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

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."

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.

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.

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.

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.

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.

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

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.

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

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

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

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.

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

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

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

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

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%

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

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

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

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

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

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

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

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

Clouds Fog

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

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

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

Atmospheric Stability Conditional instability

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

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.

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

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

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.

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.

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.

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.