive presentation.

At-risk and slow learners seem to do well with these virtual computer activities because they can easily see the results. These students should be permitted to work with their lab partners.

Conclusion and Assessment

Once all the students have completed the activity discuss with your students why or why not atmospheric moisture would make a good tool for locating the Jet Stream and by extension, be used to forecast weather system movement.

Enrichment and Reinforcement Activity

You can also learn to make your own simple instrument for measuring the relative humidity of the air here.