EVAPORATION, HUMITIY, AND FORMS OF CONDENSATION

 

I.                   General Objectives:

Explain the water cycle and its relationship to weather patterns.

 

II.                Lesson Objectives:

1)      Specify the changes in state involved in evaporation and condensation and explain why evaporation is a cooling process.

2)      Define relative humidity and show how it is measured.

3)      Demonstrate how cooling produces condensation and list some of the ways in which air becomes cooler.

4)      Describe how dew, frost, and fog form.

 

III.             Motivator:

Read “How does water disappear into the air” p.28-30 from Spencer Christian’s Weather Book (IMC)

 

IV.              Activities and Materials:

ü      Clean Hair Sample

ü      Forceps or latex gloves

ü      2 Petri dishes

ü      2 glass jars or beakers

ü      Water

ü      Paper Towel

ü      Tape

ü      Psychrometer

ü      Quiz

ü      Internet Activity: Fog in the USA

 

V.                 Procedures

1)      Water exists in all three states in the atmosphere.  Solid, snow and hail; liquid, rain; gas, water vapor.

2)      Evaporation is a cooling process.  Molecules with sufficient energy to escape the water’s surface into the atmosphere are said to evaporate. With increasing temperatures the evaporation rate increases.

3)      The amount of water vapor actually present in the air is specific humidity.  The capacity for holding water vapor roughly doubles with a rise of 11 degrees Celsius.

4)      Relative humidity is a comparison of the specific humidity to the maximum amount of water vapor the air can hold at that temperature (capacity).  It can be measured with instruments called hygrometers. 

5)      Activity: Bad Hair Day

a)      Illustrate and write a description of hair sample in journal.

b)      Divide sample into two equal parts. Place each in a separate Petri dish.

c)      Place one dish on counter or desktop and the other on a water-soaked paper towel on counter or desk.

d)      Cover both containers with a glass jar or a beaker. Place tape around bottom to seal air passages.  Illustrate set-up in journal.

e)      After 24 hours, observe hair samples. Write observation and illustrate the sample in journal.

f)        Write a formal conclusion.

g)      Analysis:  What factors were apparently involved in how humidity affects human hair? (Hint: consider: gender, race, hair color, texture, and chemical additives)

h)      Challenge: What is a hair hygrometer?

6)      Psychrometer is another form of a hygrometer.  Describe how to use the difference between wet-bulb and dry-bulb temperature to find relative humidity.  Show hygrometer and how it works.

7)      Condensation usually occurs after a warm humid sunny day cools rapidly at night.  If condenses on a surface above 0 degrees Celsius it is dew. If it condenses on a surface below 0 degrees Celsius it is frost.  For water vapor to condense it must be cooled to dew point.  This can happen in four different ways: contacting a cooler surface, radiation heat, mixing with colder air, or expanding when it rises (important in producing clouds, rain, or snow). 

8)      Needs condensation nuclei such as salt, sulfate particles, or nitrate particles for condensation to occur.

9)      Light winds mix the cold bottom air with the air a short distance form the surface.  A whole layer of air is cooled below the dew point, and fog is formed.  Advection fog results when warm moist air blows over cool surfaces.

10)   Internet Lesson: Go to http://www.usatoday.com/graphics/weather/gra/gfog/frame.htm and follow instructions on Fog in the USA handout.

 

 

VI.              Presentation Strategies:

Lecture

Laboratory

 

VII.           Vocabulary:

Evaporation- The change from liquid water to water vapor

 

Condensation- The change from water vapor to liquid water.

 

Specific Humidity- The amount of water vapor actually present in the air.  It is measured in grams of water per kilogram of air.

 

Saturated Air- Specific humidity equals air’s capacity for holding water vapor.

 

Relative Humidity- Compares specific humidity with the maximum amount of water vapor the air can hold at that temperature (its capacity).  Stated as a percent (%).

 

Hygrometer- Instrument used to measure relative humidity

 

Hair Hygrometer- An instrument that uses the principle that human hair stretches when it is humid to determine relative humidity.

 

Psychrometer- Uses the differences of temperature in wet-bulb and dry-bulb thermometer to determine the relative humidity.

 

Dew- Condensed water vapor on a surface that is above 0 degree Celsius.

 

Dew Point- The temperature at which saturation occurs.

 

Frost- Formed when water vapor condenses on a surface that is below 0 degrees Celsius.

 

Condensation Nuclei- Tiny particles on which water vapor condenses.

 

Fog- Forms when an entire layer of air is cooled below dew point and tiny droplet fill the air.

 

VIII.        Learning Strategies:

Observation

                Inference

 

IX.              Assessment:

Short quiz (see attached)

 

X.                 Extensions:

An activity involving condensation or dew point would be a nice addition to the material.

 

XI.              Resources:

 

Spaulding, N., and Namowitz, S.  (1994) Heath Earth Science. Lexington, Massachusetts: D.C. Heath and Company.

 

http://www.weather.com/education/wxclass/handouts/waterhandout1.html.  The Weather Channel.  Bad Hair Day.

 

Spencer, C. and Biracree, T. (1993) Spencer Christian’s Weather Book. New York, New York. T.D Media, Inc.