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

• Describe the movement of water through the hydrologic cycle. List and describe water’s unique properties (Section 4.1).
  
  

• 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 (Section 4.2).
  
  

• Explain the relationship between air temperature and the amount of water vapor needed to saturate air (Section 4.3).
  
  

• List and describe the ways relative humidity changes in nature. Compare relative humidity to dew-point temperature (Section 4.4).
  
  

• Describe adiabatic temperature changes and explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate (Section 4.5).
  
  

• Identify four mechanisms that cause air to rise (Section 4.6).
  
  

• Explain the relationship between environmental lapse rate and stability (Section 4.7).
  
  

• List the primary factors that influence the stability of air (Section 4.8).

Water vapor is an odorless, colorless gas that mixes freely with the other gases of the atmosphere. Unlike oxygen and nitrogen—the two most abundant components of the atmosphere—water can change from one state of matter to another (solid, liquid, or gas) at the temperatures and pressures experienced on Earth. Because of this unique property, water leaves the oceans as a gas and returns to the oceans as a liquid.

Water on Earth

• Movement of Water through the Atmosphere

• Water: A Unique Substance

Water's Changes of State

• Ice, Liquid Water, and Water Vapor

• Latent Heat

  • Evaporation and Condensation

  • Sublimation and Deposition

Humidity: Water Vapor in the Air

• How Is Humidity Expressed?

  • Absolute Humidity

  • Mixing Ratio

• Vapor Pressure and Saturation

Relative Humidity and Dew-Point Temperature

• How Relative Humidity Changes

  • How Changes in Moisture Affect Relative Humidity

  • How Relative Humidity Changes with Temperature

• Natural Changes in Relative Humidity

• Dew-Point Temperature

• How is Humidity Measured?

  • Psychrometers

  • Hair Hygrometers

  • Electric Hygrometers

Adiabatic Temperature Changes and Cloud Formation

• Adiabatic Cooling and Condensation

Processes that Lift Air

• Orographic Lifting

• Frontal Lifting

• Convergence

• Localized Convective Lifting

The Critical Weathermaker: Atmospheric Stability

• Types of Stability

  • Absolute Stability

  • Absolute Instability

  • Conditional Instability

Stability and Daily Weather

• How Stability Changes

  • Solar Heating and Stability

  • Horizontal Air Movement and Stability

  • Radiation Cooling from Clouds

• Temperature Inversions and Stability