NAME:______Class Period:______
Introduction A cloud is a visible mass of liquid droplets or frozen crystals made of water suspended in the atmosphere above the surface of the earth. Clouds form when air containing invisible water vapor rises and cools to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling. Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which reduces its capacity to hold water vapor. If the air is cooled to its dew point and becomes saturated, excess water vapor condenses into clouds.
The altitude at which this begins to happen is called the lifted condensation level, which roughly determines the height of the cloud base. Water vapor in saturated air is normally attracted to condensation nuclei (such as salt or dust particles that are small enough to be held aloft by normal circulation of the air). If the condensation process occurs below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of ice crystals. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.
Record the cloud base altitude in the table below! (the first line is filled in already) Part I: Procedure To find the cloud base altitude: find the dry bulb and the dewpoint temperature on the x-axis of the graph to the right. Follow the solid diagonal lines up from the dry bulb temperature and the dashed diagonal lines up from the dewpoint temperature. Record the altitude (on the y-axis) where the dewpoint line crosses the dry bulb line. This is the altitude at which the dewpoint and the air temperature are the same, and condensation begins to form clouds.
Air Temp Dewpoint Temp Cloud % Rel. (°C) (°C) Diff.(°C) Base (km) Humidity 30 8 22 2.8 25% 30 14 39% 20 4 36% 30 18 49% 26 20 70% 26 24 87%
Conclusions/Questions 1. What happens to the cloud base altitude when the air temperature and the dewpoint get closer together? ______2. State the relationship between the temperature difference and cloud base altitude. ______3. This relationship is: Direct Inverse (circle the correct answer) 4. Why does the temperature in the troposphere decrease as the altitude from the earth’s surface increases? ______5. What process takes place when the dewpoint and the air temperature are equal? ______6. Use p. 12 of the ESRT to figure out the relative humidity when the dewpoint equals the air temperature. What is this value (always!): ______7. What do we call the air when the dewpoint and the air temperature are equal? ______8. Check the values for relative humidity on the data table on the first page. How does the relative humidity change as the dewpoint and the air temperature get closer to each other? ______9. Since the dewpoint and the air temperature get closer together as altitude increases in the troposphere, what happens to the relative humidity of the air as altitude increases? ______10. What happens to each of these values as the air rises in the troposphere (use p.14 of the ESRT)? a. Pressure: ______b. Temperature: ______c. Dewpoint: ______d. Relative Humidity: ______11. Why does humid air (dewpoint) decrease at a lower rate than dry air (air temperature or dry bulb temperature)? (Hint: remember as humid air rises condensation starts to happen. When we get evaporation (as in the wet bulb thermometer) the temperature decreases (energy is removed from the environment). Condensation is the opposite of evaporation and what has to happen to the environment when condensation takes place?) ______
Part II: Procedure Fill in the data table below and then graph the data on the grid to the right of the table. As the air rises, the air temperature (dry bulb temperature) decreases by 1°C for every 100 m. Start with 19 °C at the surface and subtract 1 °C as you fill in the value for Air Temperature in the table below. As the air rises, the dewpoint temperature decreases by 0.17 °C for every 100 m. Start with 10 °C at the surface and subtract 0.17 °C as you fill in the value for Dewpoint in the table below. Now create a line graph on the grid to the right of the table. The x-axis is Temperature: start with 0 and go up by 2 °C until you get the 20 °C. The y-axis is Height: start with “Surface” (which is 0 meters) and go up by 100 m until you get the 1,200 m. Label the y-axis “Height in meters” and connect your sets of values with a 2 lines. Where the lines cross, the dewpoint and the air temperature are equal and condensation (cloud formation) starts.
Height Air Temp Dewpoint (m) (°C) (°C)
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100 18.0 9.83 Surface 19.0 10.0 Temperature (°C) Questions: 1. What height did the clouds form? ______2. Suppose the lines crossed at a temperature below 0 °C. What would the clouds be made of? (Hint: most clouds consist of tiny water droplets. What happens to water below 0 °C?) ______3. Suppose the lines crossed near the earth’s surface? Clouds would form, but we don’t call this clouds. What are clouds called that you can walk through? ______4. Suppose the air temperature at the surface was 16 °C and the dewpoint was 14 °C. Use your graph to find the height at which the clouds form. ______5. What happens to the height of clouds when the dewpoint and the air temperature are close together? ______