Ch-5, Part-5: Clouds and Fog

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FORMATION OF CLOUDS

• Composition of clouds:
  • Clouds are formed of liquid droplets if the air is above the freezing point
  • Clouds may comprise ice crystals is the air is at or below the freezing point
  • This occurs always with very high elevation cirrus clouds
  • Clouds mixed with liquid droplets and ice crystals at the same time can exist under certain conditions
• Formation of clouds
  • Condensation nuclei are very tiny salt particles and dust
  • Cloud particles are formed when moisture condenses into the liquid state around condensation nuclei in the atmosphere
  • Cloud particles can grow larger to form precipitation droplets

Fig 5-17: Condensation nuclei, Cloud droplets and a typical raindrop

TYPES OF CLOUDS; CLASSIFICATION

• Clouds are classified and named according to their form (shape), (shape), (shape), height, and precipitation activity/potential [F 5.18, P 152, below]  
  • Wispy = cirriform: cirrus; high clouds
  • Layered = stratiform: stratus
  • Nimbo- and -nimbus = precipitating
  • Clouds of vertical development = cumuliform
  • Middle elevation clouds have the prefix alto- appended to their names

Fig 5-18:  Cloud types

EXAMPLES OF CLOUDS

Cirriform = high, wispy, Sun shines through

• Composed of ice crystals -- very high; very cold; little moisture available to make clouds
• Examples
  • Cirrus  
    • Thin and wispy
    • The sun shines through
    • Made of ice crystals

Fig 5-18b:  Cirrus clouds

• Cirrostratus is high, somewhat layered

Fig 5-18c:  Cirrostratus Clouds

Alto = middle altitude clouds

• Altostratus -- middle altitude, layered clouds 

Fig 5-18g:  Altostratus clouds

Stratiform = layered clouds, much wider than high, at various altitudes

• Stratus is low altitude and layered
• Nimbostratus: precipitating low, layered  (nimb = precipitating)
• Stratocumulus  

Fig 5-18f: Stratus clouds

Fig 5-18e:  Nimbostratus clouds

Cumuliform -- clouds of vertical development; taller than wide

• Cumulus = "fair weather" clouds of summer

Fig 5-18h:  Cumulus clouds

• Be sure to study the Figure, below
• Cumulonimbus clouds may extend into the tropopause and the bottom of the stratosphere
• The cloud has strong updrafts and downdrafts
• Hail is produced by being repeatedly carried aloft by updrafts into below freezing air
• A single "thunderhead" cloud often has an anvil-shaped top
• Rain can be quite intense and quite localized.  It does not make broad regional coverage from a single cumulonimbus cloud
• Lightning often occurs with cumulonimbus clouds
• Tornadoes are associated with cumulonimbus clouds in regions where tornadoes occur
• Hurricanes contain many cumulonimbus clouds

Fig 5-19b:  Massive thunderhead Galveston Bay, Texas

• Multiple, contiguous cumulonimbus "thunderhead" clouds

Fig 5-18d:  Cumulonimbus "build up"

FOG

• Fog = clouds at the surface
  • Radiation fog
    • Temperature inversions produce radiation fog
    • On otherwise clear nights, longwave radiation can cool the surface very rapidly
    • Hence the air near the cold surface is also cooled
    • If the air is cooled to the saturation point, fog forms
    • Radiation fogs may be quite thin, vertically
    • It may be possible to see the Moon through the fog
    • Morning Sun can "burn off" the fog, warming the fog and surface above the 100% humidity temperature

Fig5-22: Radiation fog, central California

• Evaporation fog
  • Evaporation fogs occur when a body of water evaporates into a cold layer of air
  • The water vapor raises the humidity of the air immediately above to the saturation point, producing for
  • One place to sometimes see evaporation fogs is over the City of Moscow Sewage Treatment Plant, about a mile west of the U.I. campus
  • Evaporation fogs are very thin and wispy

Fig 5-00:  Evaporation fog

• Sea fog
  • Sea fogs occur when moist air moves over a cold ocean current
  • One example is along the coast of California

   

• Advection fog
  • This type of fog occurs when warm moist air moves horizontally over a cold, underlying surface
  • One case occurs when cold and warm ocean currents move side by side

Fig 20:  Advection fog

• Cold air settles in valley bottoms and chills to the dew point, forming fog. 
• Remember, colder air is heavier and flows where gravity will permit it to lower its elevation.

Fig 5-21:  Valley fog, Appalachian Mountains

PRECIPITATION

• Precipitation -- what is it?
  • Rain
    • Drops of water falling from the sky to the surface
  • Snow
    • Snow flakes are not frozen rain
    • Snow flakes are agglomerations of tiny, six-sided ice crystals
  • Sleet
    • Sleet is rain which freezes as it falls through a cold layer of air before it strikes the surface
  • "Ice storm"
    • Ice does not fall out of the sky!
    • This is preceded by a very cold layer of air chilling the surface well below freezing
    • Next, a warmer, moist layer of air moves in with rain
    • As the rain strikes the cold surface, it freezes, coating everything in an increasingly thick layer of ice
    • This provides great danger for both foot and motor vehicle traffic
    • Utility wires and tree limbs often break under the weight of the ice, made worse if winds accompany
  • Hail
    • Hail is made of concentric layers of ice, much like the structure of an onion
    • The layers build up in successive updrafts within a cumulonimbus cloud, which raise the growing stone upwards into below freezing temperatures
  • Rime
    • This is an accumulation of ice on objects such as trees and structures
    • This especially can occur on chilled mountain peaks when a moist cloud enshrouds them
    • The cloud moisture is chilled below freezing by the cold objects

• Dew
  • Dew is liquid water which condenses out of the air against cold surfaces
  • Examples are dew on the grass, on the tops and windows of motor vehicles
  • These surface chill rapidly via longwave radiation at night
• Frost
  • Frost is a formation of ice crystals on a cold surface
  • Frost develops like dew except the surface temperature is below freezing
  • Frost is not frozen dew
  • Frost forms directly out of water vapor by the process of deposition

Measuring precipitation

• Rain gauge  
• Snow depth and water equivalent
  • Snow is often measured in terms of its depth
  • To add snowfall to precipitation totals, the snow must be converted to its water equivalent, what it would be if melted
• Isohyets -- lines of equal precipitation
  • Maps of precipitation are constructed by drawing "contour" lines depicting annual totals
  • The lines are called isohyets, where iso- means "equal"

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