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Warm-Up

Warm-Up. Give an example of precipitation. Rain/snow/hail Does latent heat produce a temperature change? no What does a high dew point indicate? Moist air. Cloud Formation. Chapter 18. Section 2. Air Compression and Expansion.

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Warm-Up

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  1. Warm-Up • Give an example of precipitation. • Rain/snow/hail • Does latent heat produce a temperature change? • no • What does a high dew point indicate? • Moist air

  2. Cloud Formation Chapter 18. Section 2

  3. 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

  4. Cloud Formation by Adiabatic Cooling

  5. 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

  6. Orographic Lifting

  7. Frontal Wedging

  8. 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

  9. Convergence

  10. Localized Convective Lifting

  11. 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

  12. Stable Atmosphere and Adiabatic Rate

  13. Absolute Stability

  14. Absolute Instability

  15. 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

  16. Comparative Diameters

  17. Assignment • Read Chapter 18, Section 2 (pg. 510-516) • Do Section 18.2 Assessment #1-7 (pg. 516)

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