Chapter 5: Clouds

Fog: A cloud in contact with the ground

• Fog forms when condensation begins to dorm on less active nuclei

  • Usually when the RH is closer to 100%

• Fog droplets grow in size and eventually fall to the ground

• Fog is formed in two ways:

  • Cooling the air to it’s saturation point (dew point)

  • Melting and evaporation (adding more vapor content to the atmosphere)

• Fog is maintained by:

  • Forming new “fog droplets”

  • Adding moisture to the atmosphere

    • puddles, lakes, recent rains/snow melt, etc.

Clouds

• Stratus - “Layers”

• Cumulus - “Heap”

• Cirrus - “Curl of hair”

• Nimbus - “Violent Rain”

• Nimbostratus - rain clouds that are layered

• Cumulonimbus - rain clouds that show significant vertical growth

• 4 Primary groups (height levels)

  • High-level Clouds

    • Form above 20,000 feet

    • Almost exclusively “ice crystal” clouds

    • Little moisture available - thin clouds

  • Cirrus Clouds

    • Most common high cloud

    • Thin and wispy clouds

    • Move from west to east

    • Indicate the wind direction at elevation

    • Generally fair & pleasant weather

  • Cirrocumulus Clouds

    • Small, rounded, white puffs

    • Isolated, or in rows:

      • Isolated - little piles of sand

      • Rows - rippling appearance; scales of fish

    • Seldom cover a small portion of a sky

  • Cirrostratus Clouds

    • Thin, sheet-like high clouds

    • So thin, sun & moon can shine through

    • Usually produce a “halo”

    • Form ahead of approaching storms:

      • Good predictor of rain or snow withing 12-24 hours

  • Middle-level Clouds

    • Cloud Bases: 6,500 - 23,000 feet

    • Water droplets and ice crystals

    • Thicker clouds that sometime dim the sun

  • Altocumulus Clouds

    • Gray, puffy masses - waves or bands

    • Can be an “overcast cloud”

    • Sometimes look like little “castles”

      • castellanus

    • Precursor of summer afternoon thunderstorms

  • Altostratus Clouds

    • Gray, bluish color - covers the sky

    • A “milky” look

      • sun is dimly visible

    • One big (thin) blanket

    • Won’t see any shadows

    • Forms ahead of storms producing widespread rain

  • As the clouds start to lower, the percentage of precipitation increases

  • Low-level Clouds

    • Cloud Bases: Below 6,500 feet

    • Comprised of water droplets (mostly)

    • Gloomy looking sky - often in winter

  • Stratocumulus Clouds

    • Low and lumpy clouds

    • Appear in rows or patches

    • Blue sky is sometimes visible

    • “Precip-Free” type of cloud - usually

    • Easily confused with altocumulus

  • Nimbostratus Clouds

    • Dark gray, “wet-looking” cloud

    • Associated with continuous precipitation

      • (drizzle, rain and snow)

    • Intensity of precipitation is always light

    • Very low-lying clouds

      • although their tops can exceed 10,000 feet

    • Visibility is quite poor with nimbostratus'

      • due to rain and fog

  • Scud Clouds

    • Can sometimes break apart in the wind

  • Stratus Cloud

    • Uniform, grayish cloud

    • Covers the entire sky

    • Resembles fog

      • although doesn’t touch the ground

    • Light precipitation can fall from stratus

      • usually just drizzle or flurries

  • Vertically Developed Clouds

    • Cloud bases: 1,000 - 5,000 feet

    • Cloud tops: 5,000 - 80,000 feet

    • Comprised of water and ice

  • Cumulus Clouds

    • Variety of shapes - cotton ball

    • Sharp outlines and puffy

    • Base is light and gray

    • “Detached Clouds” and have dome-like tops

    • Show slight vertical growth

      • Cumulus Humilis

    • Fair weather clouds

  • Cumulonimbus Clouds

    • Continuously growing cumulus cloud

    • This is a thunderstorm

    • Base may start at 2,000 feet

    • Tops may grow to 40,000 feet and higher

    • Vertical growth depends to updrafts and downdrafts inside the thunderstorm

    • The taller a cloud grows, the more problems the cloud will have (severe weather

    • Cool the air, condense the air, becomes a cloud

  • Anvil Clouds

    • Flat anvil top of a tall cumulonimbus cloud

  • Castellanus Cloud

    • Tower-like vertical extension

  • Lenticular - shaped like a dome that hovers over a mountain

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