Download Cloud Development and Forms

Chapter 6 Lecture
Understanding
Weather and
Climate
Seventh Edition
Cloud
Development
and Forms
Redina L. Herman
Western Illinois University
© 2015 Pearson Education, Inc.
Mechanisms That Lift Air
• When air lifts, clouds develop and precipitation can
also occur.
• There are four mechanisms that cause air to lift.
These four mechanisms are reviewed in detail in the
slides that follow.
© 2015 Pearson Education, Inc.
Mechanisms That Lift Air
• Orographic uplift: Occurs when a mass of air is
deflected over or around a terrain, usually a hill or a
mountain. This upward movement of air results in
adiabatic cooling. This promotes the development of
clouds and precipitation.
• Rain shadow: Air compresses as it descends down
the terrain and results in little to no precipitation.
© 2015 Pearson Education, Inc.
Mechanisms That Lift Air
• Frontal lifting: Occurs when two air masses
converge at the front. This can occur when cold air
advances toward warm air (cold front) or when warm
air advances toward cold air (warm front). Clouds
develop as a result of these two situations.
Cold front example
© 2015 Pearson Education, Inc.
Warm front example
Mechanisms That Lift Air
• Convergence: Occurs when there is a horizontal
movement of air into a region. When air converges
along the Earth's surface, it is forced to rise since it
cannot go downward.
© 2015 Pearson Education, Inc.
Mechanisms That Lift Air
• Localized convection: Occurs when differential heating
at the surface causes air to lift. The air expands and
cools as it lifts, causing cloud development.
© 2015 Pearson Education, Inc.
Static Stability & Environmental Lapse Rate
• Static stability: Refers to the concept that all air has
a tendency to lift.
– Unstable: Air will continue to rise if given an initial upwards
push.
– Stable: Air will resist the upward push and sinks back to
the original level.
– Neutral: Air will not rise or sink. If displaced, it will rest at
the height to which it was displaced.
• Environmental lapse rate: Refers to the rate of
decrease of temperature with elevation.
© 2015 Pearson Education, Inc.
Static Stability & Environmental Lapse Rate
• Absolutely unstable
• Absolutely stable
• Conditionally unstable
– These three types of static stability will be explained in
detail in the slides that follow.
© 2015 Pearson Education, Inc.
Static Stability & Environmental Lapse Rate
• Absolutely unstable:
This occurs when a
parcel of air is lifted and
it continues to move
upward regardless of
saturation. If the
environmental lapse rate
(ELR) exceeds the dry
adiabatic lapse rate
(DALR), the air is
absolutely unstable.
© 2015 Pearson Education, Inc.
Static Stability & Environmental Lapse Rate
• Absolutely stable: This
occurs when a parcel of
air returns to its original
location after being
displaced. If the
environmental lapse
rate (ELR) is less than
the saturated adiabatic
lapse rate (SALR), the
air is absolutely stable.
© 2015 Pearson Education, Inc.
Static Stability & Environmental Lapse Rate
• Conditionally unstable: This
occurs when the
environmental lapse rate
(ELR) is between the dry
adiabatic lapse rate (DALR)
and the saturated adiabatic
lapse rate (SALR). An air
parcel become saturated at
the lifting condensation level
(LCL) and it will become
buoyant if lifted to a critical
altitude called the level of free
convection (LFC).
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Factors Affecting the Environmental Lapse
Rate (ELR)
• The ELR can change when the following occurs:
– Heating or cooling of the lower atmosphere
– Advection of cold and warm air at different levels
– Advection of an air mass with a different ELR
© 2015 Pearson Education, Inc.
Factors Affecting the Environmental Lapse
Rate (ELR)
• Heating or Cooling of the Lower Atmosphere
– Daytime heating of the Earth’s surface occurs rapidly and
leads to a steep ELR near the surface.
– The opposite occurs at night as cooling can promote the
development of a temperature inversion (air temperatures
increase with altitude in the lower portion of the
atmosphere).
© 2015 Pearson Education, Inc.
Factors Affecting the Environmental Lapse
Rate (ELR)
• Advection of cold and warm air at different levels.
© 2015 Pearson Education, Inc.
Factors Affecting the Environmental Lapse
Rate (ELR)
• Advection of an air mass with a different ELR
(environmental lapse rate).
© 2015 Pearson Education, Inc.
Cloud Types
• Generalized cloud chart
Cloud classification will be explained in more detail in the slides
that follow.
© 2015 Pearson Education, Inc.
Clouds: Cloud Coverage and Observation
• Cloud Observation by Satellite
– (a) Visible, (b) infrared, and (c) color-enhanced infrared
satellite images
© 2015 Pearson Education, Inc.