Cloud Formation - Cloudfront.net
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Transcript Cloud Formation - Cloudfront.net
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)