IT'S ALL RELATIVE [HUMIDITY]
Relative humidity is the saturation percentage of air. In the activity, students will observe the daily changes in Relative Humidity and be able to make reasonable predictions for daily weather events based on Relative Humidity readings.
Instructional Method: Experiment
Goal: To use a sling psychrometer to determine current humidity and to estimate humidity by using senses.
Objectives: Students will be able to:
- Label Relative Humidity on the hydrologic cycle map (student page).
- Identify sources of water that lead to increased Relative Humidity.
- Use current temperature, humidity and "feels like" scale to understand the current heat index.
- Observe weather events and correlate those events with classroom humidity and temperature measurements.
Explanation: 30 min.
3-5 days of data collection (10 min. each day)
Discussion: 30 min.
- Thermometer Sling psychrometer (in discovery chest)
- PDF Hydrology Worksheet
- Television (homework) or Internet (classroom)
- Graduated Cylinder
Relative Humidity (RH)
As water is evaporated and transpired into water vapor (collectively known as evapotranspiration), it is absorbed into the air. Relative Humidity (RH) is the amount of water vapor in the air relative to the maximum amount of water vapor the air can hold when completely saturated. Saturation is dependent upon the temperature of the air. Colder air cannot hold as much water as warm air, so as air cools, the vapor condenses into small water droplets. Condensation occurs when the air becomes supersaturated with water. We commonly see condensation in the form of clouds. Clouds that form at the ground level are called fog.
RH is reported as a percentage. If the air can hold 100 parts of water and if there are only 40 parts of water in the air, the RH is 40%. If the RH is recorded to be 100%, the air is saturated, and the water vapor should be condensing into droplets. When enough moisture has condensed, and the droplets have become so large that they can no longer be suspended by air currents, they fall as rain. It is important to understand that 100% relative humidity is not rain but fog. The distinction is that 100% humidity can create rain, but once the rain falls from the cloud it usually enters air of much lower humidity. For example, when thunderstorms occur in the desert, the humidity at ground level may be less than 30%. RH should never be observed as 0%, because there is always some water vapor in the air. Look at local almanacs or contact your local weather service to determine the average humidity for your geographic area.
When unsaturated warm air at a given RH cools enough, it becomes supersaturated and condenses to form water droplets. This process often happens at night when the air temperature drops and, in the morning, can be observed as dew. Dew is water vapor that has condensed out of the air at Earth's surface. Dew point is the air temperature required to saturate available water molecules in the air, thus producing condensation.
RH also has the ability to change the way we perceive temperature. A warm "muggy" day is less comfortable than a day of the same temperature with lower humidity. The heat index is what the temperature "feels like" when you take the humidity into account.
Scientists use a sling psychrometer to measure RH. A sling psychrometer is made of two thermometers, one with a dry bulb, the other with a wet bulb. As you spin the psychrometer, the dry bulb measures the air temperature. Water on the wet bulb evaporates as it spins. Evaporation requires heat energy, which makes the temperature cooler on the wet bulb thermometer. The more humidity in the air, the less water evaporates off of the wet bulb and the higher the temperature reading. If the air is very dry, most of the water evaporates off the wet bulb, and the temperature reading is much cooler. A simple math problem compares the dry bulb reading to the wet bulb reading and produces a reasonable estimate of the Relative Humidity.
In this activity we observe Relative Humidity and its relationship to current weather by observing weather conditions and recording weather data over time.
- Ask the kids what they know about humidity. What does it mean? Where does it come from? etc.
- Inform the class of today's current weather forecast.
- Introduce the sling psychrometer (enclosed in Discovery Chest) and explain how it works (see background information).
- Go outside and use a thermometer to take the current temperature and a psychrometer to take the current RH. Observe weather conditions and record them on the student page.
What is Relative Humidity? What happens to the water in the air? From where does the water in the air come? Where does it go? When was the RH the highest this week? What was the weather like when we took this high reading? Does it make a difference if the air is hot or cold? Which makes a higher RH? Where do clouds come from? Older students - What is Virga? (Virga is the evaporation of precipitation before it reaches the earth's surface, due to low relative humidity. Usually occurs in desert regions.) How does the RH effect Virga?
Fill a graduated cylinder with water and crushed ice. Ask the students to carefully measure the volume of ice and water in the cylinder. Watch as beads of condensation form on the outside of the cylinder. Ask the students from where the water is coming. Perhaps they will suspect that the cylinder is "sweating" and that the water is somehow escaping through the walls of the cylinder Ask them to measure the volume again to determine if the amount of water in the cylinder has decreased. Repeat as necessary, eventually explaining that the water that created the condensation on the outside of the cylinder was stored in the surrounding air as water vapor. It was the coolness of the cylinder that brought the immediate surrounding air below the dew point, creating dew or condensation on the outside of the cylinder. Repeat the experiment using a fan to blow air past the cylinder Ask the students if they expect more or less condensation to occur? You'll find that by keeping the air moving its harder for it to cool enough to create condensation.
Each night, have students watch the evening or morning news and keep a record of the weather forecast, RH, temperature, and heat index. Alternatively - if you have Internet access in the classroom, check the current forecast as a class before you do the following experiment to get your own humidity readings.
Included National Parks and other sites:
Utah Science Core:
Kindergarten Standard 3 Objective 1
1st Grade Standard 5 Objective 1
2nd Grade Standard 2 Objective 1,2
4th Grade Standard 1 Objective 1,2
4th Grade Standard 2 Objective 1,2,3
5th Grade Standard 1 Objective 2