Name______Hour_____ Measuring Relative Humidity and Dew Point Using a Sling Psychrometer

The amount of water vapor existing in the air is a key weather variable. Because water can absorb and release large quantities of heat energy, it is important to keep track of the amount of water vapor existing in the air. In addition to being a weather variable, since the human body is sensitive to changes in humidity, it is convenient to know the relative humidity and dew point temperature.

Purpose: Use a sling psychrometer to find the relative humidity and dew point of the air in the science classroom.

Procedure: Record all data into your science notebook 1. Dip the wet bulb wick into water keeping the dry bulb thermometer completely dry. Swing the psychrometer for the amount of time listed in the data table below. Record the temperature of both the dry and wet bulb thermometers into the table.

Time Dry Bulb Temperature (°C) Wet Bulb Temperature (°C) (sec) 5 10 more 15 more 20 more 25 more 30 more 35 more

2. Calculate the wet bulb depression. Find the lowest wet bulb temperature and subtract it from the dry bulb temperature. This resulting temperature is called the wet bulb depression. dry bulb – wet bulb = wet bulb depression

______(°C) - ______(°C) = ______(°C)

3. Use the relative humidity and dew point tables provided to identify the relative humidity and dew point temperatures of the air in the room.

Dew Point_____ Relative Humidity_____ ***Read the sections of information on dew point, relative humidity, and cloud formation before answering the following questions.

A. Explain, using your knowledge of the phase changes of water , why the wet bulb temperature dropped below the dry bulb temperature.

B. Why, in terms of heat transfer, is evaporation considered a cooling process?

C. Explain why you often feel chilly when you get out of a lake or an outdoor swimming pool?

D. Explain, using your knowledge of the phase changes of water, why the dew point temperature is called the dew point temperature.

E. Why is the dew point temperature a good temperature forecast for the overnight low temperature?

F. What does the relative humidity tell us?

G. What do you know about the relative humidity of the air if the psychrometer shows a very large wet bulb depression?

H. Explain the concept behind the Heat Index.

I. Give a detailed diagram and explanation of how a cloud forms as a result of convection.

Dew Point Temperature

Relative Humidity