EarthStorm

Relative Humidity & Dew Point

Relative humidity and dew point (or dewpoint temperature) are two parameters which describe the amount of water vapor in the atmosphere. However, dew point is a substantially better indicator of the actual water content of the air. Relative humidity is dependent not only on the amount of moisture in the air, but also on the air temperature. Dew point, on the other hand, is only dependent on the amount of moisture in the air.

Objectives

Given a 24-hour graph of air temperature, relative humidity, and dew point, the student will be able to describe, for each plot, at least one reason why the variables change (or don't change) during the day.
Given scenarios which include illustrations of the dependence of relative humidity on temperature, the student will be able to identify and describe the problems with using relative humidity as the indicator of moisture in the air.
Be able to describe the advantages of using dewpoint temperature as an indicator of moisture in the air.

Notes to the teacher

Find a warm day with calm or light winds (less than 3 meters per second on average). Obtain Mesonet or ARM air temperature, relative humidity and dewpoint temperature for a 24-hour period.

PROCEDURE

1. Using WxScope, graph the air temperature and relative humidity every 15 minutes on one graph (use 0% and 100% as the minimum and maximum, respectively, on the relative humidity axis).

2. Graph the air temperature and dew point every 15 minutes on a second graph.

3. Use the same maximum and minimum temperatures on the axes for both graphs.

QUESTIONS

1. Describe how the air temperature, dewpoint temperature and relative humidity change throughout the day.

2. Does the amount of moisture in the air change throughout the day? If so, when?

3. Is relative humidity a good indicator of moisture content? Why or why not?

4. Is dew point a good indicator of moisture content? Why or why not?

5. (Optional) Can you name a situation when the relative humidity may be high, but there is little moisture in the air?

6. (Optional; Advanced) In the winter, static charge sometimes makes a spark between your finger and a piece of metal. This occurs more frequently inside a house than outside a house. Can you explain why?

PREREQUISITES

Definitions of air temperature, dewpoint temperature and relative humidity

MATERIALS

Pencil
Graph paper
Mesonet or ARM air temperature, relative humidity and dew point data every 15 minutes for a 24-hour period
Computer
WxScope software

VOCABULARY

Dew point
Relative humidity
Static charge

CORE CURRICULUM SKILLS APPLIED IN THIS LESSON

Create an appropriate graph or chart from collected data.
Interpret line graphs.
Select the most logical conclusion for given experimental data.
Recognize and describe patterns.
Select numerical observations in a given set of events.
Identify qualitative and quantitative changes given conditions before, during and after an event.
Use appropriate Systems International (SI) units to measure events.