Transcript Cloud Formation - Cloudfront.net

Warm Up 3/14
1) Which gas is most important for understanding atmospheric
processes?
a. water vapor
c. carbon dioxide
b. oxygen
d. ozone
2) 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.
3) 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-tomoderate 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. 527528)