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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
 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
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
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
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, Section 2 (pg. 510-516)
 Do Section 18.2 Assessment #1-7 (pg. 516)