relative humidity and dew point

5/26/2018 Relative Humidity and Dew Point

1/5

Relative Humidity and Dew Point

Lab _________ Name _______________

Part 1: Relative Humidity

Latent heat of evaporation is a part of our daily lives. Every time you get out of

the shower and experience the chill that follows, is an example of evaporation and the

energy exchanges necessary for water to change state. The moisture evaporating from

your skin uses the heat energy from your body to change state from a liquid to a gas.

Relative humidity of the air directly affects the rate of evaporation. On a winter day

when the air has a much lower volume of water vapor in it, you will experience a greater

chill than on a humid summer day, even if the temperaturein the house is thesame. This

principle can be used to determine the humidity in the atmosphere by using a sling

psychrometer and comparing the wet and dry bulbtemperatures.

On hot, humid summer days there is an abundance of water vapor in the air. The

amount of water vapor the air can hold depends upon the temperature of the air, in that

warm air can hold more water vapor than cold air. When warm humid air rises, it

expands and begins to cool adiabatically. This drop in temperature causes the relative

humidity to increase. If saturation occurs and there are condensation nucleipresent,

water vapor will begin to condense, forming clouds.

The dew point temperature is a way to measure the amount of water vapor in the

atmosphere. The dew point is the temperature at which water droplets first begin to form

on a cool clean surface, or on condensation nuclei in the atmosphere. If the air is

saturated, this means that the air temperature and the dew point are one in the same. This

is stated as 100% humidity.


2/5

Objectives: Calculate relative humidity from data collected with a sling psychrometer.

Use the dew point table to convert the wet bulb depression and air temperature values todew point temperature. Relate dew point to relative humidity.

Materials

Water Sling Psychrometer Text book

Procedure

1. Measure and record the temperature inside the room by using the slingpsychrometer. Sling it until your temperature reading is precise.

2. Wet the muslin of the psychrometer with water. Sling the psychrometer over yourhead for one minute. Be sure you are aware of what and who is around you

before hand.

3. After one minute, read the wet bulb temperature. Spin the psychrometer aroundfor another thirty seconds, and again check the wet bulb temperature. If the

temperature has changed, continue to spin until a constant temperature is reached.Record this temperature on your data table.

4. Repeat this procedure 3 times.

5. Repeat steps 1-4 outside.6. Determine the relative humidity and dewpoint values for inside and out by using

the tables in your text book.


3/5

Data Table

Questions1. Compare the relative humidities inside and out. Write a general statement

expressing any similarities or differences.

_____________________________________________________________________

__________________________________________________________________________________________________________________________________________

_____________________________________________________________________

2. Why did you sling the psychrometer until you had a constant temperature on thewet and dry bulbs?

____________________________________________________________________________________________________________________________________

__________________________________________________________________

__________________________________________________________________

3. If the air temperature increased and the amount of water vapor were to remainconstant, what would happen to the wet-bulb temperature? Does this mean the

relative humidity would increase or decrease?__________________________________________________________________

__________________________________________________________________

____________________________________________________________________________________________________________________________________


4/5

4. Explain why the air in the winter seems so dry (inside and outside) compared tothe air in the summer. What can we do to overcome these uncomfortableextremes?_________________________________________________________

______________________________________________________________________________________________________________________________________________________________________________________________________

5. Describe other processes observed in your daily lives where latent heat ofevaporation can be observed.

__________________________________________________________________

__________________________________________________________________

____________________________________________________________________________________________________________________________________

6.

When air rises it cools 1 degree Celsius for every 100 meters. At what heightwould the air in the room begin to condense to form clouds? Show all of the

work.

7. Repeat #6 for the outside air. Are any clouds visible at this height?

8. What conditions and/or situation(s) would be necessary for water vapor tocondense on to your skin? (remember: what is skintemperature?)______________________________________________________

_________________________________________________________________

9. What effect does condensation have on the contents of a cold can on a hot day?When will the condensation begin to evaporate?__________________________________________________________________

__________________________________________________________________

____________________________________________________________________________________________________________________________________


5/5

10.What is meant by the term saturation? Under what conditions does the airbecome saturated?

__________________________________________________________________

____________________________________________________________________________________________________________________________________

__________________________________________________________________

Conclusions:

Based on your observations in this lab (or your own experiences), write a statement

relating Dew Point, Relative Humidity, and Temperature.

relative humidity and dew point

Documents