Transcript Understanding Weather and Climate Ch 6

Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 2. Water in the
Atmosphere
Chapter 6
Cloud Development and Forms
Introduction
Clouds are instrumental to the Earth’s energy and moisture
balances
Most clouds form as air parcels are lifted and cooled to
saturation
Mechanisms that Lift Air
Orographic Lift occurs as air is displaced over topographic
barriers such as mountains and hills
On the windward side of the barrier, air is displaced toward higher
altitudes and undergoes adiabatic cooling, possibly to saturation
On the leeward side, descending air warms through compression
leading to a dry rainshadow
Exemplified by the Sierra Nevada mountain range and the dry
eastward interior
Orographic uplift (left) and
orographically induced clouds
(below)
Frontal Lifting
When boundaries between air of unlike temperatures (fronts)
migrate, warmer air is pushed aloft
This results in adiabatic cooling and cloud formation
Cold fronts occur when warm air is displaced by cooler air
Warm fronts occur when warm air rises over and displaces cold air
A cold front (a) and
a warm front (b)
Convergence
Atmospheric mass is non-uniformly distributed over Earth
Air advects from areas of more abundant mass to areas of less
mass
Air moving into these low pressure regions converges
Stimulates rising motions and adiabatic cooling
Localized Convection
Localized surface heating may lead to spatially limited free
convection
Vertical motions are stimulated from the surface upward resulting
in towering clouds and a chance for intense precipitation over
small spatial scales
Static Stability and the Environmental Lapse Rate
Static stability refers to atmospheric conditions as they relate to
vertical air motions
An atmosphere which supports upward motions = statically
unstable
• A parcel pushed up in this condition will rise freely
An atmosphere which resists vertical motions = statically stable
• A parcel pushed up in this condition will return to the origination
point
An atmosphere that promotes neither situation = statically neutral
• A displaced parcel will simply remain at rest
Static stability is related to temperature controlled buoyancy
Positive buoyancy in a parcel = less density than surrounding
conditions, statically unstable situation
Negative buoyancy in a parcel = greater density than surrounding
conditions, a statically stable situation
Absolutely unstable, and unsaturated, air
Absolutely unstable, but saturated, air
Temperature and saturation conditions of rising parcels as they
relate to ambient air lead to absolutely unstable, absolutely stable,
and conditionally unstable atmospheres
Absolutely Unstable Air
When parcel temperature, considering the dry adiabatic lapse rate
(DALR), is greater than the ambient air, taking into account the
environmental lapse rate (ELR), positive buoyancy occurs
Vertical motions are supported throughout the atmosphere and
buoyancy increases with height
Parcel cooling rates are less than that of the ambient air
• Whether the parcel is saturated or not
Absolutely stable, unsaturated, air
Absolutely stable, but saturated, air
Absolutely Stable Air
When the ELR is less than the saturated adiabatic lapse rate
(SALR)
Negative buoyancy results as vertical motions are discouraged
No matter where a parcel is lifted, it will always be cooler than the
ambient air
Conditionally Unstable Air
When the ELR is between the DALR and the SALR
Initially the atmosphere resists vertical motions
If a parcel is forced to rise and saturation occurs, parcel cooling at
the lesser SALR will eventually create a situation where the parcel
temperature will exceed that of the ambient air
The parcel will accelerate upward under a positive buoyancy
situation
Parcel buoyancy is dependent on lifting to the level of free
convection
A conditionally unstable situation
with unsaturated air
A conditionally unstable situation
with saturated air
Factors Influencing the ELR
The ELR changes temporally and spatially as surface temperatures
change
Heating or Cooling of the Lower Atmosphere
During the day, surface insolation gains result in greater heating
near the surface than aloft
This leads to a greater ELR through the lower atmosphere
The ELR is greatest on clear days, above dry, low albedo surfaces
At night, the situation reverses as terrestrial radiation loss causes
near surface chilling = a temperature inversion
A diurnal profile of the ELR
Advection of Cold and Warm Air at Different Levels
Vertical temperature profiles may lead to varying wind direction
with height
Advection of cold or warm air at different levels promotes such
conditions and affects the ELR
Advection of an Air Mass with a Different ELR
Air masses are quantities of air with similar temperature and
moisture characteristics
Those conditions are maintained as the air mass advects to other
locations, although modifications occur
As temperatures change with the advection, so does the ELR
Advection influences on
the ELR
ELR changes with air
mass replacement
Limitations on the Lifting of Unstable Air
Rising parcels are limited by atmospheric stability and entrainment
A Layer of Stable Air
A rising parcel may reach a stable upper air environment
The parcel cooling rate will exceed that of the ambient air
The parcel will slowly cease ascension and come to rest at some
equal temperature level
Entrainment
Ambient air intrusions into parcels which limits vertical cloud
development
Causes evaporation along cloud boundaries
The evaporation process uses latent heat which cools the cloud
margins and reduces buoyancy
Extremely Stable Air: Inversions
An atypical situation when temperature increases with height
through the troposphere
A negative buoyancy situation
Develop from a variety of situations
Most commonly they relate to terrestrial radiation loss on clear,
calm nights
Near surface air chills diabatically more rapidly than air aloft
Radiation fogs are a common indicator of a radiation inversion
Frontal inversions form along cold/warm air boundaries as air
associated with a front wedges into unlike air at some angle
• Sleet and freezing rain are commonly associated with this situation
Subsidence inversions occur when air sinks toward the surface and
undergoes adiabatic warming
• Frequently found on the eastern sides of migratory or semi-permanent
high pressure areas and in lee of mountains
Profile of a frontal inversion
Profile of a subsidence inversion
Cloud Types
Occur in an unlimited variety of size, shape, and composition
Subdivided into classes based on appearance and/or height
High Clouds
Bases above 6000 m (19,000 ft)
Composed of ice
Cirrus is the most common
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Wispy appearance due to low water content and cold temperatures
Fall streaks may appear below as ice crystals descend
Mares’ tails - horizontal swirls, occur in turbulent conditions
Cirrostratus occurs when cirrus thickens and stretch across the sky
May form a halo about the sun or moon as entering light is refracted
22o by cloud ice crystals
• Cirrocumulus occurs due to thickening causing a billowy appearance
which resembles fish scales - a mackerel sky
Cloud types
Cirrus
Cirrus clouds with fall streaks
Middle Clouds
Bases between 2000 and 6000 m (6-19,000 ft)
Largely composed of liquid drops
Carry the “alto” prefix
Altostratus is typically thick enough to almost fully obscure the
sun or moon and blanket the sky from horizon to horizon
Altocumulus, typically typified by a banded arrangement of
billowy clouds
Low Clouds
Bases below 2000 m (6,000 ft)
normally composed of liquid water
Most commonly, stratus clouds which blanket large spatial areas
• Shallow vertical extent (0.5-1 km)
• Precipitation associated with nimbostratus is usually very light
• Stratocumulus are low, layered clouds with some vertical
development
Altocumulus
Stratus
Stratocumulus
Clouds with Vertical Development
High vertical velocities in air that is unstable or conditionally
unstable
Cumulus clouds occur
Cumulus humilis, or fair weather cumulus
• Develop primarily from convection
• Usually evaporate shortly after formation and are vertically limited
Cumulus congestus
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More organized development as cloud towers appear
Each tower is indicative of uplift cells
Cells are short lived but are constantly replaced
Each tower progresses higher
Cumulonimbus
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Most violent of all clouds = thunderstorms
Indicate inherently unstable conditions
From base to top may extend fully through the troposphere
Anvil top may form as ice crystals are blown horizontally
Cumulus humilis
Cumulus congestus
Formation of fair weather cumulus
Cumulonimbus
Unusual Clouds
Not easily categorized as they occur in a variety of situations
Lenticular clouds form as a result of turbulence downwind of
mountain ranges
• Exhibit a lens shape
Banner clouds are similar to lenticular but are anchored to
individual mountain peaks
Mammatus, or sack-like protrusions from the base of a cloud,
indicate low level turbulence common in cumulonimbus clouds
Nacreous clouds, composed of supercooled water or ice, are
stratosphere clouds sometimes called mother of pearl clouds
Noctilucent clouds form in the mesosphere and are typically
illuminated after sunset
Lenticular
Banner cloud
Nacreous
Noctilucent
Cloud Coverage
When clouds comprise more than 9/10th of the sky = overcast
When coverage is between 6/10th and 9/10th = broken
When coverage is between 1/10th and 6/10th = scattered
Cloud coverage less than 1/10th = clear
End of Chapter 6
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt