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From: Monster Storms Mission 3: The Chase (pp: 52,53)
It's Not Just the Heat, It's the Dew Point!

In this lab activity, students measure dew point, then design an experiment to gather information about the relationship between dew point, air temperature, and weather.
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Dry Line Map
Map showing a dry line in the united states near which tornadoes form
A dry line often appears where dew points east of the line are above 50°F (10°C) with winds from the southeast, and dew points west of the dry line are generally below 40°F (4°C). Air temperatures also can differ east and west of the line. In the spring, temperatures east of the dry line are usually between 70° and 80°F (21° to 27°C), and temperatures west of the line are typically above 85°F (29°C). Tornadic thunderstorms often develop just east of a dry line.
When looking for tornadoes to chase, Tim Samaras needs to know about dew point. Knowing where high and low dew points exist helps define where severe storms and tornadoes are likely to occur.

Tim uses temperature and dew point data to find a dry line, a particular margin between two air masses of different characteristics.

In order for a thunderstorm to form, the air mass ahead of the dry line needs to have plenty of water vapor. This condition is indicated by a high dew point temperature. The air mass behind the dry line needs to have less water vapor. As this drier mass pushes ahead, it acts like a wedge, driving the high dew point air upward. As this vapor-rich air ascends, it cools rapidly and releases its store of energy. It is this energy transported aloft that drives the formation of the powerful storm systems.

In this lab, you will measure dew point to determine the impact this temperature has on weather. In the map shown, air temperatures appear over dew point temperatures, both of which are more commonly recorded in English units (°F) in the United States.

Materials

Lab Prep

Answer these questions before you go into the field with your dew point tool.

  1. Build your dew point tool. Practice measuring the dew point in your classroom several times. Where did the moisture on the outside of the can come from? Did you and your teammates observe the same dew point temperature?

  2. From your classroom measurements, what would you say about the water vapor content of your room? Do you have high or low humidity? What could you do to lower the amount of water vapor in your classroom? What could you do to increase it?

  3. What is the relationship between dew point and humidity?

  4. What is the relationship between the rising of warm, moist air and cloud formation?

  5. Research other types of boundaries between air masses. Collectively, these are called fronts. Complete the definition table in the Lab 1 Data Sheet for the fronts you research. Indicate and compare the characteristics of the air masses you would expect to see ahead of and behind each front.

Make Observations
  1. Use the dew point tool to measure the amount of water in the air in several locations in your school over several days. Before you take measurements, think about and answer these questions:

    1. In which places would you expect to see differences? What characteristics make you think this?

    2. What time(s) of day will be best to take measurements? Does time matter? Why?

    3. How many times and from how many places do you need to collect data before you can use your findings to make predictions? Discuss this with your mission group to decide your answers. Make sure you can justify your answers to your teacher!

  2. Design an experiment using your dew point tool, the data you collected, and a map of the school to determine where dry lines exist.

    1. What data, in addition to air temperature and dew point temperature, do you need to collect?

    2. Where do you expect to see dry lines?

  3. Design an experiment to observe the relationship (if any) between dew point, air temperature, and weather.  Observe clouds and make note of the type of weather that occurs before and after temperature and dew point changes.

    1. Decide as a class how long the experiment should be conducted.

    2. Decide as a class where you should take your measurements and make your cloud cover observations.

    3. After collecting your data, discuss changes you observed in the weather and how dew point and air temperature might be related to your observations.
student argonauts collecting weather data

Interpret Data
  1. In your school, you probably found dry lines, but you didn’t find tornadoes. Explain why thunderstorms do not form inside your school. Consider what other weather conditions are necessary for these storms to form.

  2. Why might dew point in an area change?

  3. If you observed dew point changes, how often did they occur? Why did these changes happen?

  4. How does dew point impact your life other than indicating the potential of storm formation?

  5. Tornadoes can form near fronts and dry lines. What conditions do each of these boundaries have in common that make the tornado formation possible?

   Journal Question  
You know that dew point is the temperature at which the air is saturated and that relative humidity is the percentage of how saturated the air is for a measured air temperature. Why would scientists use dew point, rather than relative humidity, to determine where storms might be found?


 

 
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