Problemsolving Lab

Interpret the Graph

How do you calculate relative humidity?

Relative humidity is the ratio of the actual amount of water vapor in a volume of air relative to the maximum amount of water vapor needed for that volume of air to reach saturation. Use the graph at the right to answer the following questions.

Think Critically

1. Compare the maximum amount of water vapor 1 m3 of air could hold at 15°C and 25°C.

2. Calculate the relative humidity of

1 m3 of air containing 10 g/m3 at 20°C.

3. Analyze Can relative humidity be more than 100 percent? Explain your answer.

Data and Observations

Humidity Changes with Temperature

Dew point Another common way of describing the moisture content of air is the dew point. The dew point is the temperature to which air must be cooled at constant pressure to reach saturation. The dew point is often called the condensation temperature because it is the temperature at which water vapor in air condenses into water called dew. If the dew point is nearly the same as the air temperature, then the relative humidity is high.

Latent heat As water vapor in the air condenses, thermal energy is released. Where does this energy come from? To change liquid water to water vapor, thermal energy is added to the water by heating it. The water vapor then contains more thermal energy than the liquid water. This is the energy that is released when condensation occurs. The extra thermal energy contained in water vapor compared to liquid water is called latent heat.

When condensation occurs, as in Figure 11.15, latent heat is released and warms the air. At any given time, the amount of water vapor present in the atmosphere is a significant source of energy because it contains latent heat. When water vapor condenses, the latent heat released can provide energy to a weather system, such as a hurricane, increasing its intensity.

Condensation level An air mass can change temperature without being heated or cooled. A process in which temperature changes without the addition or removal of thermal energy from a system is called an adiabatic process. An example of an adiabatic process is the heating of air in a bicycle pump as the air is compressed. In a similar way, an air mass heats up as it sinks and cools off as it rises. Adiabatic heating occurs when air is compressed, and adiabatic cooling occurs when air expands.

Investigate Dew Formation

How does dew form? Dew forms when moist air near the ground cools and the water vapor in the air condenses into water droplets.

Procedure

1. Read and complete the lab safety form.

2. Fill a glass about two-thirds full of water. Record the temperature of the room and the water.

3. Add ice cubes until the glass is full. Record the temperature of the water at 10-s intervals.

4. Observe the outside of the glass. Note the time and the temperature at which changes occur on the outside of the glass.

5. Repeat the investigation outside. Record the temperature of the water and the air outside.

Analysis

1. Compare and contrast what happened to the outside of the glass when the investigation was performed in your classroom and when it was performed outside. If there was a difference, explain.

2. Relate your observations to the formation of dew.

Evaporation-Condensation Equilibrium tttt

Water molecules begin to evaporate.