ATMS 373 Mountain Meteorology

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ATMS 373- Mountain Meteorology

• Packet3
• Clouds and fogs

http//www.ils.unc.edu/parkproject/webcam/webcam.h
tml
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ATMS 373- Mountain Meteorology

• Outline
• Clouds and fogs
• (Chap. 7)
• Review of cloud classification
• Influence of mntns on circulations and cloud
  formation
• Clouds and fogs unique to mountainous regions

http//www.ils.unc.edu/parkproject/webcam/webcam.h
tml
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ATMS 373- Chapter 7

• (7.1) Clouds
• provide information about
• Winds
• Stability
• Moisture content, phase
• Approaching weather systems

http//are.berkeley.edu/perloff/PHOTO/clouds.htm
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ATMS 373- Chapter 7

• Clouds
• Form when
• Moist air is cooled
• Water vapor is added

http//www.comet.ucar.edu/
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ATMS 373- Chapter 7

• Clouds
• Water vapor is added
• Evaporation or sublimation of falling
  precipitation

http//www.fsplanet.com/thunderstorms3.jpg
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ATMS 373- Chapter 7

• Clouds
• Moist air is cooled
• Air moving over a cold surface
• Lifting (adiabatic cooling)
• Low pressure systems
• Flow over mountains

http//www.comet.ucar.edu/
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ATMS 373- Chapter 7

• Clouds (7.1.1) classification of clouds
• Appearance
• Altitude of bases

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ATMS 373- Chapter 7

• Clouds Appearance
• Stratiform
• Horizontal dimension much greater than vertical
  dimension
• Cumuliform
• Roughly equal horizontal and vertical dimensions

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ATMS 373- Chapter 7

• Clouds Altitude
• High (cirro-)
• Composed primarily of ice
• Middle (alto-)
• Multi-phase clouds
• Low (no prefix)
• Composed primarily of liquid water

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ATMS 373- Chapter 7

• Clouds observation challenges
• Identification
• Estimating cloud base altitudes
• Nighttime observations
• Ceilometer
• Radar
• Pilot reports

http//www.radar.mcgill.ca/data_examples_ceilo.htm
l
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ATMS 373- Chapter 7

• Clouds (7.1.2) association with fronts
• flat topography
• mountains

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter11/cf_xsect.html
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ATMS 373- Chapter 7

• Clouds association with fronts
• flat topography
• Warm front
• (1) cirrus
• (2) altostratus, altocumulus
• (3) stratocumulus, stratus, fog
• Widespread, persistent precipitation
• Showery weather if convective elements embedded
  in stratiform clouds

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ATMS 373- Chapter 7

• Clouds association with fronts
• flat topography
• Cold front
• (1) cirrus
• (2) deep cumuliform clouds (nimbostratus,
  cumulonimbus)
• Showery, sometimes heavy, precipitation

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ATMS 373- Chapter 7

• Clouds association with fronts
• Mountains
• Progression of clouds is more complicated
• Additional clouds form as air is lifted over the
  mountains
• Low clouds associated with fronts are often not
  visible from lee slopes
• Increasing sky coverage, thickening, and lowering
  of high- and middle-level clouds are best
  indicators of frontal approach in mntns

http//improve.atmos.washington.edu/improve2/IOP/w
x_images/011128-011129images/flighttracks011128_22
05.gif
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ATMS 373- Chapter 7

• Clouds (7.1.3) influence of mountains on
• circulations around pressure centers
• cloud formation

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ATMS 373- Chapter 7

• Clouds influence of mountains on
• Clouds and precipitation occur on the upslope
  flow side of high and low pressure centers
• Air parcels are cooled due to adiabatic expansion

Look up the Denver cyclone for an interesting
mountain-forced surface pressure feature
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ATMS 373- Chapter 7

• Clouds influence of mountains on
• Exact predictions of the movement of pressure
  centers are important for making accurate
  precipitation forecasts in mountain areas
• Complexity of mountain geometry makes
  precipitation forecasts challenging

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ATMS 373- Chapter 7

• Clouds influence of mountains on
• Clear skies occur on the downslope flow side of
  high and low pressure centers
• Air parcels are warmed due to adiabatic
  compression

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Several types of clouds form primarily or only in
  mountainous terrain
• Good visual indicators of mountain winds

http//www.archives.gov/research/ansel-adams/image
s/aae17.jpg
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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Foehn (Chinook) wall clouds
• Chinook arch clouds
• Rotor clouds
• Lenticular clouds
• Cap clouds
• Banner clouds
• Billow clouds
• Fracto- cumulus and stratus clouds
• Jet stream cirrus

http//www.freenaturepictures.com/pictures/mountai
ns-over-clouds.html
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ATMS 373- Chapter 7

• Clouds (7.1.4) clouds that form over mountains
• Foehn (Chinook) wall clouds
• Wall is on leeward edge of extensive cloud sheet
• Cloud base below ridge on windward side
• Appears stationary, however, air is rushing
  through it continuously
• Can persist for hours or days
• Often indicates the approach of a storm
• Strong gusty winds on both sides of mntn
• Hazard to aircraft (turbulence)

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Chinook arch clouds
• Forms in lee of long mntn barriers at the
  beginning of a foehn (or chinook) windstorm
• Extensive altostratus downwind of mntns over the
  plains
• Appears to form an arch over the mountains
  (its not golden, so there are no french fries)

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Rotor clouds
• Stratocumulus or altocumulus cloud that forms
  downwind of a mntn barrier in the crest of a
  large-amplitude atmospheric wave
• Indicates strong winds and turbulence
• Form on a line parallel to the mntn barrier with
  bases near mntn-top level

http//rst.gsfc.nasa.gov/Sect14/Sect14_1d.html
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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Rotor clouds
• Appear to rotate about a horizontal axis parallel
  to the mntns
• Significantly higher winds at the top of the
  cloud
• Turbulence is a severe hazard to aircraft
• Glider demolished during Sierra Wave Project

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Lenticular clouds
• Source of first UFO sighting
• Quasi-stationary, forming in the crests of waves
  in the atmosphere
• Can form at any height
• Can form stacks of lenticular clouds
• Air flows continuously through a lenticular cloud
  (see figure to right)
• Condensation path B to D
• Evaporation path D to F

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Lenticular clouds
• Indicate moisture in moderately stable to
  strongly stable layers
• Associated with strong winds, especially near
  mntn crests
• Changes in cloud appearance indicate changing
  atmospheric moisture or the movement of the
  weather system

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Cap clouds
• A lenticular cloud that forms over a mntn peak
  with the cloud base below the peaks summit
• Formation and maintenance similar to lenticular
  clouds
• Common in maritime mntn ranges
• Often precursors of moist air masses approaching
  the mntn

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Banner clouds
• Plumes that form downwind from upper lee slopes
  of isolated, steep-sloped mntn peaks with sharp
  ridges
• Serve as a natural wind vane
• Sometimes the only cloud present on a fine
  weather day

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Banner clouds
• Air flow around peak produces low pressure on lee
  side, somewhat below the summit
• Upward vertical motion on lee slope
• Cloud forms if there is sufficient moisture
• Evidence that leeside horizontal eddies are often
  present (birds, snow plumes)
• Common in lee of Matterhorn (not the one found in
  DisneyWorld) and in the coastal ranges of the U.S.

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Billow clouds
• Form when vertical wind shear occurs across a
  sharp change in temperature in a cloudy
  atmosphere
• Wind shear may cause waves to curl over and break
  (like ocean waves)
• Kelvin-Helmholtz waves atmospheric waves in
  which billow clouds form
• Can also form in absence of moisture
• Can cause clear air turbulence
• Hazard to aircraft

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Billow clouds
• Most frequently superimposed on lenticular clouds
• Sometimes propagate across the top of lenticular
  clouds (see figure to right)

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Fracto- cumulus and stratus clouds
• Ragged clouds, small and irregular in appearance
• Produced by condensation and mixing in
  near-saturated air after rainstorms
• Often the remnants of clouds that are torn
  apart by turbulence in the vicinity of terrain
  obstacles
• Good indicators of strong turbulent winds and
  instability that may be hazardous to aircraft

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ATMS 373- Chapter 7

• Clouds clouds that form over mountains
• Jet stream cirrus
• Indicate
• significant wind shear
• presence of jet stream
• Wind shear is evident in the distortion of
  fallstreaks when they are present

Note the description in the textbook of various
types of optical effects often present in
orographic clouds (coronas, iridescence, halos,
and Brocken specter)
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ATMS 373- Chapter 7

• (7.2) Fogs
• clouds based at the ground

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ATMS 373- Chapter 7

• Fogs
• Heavy fog occurs most frequently on coastlines or
  in coastal mountainous areas where good moisture
  sources are present
• East side of
• Appalachians (NC, VA, WV)
• West side of
• Cascades (OR)
• Sierra Nevada (CA)

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ATMS 373- Chapter 7

• Fogs, types
• Upslope
• Radiation
• Advection
• Evaporation-mixing
• Ice
• All types can significantly reduce visibility and
  pose a hazard to all forms of transportation

http//www.victoriaweather.ca/clouds.php?imagefog
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ATMS 373- Chapter 7

• Fogs, types
• Upslope
• Develops when moist air is cooled by being lifted
  up terrain slopes
• Common following rainstorms
• Often accompanied by strong solar insolation ?
  evaporates water from the moist ground (producing
  evaporation-upslope fog)
• Fog tendrils result with non-uniform moisture
  sources are perturbed winds
• Occurs occasionally over the gentle slope of the
  Great Plains

http//www.dask.org.tr/english/want_to_know/meteor
ology/weather/weather.htm
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ATMS 373- Chapter 7

• Fogs, types
• Radiation
• Develops when nighttime outgoing longwave
  radiation cools the near-surface air below its
  dew-point temperature
• Common in valleys and basins of continental
  mountainous areas
• Top of fog deck marks the bottom of a nocturnal
  temperature inversion

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ATMS 373- Chapter 7

• Fogs, types
• Radiation
• Most frequent at sunrise
• Nighttime cooling is at a maximum
• Relative humidity reaches its maximum value
• Intermediate dissipation stage where fog lifts
  to become a stratus cloud deck
• Rising wind speeds
• Solar heating of the slopes

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ATMS 373- Chapter 7

• Fogs, types
• Advection
• Develops when warm, moist air flows over a cold
  surface and cools from below until saturation is
  reached
• Common along the Pacific Coast in the U.S. during
  the summer

http//www.digitalfieldguide.com/blog/776
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ATMS 373- Chapter 7

• Fogs, types
• Evaporation-mixing
• Develops when water evaporates and mixes with the
  adjacent air, raising the mixtures dew point
• Steam fog is a type of evaporation-mixing fog
• Vertical extent ? mixing results from atmospheric
  instability and evolution of buoyant plumes
• Allows one to see your breath on cold days

http//www.uwsp.edu/geo/faculty/ritter/images/atmo
sphere/clouds/fog_on_southern_lake_NOAA_wea00153.j
pg
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ATMS 373- Chapter 7

• Fogs, types
• Ice
• Consists of small ice crystals that form in
  extremely cold air (air temperatures below -20oF)
• Form frequently in valleys that have open sources
  of water vapor
• Commonly observed in the Fraser Plateau, the
  Yukon Plateau, the Northwest Territories, and in
  the interior of Alaska

http//www.iarc.uaf.edu/gallery/main.php?g2_viewc
ore.ShowItemg2_itemId1417