Warm Up 3/14 Which gas is most important for understanding atmospheric processes? a. water vapor c. carbon dioxide b. oxygen d. ozone What is true about warm, saturated air? a. It contains less water vapor than cold air. b. It contains more water vapor than cold air. c. It does not contain any water vapor. d. It contains the same amount of water vapor as cold air. If the water-vapor content of air remains constant, lowering air temperature causes a(n) ____. a. decrease in relative humidity b. increase in relative humidity c. increase in evaporation d. temperature inversion Answers: 1) a. 2) b. 3) b.

Cloud Formation Chapter 18. Section 2

Air Compression and Expansion Temperature changes that happen even though heat isn’t added or subtracted are called adiabatic temperature changes When air is allowed to expand, it cools, and when it is compressed, it warms Any time a volume of air moves upward, it passes through regions of successive lower pressure Dry Adiabatic Rate – the rate of adiabatic warming or cooling in unsaturated air (1ºC/100 m) Wet Adiabatic Rate – the rate of adiabatic temperature change in saturated air; it is always less than the dry adiabatic rate

Cloud Formation by Adiabatic Cooling

Concept Check What happens to heat stored in water vapor when it is cooled to its dew point? Latent heat is released.

Orographic Lifting and Frontal Wedging Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting Orographic Lifting – mountains acting as barriers to the flow of air, forcing the air to ascend Many of the rainiest places on Earth are located on windward mountain slopes By the time the air has reached the leeward side of the mountain, much of its moisture has been lost Front – the boundary between two adjoining air masses having contrasting characteristics The cooler denser air acts as a barrier over which the warmer, less dense air rises

Orographic Lifting

Frontal Wedging

Convergence and Localized Convective Lifting Whenever air in the lower atmosphere flows together, lifting results, this is called convergence This leads to adiabatic cooling and possibly cloud formation On warm summer days, unequal heating of Earth’s surface may cause pockets of air to be warmed more than the surrounding air Consequently, this warmer, less dense packet will move upward These rising parcels of warmer air are called thermals The process that produces rising thermals is localized convective lifting When warm parcels rise above the condensation level, clouds form

Convergence

Localized Convective Lifting

Concept Check What are thermals? Thermals are rising parcels of air that are warmer than surrounding air.

Stability Stable air is any air which resists vertical movement due to density differences Stable air tends to remain in its original position, while unstable air tends to rise Air stability is determined by measuring the temperature of the atmosphere at various heights The rate of change of air temperature with height is called the environmental lapse rate Temperature Inversion – a layer where the temperature increases with height; the most stable conditions for air Clouds associated with the lifting of unstable air are towering and often generate thunderstorms and tornados

Stable Atmosphere and Adiabatic Rate

Absolute Stability

Absolute Instability

Concept Check What types of weather can result when stable air rises? Clear weather may occur or light-to-moderate precipitation.

Condensation Recall that condensation happens when water vapor in the air changes to a liquid in the form of dew, fog, or clouds For any of the forms of condensation to occur, the air must be saturated Generally, there must be a surface for water vapor to condense on Condensation Nuclei – tiny bits of particulate matter that serve as surfaces on which water vapor condenses When condensation takes place, the initial growth rate of cloud droplets is rapid It diminishes quickly because the excess water vapor is absorbed by numerous competing particles This results in the formation of a cloud consisting of millions upon millions of tiny water droplets

Comparative Diameters

Assignment Read Chapter 18 (pg. 504 – 522) Do Chapter 18 Assessment #1-30 (pg. 527-528)