1. McKnight's Physical Geography 11e
Lectures
Chapter 6
Atmospheric Moisture
Andrew Mercer
Mississippi State University
© 2014 Pearson Education, Inc.

2. Learning Goals of This Chapter
Using the hydrologic cycle, explain the transfer of water to and from the atmosphere.
Explain the nature of the water molecule.
Explain the significance of the liquidity of water.
Explain the unique qualities and significance of ice expansion.
Describe the surface tension of water and its significance.
Explain the capillarity of water and its significance.
Explain the significance of water as the "universal solvent."

3. Learning Goals of This Chapter
Explain the significance of water's specific heat.
Identify the six phase changes that can be made by water and whether they are cooling or warming processes.
Explain the role of latent heat in condensation.
Explain the role of latent heat in evaporation.
Identify the conditions that increase the rate of evaporation.
Identify the conditions that decrease the rate of evaporation.
Differentiate between evapotranspiration and potential evapotranspiration and their importance.

4. Learning Goals of This Chapter
Explain the concepts that give rise to relative humidity.
Calculate the relative humidity given the air temperature and the actual amount of water vapor in the air.
Explain the impacts on relative humidity of cooling air to the dew point temperature.
Explain the humidity impacts of cooling air below its original dew point temperature and then warming it.
Identify the factors that change relative humidity and the situations that result in these changes.
Identify the two conditions necessary for condensation to occur.

5. Learning Goals of This Chapter
Differentiate between the normal condensation processes and situations with supercooled water and the significance.
Differentiate between the dry adiabatic rate and saturated adiabatic rate.
Identify the difference between the adiabatic rates, the average lapse rate, and the environmental lapse rate.
Given the elevations, temperature, and moisture content of the air, calculate the temperatures of the air as it moves across a mountain.
Identify and describe the three cloud forms.
Identify and describe the four cloud families.

6. Learning Goals of This Chapter
Identify and describe the 10 cloud types.
Define fog, and name and describe the four types.
Differentiate between air stability, instability, and conditional stability.
Explain the stability situations that influence cloud formation and precipitation.
Describe and differentiate between the two processes that can produce precipitation.
Identify each of the types of precipitation, and describe the conditions that form them.

7. Learning Goals of This Chapter
Identify and explain the four types of atmospheric lifting.
Identify the three types of regions with high annual precipitation, and explain the atmospheric processes acting on these locations.
Identify the three regions of low annual precipitation, and explain the atmospheric processes acting on these locations.
Explain the impact of the seasonal shifting of wind belts on precipitation patterns.
Explain the formation of acid rain.
Explain the effects of acid rain on the environment.

8. Atmospheric Moisture The Nature of Water: Commonplace but Unique
The Hydrologic Cycle
Phase Changes of Water
Water Vapor and Evaporation
Condensation
Adiabatic Processes
Clouds
Atmospheric Stability
Precipitation
Atmospheric Lifting and Precipitation
Global Distribution of Precipitation
Acid Rain

9. The Nature of Water: Commonplace but Unique
Water is common on Earth’s surface, occupying 70% of the total surface.
Pure water has no taste, color, or smell.
It floats in its own melt.
It has a high heat capacity.
It can actually defy gravity by flowing upward in narrow openings.

10. The Hydrologic Cycle
Transfer of water between oceans, land surface water systems, ground water systems, and through the atmosphere

11. The Hydrologic Cycle Chemistry of water Atoms and molecules
Two hydrogen and one oxygen molecule (H2O)
Covalent bonds
Electrical polarity of water molecule
Hydrogen bonds

12. The Hydrologic Cycle Important properties of water
Exists as a liquid at most points on Earth’s surface
Expands when it freezes; less dense than liquid water; ice floats in water
Hydrogen bonding creates surface tension, a “skin” of molecules giving water a stickiness quality
Capillarity
Good solvent
High specific heat

13. Phase Changes of Water Water typically exists in three states:
Solid – ice
Liquid – liquid water
Gas – water vapor
Latent heat is required to convert water to its different phases.

14. Phase Changes of Water Phase change processes
Condensation – gas to liquid
Evaporation – liquid to gas
Freezing – liquid to solid
Melting – solid to liquid
Sublimation – solid to gas and gas to solid
Latent heat required for each process
Latent heat as a source of atmospheric energy

15. Water Vapor and Evaporation
Properties of water vapor
Colorless, odorless, invisible
Air feels sticky
Evaporation
Warmer temperatures evaporate more water
Vapor pressure
Windiness reduces evaporation
Evapotranspiration

16. Measures of Humidity Humidity – amount of water vapor in the air
Absolute humidity – mass of vapor for a given volume of air
Specific humidity – mass of water vapor for a given mass of air
Vapor pressure – contribution of water vapor to total atmospheric pressure

17. Measures of Humidity
Relative humidity – how close the air is to saturation
Saturation represents the maximum amount of water vapor the air can hold
Saturation depends on temperature
Saturation vapor pressure

18. Measures of Humidity Relative humidity – example calculation
Assume air at 20°C has 10 g of water vapor per kg of dry air
To calculate relative humidity, use the curve to get saturation conditions at 20°C (15 g/kg)
RH = (10g/15g) X 100% = 66.7%

19. Measures of Humidity
Temperature and relative humidity are inversely related (see figure)
Dewpoint temperature
Sensible temperature

20. Condensation Conversion of vapor to liquid water
Surface tension makes it nearly impossible to grow pure water droplets
Need supersaturated air
Need particle to grow droplet around, a cloud condensation nuclei
Liquid water can persist at temperatures colder than 0°C without a nuclei – supercooled

21. Adiabatic Processes Dry adiabatic lapse rate
Lifting condensation level (LCL)
Saturated adiabatic lapse rate
Parcel lapse rates versus environmental lapse rate

22. Adiabatic Processes Importance of adiabatic processes
Example of the air parcel moving up and down a mountain range
Implications on climate

23. Clouds Definition of clouds Influence on radiant energy
Classification (three primary cloud forms)
Cirrus clouds

24. Clouds
Stratus clouds (see figure)
Cumulus clouds (see figure)

25. Clouds
Can get a mix of the types as well
Cirrostratus
Nimbostratus

26. Clouds Cloud types High clouds (over 6 km)
Middle clouds (from 2 to 6 km)
Low clouds (less than 2 km)
Clouds of vertical development
Grow upward from low bases to heights of over 15 km occasionally

27. Clouds
Fog

28. Clouds Dew and frost Role of clouds on climate change
Moisture condensation on surfaces that have been cooled to saturation
Frozen dew is frost
Role of clouds on climate change

29. Atmospheric Stability
Definition of buoyancy
Stable air – parcel is negatively buoyant; will not rise without an external force
Unstable air – parcel is positively buoyant; will rise without an external force
Condition instability

30. Atmospheric Stability
Determination of stability via temperature and lapse rate
Stable (see top figure)
Unstable (see bottom figure)

31. Atmospheric Stability
Conditional instability

32. Precipitation Originates from clouds
Condensation insufficient to form rain drops
Other processes important
Collision/Coalescence – tiny cloud drops collide and merge to form larger drops

33. Precipitation Ice crystal formation Bergeron process
Ice crystals and supercooled droplets coexist in cold clouds.
Ice crystals attract vapor; supercooled drops evaporate to replenish the vapor.
Ice crystals fall as snow or rain.

34. Precipitation Types of precipitation Rain – liquid water
Snow – cloud ice crystals
Sleet – snow melted and frozen again before hitting land; ice pellets
Glaze (freezing rain) – water falls as liquid; freezes to surfaces

35. Precipitation Types of precipitation (cont.) Hail
Requires strong updrafts
Roughly concentric layers of clear and cloudy ice
Size depends primarily on amount of supercooled water in cloud and strength of updraft

36. Summary
Moisture can impact the landscape in a variety of ways, including fog, haze, and precipitation.
Water has a number of unique properties.
The hydrologic cycle shows the balance between water removed from the oceans and water returned by precipitation.
Water vapor is the gas form of water.
Evaporation rates change as surrounding atmospheric conditions change.
There are several measures of vapor content in the atmosphere.

37. Summary
Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force.
Friction slows the wind and turns it toward lower pressure.
Wind patterns around high and low pressure systems are anticyclonic and cyclonic, respectively.
Areas of divergence at the surface are associated with sinking motion, convergence at the surface with rising motion.
Close isobar spacing indicates faster winds.

38. Summary
There are several measures of vapor content in the atmosphere, called humidity measurements.
Condensation is the process by which vapor is converted to liquid.
Adiabatic processes explain changes in parcel temperature without the addition or subtraction of heat to the parcel.
Clouds are a visual identification of saturation.
Air has buoyancy associated with it that describes its stability.
Many processes are responsible for precipitation.

39. Summary There are five primary types of precipitation.
Atmospheric lifting occurs through four primary mechanisms.
The most highly variable rainfall worldwide occurs over deserts.
Tropical regions are generally wet.
Acid rain affects the Northeast and results from compounds released into the air by humans.