Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

12.1 Summarize the six processes by which water changes from one state of matter to another. For each, indicate whether energy is absorbed from or released to the environment.

12.2 Explain the relationship between air temperature and the amount of water vapor needed to saturate air.

12.3 Describe adiabatic temperature changes and explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate.

12.4 Identify four mechanisms that cause air to rise.

12.5 Explain the relationship between environmental lapse rate and stability.

12.6 Name and describe the 10 basic cloud types, based on form and height. Contrast nimbostratus and cumulonimbus clouds and their associated weather.

12.7 Identify the basic types of fog and describe how each forms.

12.8 Describe the Bergeron process and explain how it differs from the collision–coalescence process.

12.9Describe the atmospheric conditions that produce rain, snow, sleet, freezing rain, and hail.

12.10 List the advantages of using weather radar versus a standard rain gauge to measure precipitation.

Weather Wise: Lenticular Invasion

Concept Checks 

12.1 Summarize the processes by which water changes from one state of matter to another. Indicate whether heat is absorbed or released in each case. Why is evaporation called a cooling process? What is a common example of sublimation?

12.2 List three measures used to express humidity. If the amount of water vapor in the air remains unchanged, how does a decrease in temperature affect relative humidity? Define dew-point temperature.

12.3 What name is given to the processes whereby the temperature of air changes without the addition or subtraction of heat? Why does air expand as it moves upward through the atmosphere? At what rate does unsaturated air cool when it rises through the atmosphere?

12.4 What is meant by orographic lifting?How does frontal lifting cause air to rise? Define convergence. Identify two types of weather systems associated with convergence. Describe convective lifting.

12.5 Explain the difference between the environmental lapse rate and adiabatic cooling. How is the stability of air determined? Describe conditional instability. Describe the weather associated with unstable air.

12.6 What are the two criteria by which clouds are classified? Why are high clouds always thin in comparison to low and middle clouds? List the 10 basic cloud types and describe each based on its form (shape) and height (altitude).

12.7 Distinguish between clouds and fog. List five main types of fog and describe how each type forms.

12.8 Describe the Bergeron process. Explain how snow that formed high in a towering cloud might produce rain. Briefly summarize the collision–coalescence process.

12.9 Compare and contrast rain, drizzle, and mist. Describe sleet and freezing rain. Why does freezing rain result on some occasions and sleet on others? How does hail form? What factors govern the ultimate size of hailstones?

12.10 Although any open container can serve as a rain gauge, what advantages does a standard rain gauge provide? Why is weather radar useful in detecting precipitation?

Concepts in Review:  Moisture, Clouds, and Precipitation

12.1 Water’s Changes of State

Summarize the six processes by which water changes from one state of matter to another. For each, indicate whether energy is absorbed from or released to the environment.

Key Terms: calorie, latent heat, evaporation, condensation, sublimation, deposition

• Water can change from one state of matter (solid, liquid, or gas) to another at the temperatures and pressures experienced near Earth’s surface.

• The processes involved in changes of state include evaporation, condensation, melting, freezing, sublimation, and deposition. During each change, latent (hidden, or stored) heat is either absorbed or released.

QUESTION: Label the accompanying diagram with the appropriate terms for the changes of state that are shown.

12.2 Humidity: Water Vapor in the Air

Explain the relationship between air temperature and the amount of water vapor needed to saturate air.

Key Terms: humidity, saturation, vapor pressure, mixing ratio, relative humidity, dew-point temperature (dew point), hygrometer, psychrometer

• Humidity is the amount of water vapor in the air. The methods used to quantitatively express humidity include (1) mixing ratio, the mass of water vapor in a unit of air compared to the remaining mass of dry air; (2) relative humidity, the ratio of the air’s actual water-vapor content to the amount of water vapor required for saturation at that temperature; and (3) dew-point temperature.

• Relative humidity can change in two ways: (1) when the amount of moisture in the air increases or decreases or (2) when the temperature of the air rises or falls.

• The dew-point temperature (or simply, dew point) is the temperature to which a parcel of air must be cooled to the point of condensation.

• A variety of instruments called hygrometers are used to measure relative humidity.

QUESTION: Refer to the accompanying photo and explain how the relative humidity inside this house would compare to the relative humidity outside the house on a winter day.

12.3 Adiabatic Temperature Changes and Cloud Formation

Describe adiabatic temperature changes and explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate.

Key Terms: adiabatic temperature change, parcel, dry adiabatic rate, lifting condensation level (condensation level), sensible heat, wet adiabatic rate

• When air expands, it cools, and when air is compressed, it warms. Temperature changes produced by means of changes in air pressure, in which thermal energy is neither added nor subtracted, are called adiabatic temperature changes.

• When air rises, it expands and cools adiabatically. If air is lifted sufficiently, it will cool to its dew-point temperature, and clouds will develop.

12.4 Processes That Lift Air

Identify four mechanisms that cause air to rise.

Key Terms: orographic lifting, front, frontal lifting (wedging), convergence, localized convective lifting (convective lifting)

• Four mechanisms that cause air to rise are (1) orographic lifting, where air is forced to rise over elevated terrain such as a mountain barrier; (2) frontal lifting, where warmer, less-dense air is forced over cooler, denser air along a front; (3) convergence, a pileup of horizontal airflow resulting in an upward flow; and (4) localized convective lifting, where unequal surface heating causes localized pockets of air to rise because of their buoyancy.

12.5 The Critical Weathermaker: Atmospheric Stability

Explain the relationship between environmental lapse rate and stability.

Key Terms: environmental lapse rate, absolute stability, absolute instability, conditional instability, level of free convection

• Stable air resists vertical movement, whereas unstable air rises because of its buoyancy.

• The three fundamental conditions of the atmosphere are (1) absolute stability, (2) absolute instability, and (3) conditional instability.

• In general, when stable air is forced aloft, the associated clouds cover most of the sky and produce light to moderate precipitation that may continue all day. In contrast, clouds associated with unstable air are towering and frequently accompanied by heavy rain.

QUESTION: Describe the atmospheric conditions that were likely associated with the development of the towering cloud shown in the accompanying photo.

12.6 Cloud Formation

Name and describe the 10 basic cloud types, based on form and height. Contrast nimbostratus and cumulonimbus clouds and their associated weather.

Key Terms: cloud, cloud condensation nuclei, hygroscopic nuclei, nimbus, high clouds, middle clouds, low clouds, clouds of vertical development, cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, stratus, stratocumulus, nimbostratus, cumulus, cumulonimbus

• Clouds form in the atmosphere when water vapor condenses on cloud condensation nuclei. This produces tiny cloud droplets that are held aloft by the slightest updrafts.

• Clouds, visible aggregates of minute droplets of water and/or tiny crystals of ice, are one form of condensation.

• Clouds are classified on the basis of two criteria: form and height. The three basic cloud forms are cirrus (high, white, and thin), cumulus (globular, individual cloud masses), and stratus (sheets or layers).

• Cloud heights can be high, with bases above 6000 meters (20,000 feet); middle, from 2000 (6500 feet) to 6000 meters; or low, below 2000 meters. Clouds of vertical development have bases in the low height range and extend upward into the middle or high range.

QUESTION: Which of the three basic cloud forms (cirrus, cumulus, or stratus) is illustrated by each of the accompanying images, A–C?

12.7 Types of Fog

Identify the basic types of fog and describe how each forms.

Key Terms: fog, radiation fog, advection fog, upslope fog, steam fog, frontal fog (precipitation fog)

• Fog is a cloud with its base at or very near the ground.

• Fogs formed by cooling include radiation fog, advection fog, and upslope fog. Fogs formed by the addition of water vapor are steam fog and frontal fog.

QUESTION: Identify the fog type shown in the accompanying image and describe the mechanism that generated it.

12.8 How Precipitation Forms

Describe the Bergeron process and explain how it differs from the collision–coalescence process.

Key Terms: Bergeron process, supercooled, freezing nuclei, collision–coalescence process

• For precipitation to form, millions of cloud droplets must coalesce into drops that are large enough to reach the ground before evaporating.

• Two mechanisms have been proposed to explain the formation of precipitation. They are the Bergeron process, which produces precipitation from cold clouds, and the warm-cloud process called the collision–coalescence process.

12.9 Forms of Precipitation

Describe the atmospheric conditions that produce rain, snow, sleet, freezing rain, and hail.

Key Terms: rain, drizzle, mist, snow, sleet, freezing rain (glaze), hail, rime

• The two most common and familiar forms of precipitation are rain and snow.

• Sleet consists of spherical to lumpy ice particles that form when raindrops freeze while falling through a thick layer of subfreezing air. Freezing rain results when supercooled raindrops freeze upon contact with cold objects.

• Rime consists of delicate frostlike accumulations that form as supercooled fog droplets encounter objects and freeze on contact.

• Hail consists of hard, rounded pellets or irregular lumps of ice produced in towering, cumulonimbus clouds, where frozen ice particles and supercooled water coexist.

12.10 Measuring Precipitation

List the advantages of using weather radar versus a standard rain gauge to measure precipitation.

Key Terms: standard rain gauge, tipping-bucket gauge, weather radar

• Common instruments used to measure rain include the standard rain gauge, which measures the total amount of precipitation between readings, and the tipping-bucket gauge, which is automated and records both the amount and intensity of rain. The two most common measurements of snow are depth and water equivalent.

• Weather radar has given meteorologists an important tool to track storm systems and precipitation patterns, even when the storms are as far as a few hundred kilometers away.