Name Class Date. Water molecules condense around the salt crystal. More water molecules condense around the original small droplet.

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1 CHAPTER 20 2 Clouds and Fog SECTION Water in the Atmosphere KEY IDEAS As you read this section, keep these questions in mind: What conditions are necessary for clouds to form? What are the four processes of cooling that can lead to the formation of clouds? What are the characteristics of the three major types of clouds? What are four ways in which fog can form? How Do Clouds Form? A cloud is a collection of water droplets or ice crystals in the air. A cloud forms when water vapor condenses around small particles in the air. These particles are called condensation nuclei (singular, condensation nucleus). The troposphere contains millions of tiny particles that act as condensation nuclei. They may be particles of dust, ice, or salt suspended in the air. READING TOOLBOX Predict Before reading this section, write each heading on a piece of paper. Below each heading, write what you think you will learn. Salt crystal Water vapor molecules Water droplet Water molecules condense around the salt crystal. More water molecules condense around the original small droplet. 1. Identify What is the condensation nucleus in this figure? In order for clouds to form, the air must be saturated with water vapor. The air is saturated with water vapor when it has reached its dew point. Remember that when the temperature drops below the dew point, water droplets can form. These droplets condense around the condensation nuclei. Therefore, in order for clouds to form, the air s temperature must drop below its dew point. There are four ways that air cools enough that clouds form: adiabatic cooling, mixing, lifting, and advective cooling. 2. Identify What are four ways that air cools enough for clouds to form? Holt McDougal Earth Science 301 Water in the Atmosphere

2 3. Explain Why does air cool when it rises? How Does Adiabatic Cooling Cause Clouds to Form? As a body of air rises, the atmospheric pressure on it decreases. Because of the lower pressure, the molecules in the body of air move farther apart. Therefore, fewer molecules collide with each other and the air becomes cooler. This process is called adiabatic cooling. The air close to Earth is warmer and less dense than the air high in the atmosphere. Therefore, the air close to Earth rises. As it rises, it becomes cooler because of adiabatic cooling. When it cools below the dew point, the vapor in the air condenses and clouds form. The altitude at which this happens is called the condensation level. 4. Identify What is the condensation level in the figure? 1,000 m 500 m 10ºC Dew point = 10ºC 15ºC Dew point = 11ºC 20ºC 0 m Dew point = 12ºC Rising, expanding, and cooling air In this figure, the temperature and the dew point are the same at an altitude of 1,000 m. Above that altitude, condensation begins and clouds form. How Does Mixing Cause Clouds to Form? Some clouds form when two bodies of moist air mix together. If the two bodies of air have different temperatures, the mixing will cause the temperature of the air to change. The mixed body of air may cool to below its dew point. When that happens, clouds form. How Does Lifting Cause Clouds to Form? When air is forced to rise, it expands and cools. If it cools to below its dew point, clouds will form. There are two main situations that force air to rise. First, a moving body of air can meet sloping ground, such as a mountain. When this happens, the air will be forced upward. Second, a body of cold, dense air can push a less dense body of air upward. When that happens, storm clouds often form. Holt McDougal Earth Science 302 Water in the Atmosphere

3 How Does Advective Cooling Cause Clouds to Form? When a body of air moves over a cold surface, the cold surface absorbs heat from the air. This causes the air to cool. If the temperature of the air falls below its dew point, clouds can form. This process is called advective cooling. How Do Scientists Classify Clouds? Clouds are classified by shape and by altitude. The three main cloud shapes are stratus clouds, cirrus clouds, and cumulus clouds. 5. Define When does advective cooling occur? STRATUS CLOUDS A stratus cloud is relatively thin and flat. The word stratus means sheetlike. Stratus clouds form when a layer of warm, moist air lies above a layer of cool air. When the warm air reaches its dew point, wide stratus clouds form. Stratus clouds cover large areas of sky and often block out the sun. Very little precipitation falls from most types of stratus clouds. However, nimbostratus clouds can cause heavy precipitation. 6. Describe What shape is a stratus cloud? CIRRUS CLOUDS A cirrus cloud looks like it is made of thin strands. Cirrus clouds form at altitudes above 6,000 m. Because the air is so cold at such high altitudes, cirrus clouds are made of ice crystals. Because cirrus clouds are thin, light can pass through them easily. 7. Infer What are the clouds in this picture made of? These wispy cirrus clouds form high in the atmosphere. Holt McDougal Earth Science 303 Water in the Atmosphere

4 CUMULUS CLOUDS A cumulus cloud has a dark bottom and a puffy top that looks like a cotton ball. The word cumulus means piled. Cumulus clouds form when warm, moist air rises and cools. As the cooling air reaches its dew point, clouds form. On hot, humid days, cumulus clouds can be very tall. High storm clouds, called cumulonimbus clouds, can form. The prefix nimbo- and the suffix -nimbus mean rain. Cumulonimbus clouds often form before a thunderstorm. Critical Thinking 8. Contrast What is the main way that these cumulus clouds are different from the cirrus clouds on the previous page? These fluffy cumulus clouds look like cotton balls in the sky. 9. Identify At what altitude would a cirrostratus cloud probably be found? ALTITUDE AND CLOUD TYPES Scientists also classify clouds by their altitude, or height. There are three main altitude groups: low clouds, middle clouds, and high clouds. Scientists use prefixes to indicate the height of a cloud. The table below summarizes the three different altitude groups. Cloud Group Altitude Prefix Example Low 0 2,000 m strato- stratocumulus Middle 2,000 6,000 m alto- altostratus High above 6,000 m cirro- cirrocumulus You can understand a lot about a cloud by its name. For example, you can divide the word cirrostratus into the prefix cirro- and the root stratus. The prefix shows that a cirrostratus cloud must have a high altitude. The stratus root shows that it is relatively thin and flat. The figure on the next page shows several different types of clouds. Holt McDougal Earth Science 304 Water in the Atmosphere

5 8,000 m Cirrocumulus Cirrostratus Cirrus Talk About It Classify With a partner, discuss the different clouds in the figure. What does the name of each cloud tell you about the cloud s properties? Use the information on the previous page to help you. 6,000 m 4,000 m Altostratus Altocumulus Cumulus 2,000 m Stratocumulus Nimbostratus Stratus Cumulonimbus Critical Thinking 10. Apply Concepts Which two clouds in the figure probably produce a lot of precipitation? Scientists classify clouds according to their altitudes and shapes. The name of a cloud can give you clues about what type of cloud it is. How Does Fog Form? Fog is similar to clouds because it forms when water vapor in the air condenses. However, fog forms very near Earth s surface. The table below describes four types of fog and how they form. Type of Fog Cause Description Radiation fog Advection fog Upslope fog The ground becomes cooler at night, so the air near the ground cools. Warm, moist air moves across a cool surface. Air rises and cools as it passes along sloping land. thickest in valleys because dense, cold air sinks thick around cities because water condenses on dust and smoke particles forms along coasts because the air over the water is cooler than the air over the land forms near mountains and other sloping land Critical Thinking 11. Connect Why might advection fog and advective cooling share the same root? Steam fog Cool air moves over a warm body of water. forms over rivers and other bodies of water Holt McDougal Earth Science 305 Water in the Atmosphere

6 Section 2 Review SECTION VOCABULARY adiabatic cooling the process by which the temperature of an air mass decreases as the air mass rises and expands advective cooling the process by which the temperature of an air mass decreases as the air mass moves over a cold surface cirrus cloud a feathery cloud that is composed of ice crystals and that has the highest altitude of any cloud in the sky cloud a collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs condensation nucleus a solid particle in the atmosphere that provides the surface on which water vapor condenses cumulus cloud a low-level, billowy cloud that commonly has a top that resembles cotton balls and a dark bottom fog water vapor that has condensed very near the surface of Earth because air close to the ground has cooled stratus cloud a gray cloud that has a flat, uniform base and that commonly forms at very low altitudes 1. Describe In your own words, describe adiabatic cooling. 2. Explain Why must the air drop below its dew point for clouds to form? 3. Describe What are two situations that can cause lifting? 4. Apply Concepts Fill in the spaces in the table to describe different types of clouds. Name Altitude Shape Precipitation? Cirrostratus no Altocumulus puffy Nimbostratus Cumulonimbus low low to middle 5. Compare How is radiation fog different from advection fog? Holt McDougal Earth Science 306 Water in the Atmosphere