Investigating Moisture in the Atmosphere

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Objective

SWBAT explain and compare types of humidity, its relationship to dew point, the tools used to measure both, and how air temperature and humidity together impact human comfort.

Big Idea

Students make direct measurements of dew point and estimate relative humidity in this hands-on lesson.

Getting Started

Upon completion of this lab, the student should be able to:

  1. Define relative humidity, absolute humidity, specific humidity and vapor pressure.
  2. Define and explain what the dew point represents.
  3. Determine relative humidity and dew point using a sling psychrometer and psychrometric tables.
  4. Explain the relationship between air temperature and relative humidity.
  5. Explain how relative humidity affects human comfort.

Materials:

  • Sling psychrometer
  • Psychrometric tables

 

Introduction:

Moisture in the atmosphere, in its myriad forms, is one of the most important variables that must be considered in the study of the meteorology. You will investigate water vapor in the atmosphere (humidity) and will attempt to apply your findings to circumstances found in everyday life.

 

Relative Humidity Defined

Humidity is defined as the amount of water vapor in the atmosphere. It is measured in several ways.

 In one method, the mass of the water vapor is measured per unit volume, usually a cubic meter (m3). This measure of humidity is referred to as absolute humidity and reported as grams H2O/m3.

The humidity may also be expressed as a mixing ratio. The mixing ratio represents the mass of water vapor in the atmosphere measured in grams per kilogram mass (g/kg) of dry air. 

Specific humidity, yet another way to measure and report humidity, is essentially the same as the mixing ratio, differing only in that the sample mass of air includes the mass of H2O vapor contained within it.

While each of the methods just described to report humidity involve measuring the mass of H2O vapor within a sample unit of air, there is yet another way to measure the amount of water vapor in the atmosphere that utilizes a different approach. The concept of partial vapor pressure breaks out the amount of the total atmospheric pressure attributable solely to H2O vapor. For instance if the atmospheric pressure is 1000 millibars, and H2O vapor represents 3% of the mass of the air (which is manifested as air pressure) we could describe the amount of H2O vapor present by reporting the actual vapor pressure as 30 millibars.

Relative humidity, the most widely reported measure of humidity, is not a direct measure of the amount of water vapor in the atmosphere, but rather the ratio of the air's water vapor content to its water vapor capacity at a given temperature. The capacity is the amount of H2O vapor needed to produce saturation at that particular temperature.

A formula for finding relative humidity is:

amount of water vapor in atmosphere (per unit)         X 100

water vapor capacity of atmosphere (per unit) 

Elicit

5 minutes

I begin this lesson using the probe Where Did the Water Come From from Uncovering Student Ideas in Science (Volume 3) - Another 25 Formative Assessment Probes by Page Keeley, Francis Eberle, and Chad Dorsey (2008). 

In it students work to recognize that condensation comes from water vapor in the air. 

Engage

5 minutes

After students complete the probe have them share out their ideas. 

You may want to have a cold unopened can of soda sitting out in front of students so that they can see the condensation form on the out side. Also have an unopened room temperature can for comparison. 

Ask them to explain where the water on the outside comes from. They should have some ideas about water vapor in the air. 

Ask them to describe how the air around them feels on a clear, cool day versus an overcast rainy day. Depending on where you live the local weather may help you here. I live in the Northeast where weather changes are frequent and dramatic at times so have lots of opportunity to relate our learning to the atmosphere around us.

Explore

20 minutes

Since this lab involves a lot of data collection, measurements, calculations and use of data tables, I have found that that it is important to take the time to show students how to use the sling psychrometers, make the calculations, and use both the RH and Dew point tables. 

Be careful that you don't just go through the motion in this lab. Even my brightest 8th graders struggle with these concepts. 

 

NOTE: Care must be taken with these instruments, as they are very fragile and easily damaged.

 PRODECURE:

  1. Attach a 1” piece of muslin around the bulb of one of the two glass thermometers.  This will serve as your wet-bulb.  The other thermometer will serve as your dry-bulb.
  2. Soak the muslin with enough pure distilled water so that it is not dripping wet.
  3. Carefully whirl the two thermometers in the air for about 30 seconds, or until the wet-bulb temperature stops falling and remains constant.
  4. Read and record the temperature of the dry bulb and the wet bulb.
  5. Record the difference in temperature between the dry bulb and the wet bulb.
  6. On the Dew Point Chart, find the dry bulb temperature along the left column.
  7. Next find the difference between dry-bulb and wet bulb temperatures along the top row of the Dew Point chart
  8. Follow the row containing the dry bulb temperature until you reach the intersection of the column containing the dry bulb and wet bulb difference.  This is your dew point.
  9. Record the Dew Point on your chart
  10.  On the Relative Humidity Chart, find the dry bulb temperature along the left column.
  11.  Next, find the difference between dry bulb and wet bulb temperatures along the top row of the Relative Humidity chart. 
  12.  Follow the row containing the dry bulb temperature until you reach the intersection of the column containing the dry bulb and wet bulb difference.  This is your relative humidity.

Explain

20 minutes

Expect your students to struggle a bit this lesson. There are quite a few moving parts (literally and figuratively) so give yourself a couple of days to explore the content if you can. 

The calculations and graphing are important skill builders so be sure to emphasize the importance of using correct unit and labels. 

Note: In the second image below you will notice that my students added an additional column to the data table titled NODE. This was data we collected using the NODE+clima sensor. These are handheld wireless sensors that are easy to use and cost about $100 each. The Clima and other add on sensors are additional.

There is more information about the NODE and other available sensors on the Vernier website. 

Extend

15 minutes

Here are some additional questions to extend student thinking

  1. Comment on any differences noted in the relative humidity values you determined for the various locations. Can you give any explanations for the differences?
  2. How does air conditioning alter the relative humidity of a room? (Hint: What does an air conditioner do in addition to cooling the air?)
  3. How does heating a room alter the relative humidity?
  4. Can you think of any circumstances that may cause the mixing ratio in a room to change?
  5. Given the fact that the molecular weight of water vapor is less than that of dry air, what effect does increasing humidity have on the density of the atmosphere? Does this run counter to what your intuition tells you?
  6. How does relative humidity affect the comfort of people? Can you explain the physiological reasons for this?
  7. The diurnal (daily) relationship between temperature and humidity is such that the lowest humidity should occur in the afternoon hours. It is during these times, however, when it often seems to be most humid and uncomfortable. What could be the reason for this perceived inconsistency?
  8. When dew forms on outdoor objects, can it be assumed that the atmosphere is saturated? If so, why is there often no fog accompanying the dew?