Comprehensive Study Guide on Atmospheric Phenomena and Cloud Classification

Contributors and Document Identification

• Project Contributors and Credits: The information provided in this study guide was compiled by Environment Canada and features contributions from the following individuals:

  • Bob Stainer, Louis Blais, Marie Dallaire, Peter Elms, Wade Szilagyi, Phil Chadwick, Robert Elms, Alain Deguire, Claude Abel, Thorben Bieger, Louise Caron, Craig Machtans, and John Parker.

• Document References:

  • Catalogue no. En56-134/1999E

  • ISBN 0-662-27621-3

  • Reprint Date: 2010

  • Availability: Also available in French (*
    galement disponible en franais*).

Introduction to Cloud Observation

• Historical Context: For thousands of years, humans have forecast weather by watching the sky. Key observation factors include cloud types, movement patterns, and development cycles.

• Integrated Forecasting: A professional forecast should never be based on a single weather element alone. Recognizing various clouds and understanding the mechanics behind their growth helps in detecting impending shifts in weather patterns.

• Observation Strategy: Cloud observations should be combined with other trends for accuracy:

  • Temperature trends.

  • Wind direction.

  • Barometric pressure.

The Atmosphere and Cloud Formation Mechanisms

• The Troposphere: This is the lowest layer of the atmosphere.

  • It extends from the earth’s surface to approximately $12\,km$ above it.

  • Most weather phenomena occur within this layer.

• Conditions for Formation: Near the earth, air is comparatively warm and moist. It becomes cooler and drier as altitude increases.

  • Condensation: Occurs when warm, moist air cools, causing water vapor to condense into tiny visible water droplets.

  • Sublimation: Occurs if temperatures aloft are cold enough, causing water vapor to turn directly into ice crystals.

• Lifting Mechanisms: The cooling process required for cloud formation is usually triggered by air being lifted through one of several mechanisms:

  • Orographic Lifting: Winds driving air up a slope or a mountain.

  • Frontal Lifting: A cooler air mass undercutting a warmer air mass.

  • Turbulence: Eddying motion caused by winds blowing over rough terrain.

  • Convection: Convective currents of rising warm air resulting from the uneven heating of the earth's surface.

• Advection: This is the process where cooling of an air mass results from warm air moving over a cooler surface, leading to the formation of low clouds via contact cooling.

Classification and Naming of Clouds

• Cloud Families: Clouds are divided into four primary families:

  • Low Clouds: Categorized by the height of their bases.

  • Middle Clouds: Categorized by the height of their bases.

  • High Clouds: Categorized by the height of their bases (ranging from $6\,km$ to $12\,km$).

  • Convective Clouds: Named for the process of formation (convective currents of rising warmer air); their bases overlap both low and middle height ranges.

• Latin Nomenclature Rules: Cloud names are derived from five Latin roots:

  • Cirrus: Meaning "curl" or "lock of hair."

  • Stratus: Meaning "stretched out" or "layered."

  • Cumulus: Meaning "heap."

  • Alto: Meaning "middle."

  • Nimbus: Meaning "rain."

• General Geographic Variance: Cloud bases generally follow temperature patterns. Bases are lower in winter and Arctic regions, and higher in summer and southern regions.

High-Level Clouds (High Family)

• General Characteristics: Typically found between $6\,km$ and $12\,km$. Composition is mainly ice crystals.

• Cirrus (1):

  • Appearance: Wispy streaks.

  • Weather Indication: Often seen on good-weather days. A blanket of cirrus invading the sky (especially from the west or southwest) is the first sign of a weather change.

  • Origin: Can evolve from the upper part of thunderstorms.

  • Average Base Height: $9\,km$ in summer; $8\,km$ in winter.

• Cirrocumulus (2):

  • Appearance: Extensive sheets resembling ripples in sand.

  • Weather Indication: Often seen on the fringes of low-pressure systems.

  • Average Base Height: $9\,km$ in summer; $8\,km$ in winter.

• Cirrostratus (3):

  • Appearance: A thin veil or sheet of ice crystals covering the sky.

  • Identification: Easily identified by the halo it produces around the sun or moon.

  • Weather Indication: Indicates a large weather system is approaching; precipitation may occur within $24\,hours$.

  • Average Base Height: $8\,km$ in summer; $6\,km$ in winter.

Middle-Level Clouds (Middle Family)

• General Height range: Typically found between $2\,km$ and $6\,km$. Composition consists of droplets or ice crystals. Average base height is generally $5\,km$.

• Altocumulus (4):

  • Appearance: Neat rolls or long bands across the sky caused by an extensive layer of rising air.

  • Weather Indication: If it thickens into a solid bank moving rapidly from south or west, precipitation may fall within $12\,hours$.

• Altostratus (5):

  • Appearance: A gray or blue sheet. It is thin enough for the sun/moon to be visible but thick enough to prevent objects on the ground from casting a shadow.

  • Identification: Unlike cirrostratus, it does not produce halos. Usually evolves from cirrostratus.

  • Weather Indication: Thickening indicates precipitation is nearby.

  • Average Base Height: $2.5\,km$ (higher if the sun is visible).

• Altocumulus Lenticularis (Lenticular) (6):

  • Appearance: Shaped like discs or lenses.

  • Formation: Found downwind of mountains/hills when strong winds drive moist air over landforms, triggering a wave-like wind pattern aloft.

  • Significance: Indicates turbulence for aircraft, though looks innocuous from the ground.

Low-Level Clouds (Low Family)

• General Height Range: Bases from near the surface up to $2\,km$. Mostly droplets or ice crystals.

• Stratus (7):

  • Appearance: Featureless, thin layer with little shape or definition due to slow internal air movement.

  • Base Height: Near surface up to $450\,m$ (typically below $300\,m$).

  • Precipitation: Tiny droplets drift to earth as drizzle.

• Stratocumulus (8):

  • Appearance: Parallel rolls with flat bottoms or a merged layer of former cumulus clouds.

  • Precipitation: Weak intensity if produced from this cloud alone.

• Nimbostratus (9):

  • Appearance: Dense, gray cloud layer covering the entire sky, blocking the sun entirely.

  • Process: Great vertical extent allows droplets/crystals to collide and grow.

  • Precipitation: Produces steady, all-day rain or snow. Often leads to "ragged" clouds forming beneath it.

• Fog (10):

  • Definition: A thin layer of stratus cloud formed at ground level.

  • Conditions: Usually forms when there is little to no wind. Consists of water droplets (or ice crystals in winter) suspended in air.

Convective Clouds (Convective Family)

• General Height Range: Bases between $450\,m$ and $3\,km$. Bases overlap with low and middle heights.

• Cumulus (11):

  • Appearance: Detached white puffs with well-defined bases and little vertical extent.

  • Weather Indication: "Fair-weather" clouds. Flattened tops indicate stable air near the cloud top. In unstable air, they evolve into larger types.

• Towering Cumulus (12):

  • Appearance: Bulging cauliflower-like tops indicating strong internal currents.

  • Precipitation: Produces showers or bursts of precipitation. Rapid/early formation in the day increases the likelihood of precipitation.

• Cumulonimbus (13):

  • Appearance: Great vertical extent, dark ominous base, and a flattened top spread out like an anvil.

  • Significance: Most dangerous cloud type. Capable of producing heavy rain, high winds, hail, and tornadoes.

• Cumulonimbus Mammatus (14):

  • Appearance: Pouch-like structures forming on the underside of a cumulonimbus anvil.

  • Significance: Caused by subsiding air; themselves harmless, but the parent cloud may produce severe weather.

Precipitation and Forecasting Rules

• Rules of Thumb:

  • Diversity: The more cloud types present at once, the greater the likelihood of precipitation.

  • Indicators: Increasing east winds at the surface + falling pressure + thickening clouds from the southwest (moving from cirrostratus to altostratus) = high chance of steady precipitation.

• Precipitation Types and Cloud Sources:

  • Drizzle: Produced by Stratus, Nimbostratus.

  • Snow Grains: Produced by Stratus, Nimbostratus.

  • Rain (Continuous/Intermittent): Produced by Nimbostratus, Altostratus, Altocumulus, Stratocumulus.

  • Rain Showers: Produced by Towering Cumulus, Cumulonimbus.

  • Snow (Continuous/Intermittent): Produced by Nimbostratus, Altostratus, Altocumulus, Stratocumulus.

  • Snow Flurries: Produced by Stratus, Nimbostratus, Altostratus, Altocumulus, Stratocumulus.

  • Ice Pellets: Produced by Nimbostratus, Altostratus, Altocumulus, Stratocumulus, Towering Cumulus, Cumulonimbus.

  • Large Hail: Feature of Cumulonimbus clouds only.

Other Atmospheric Phenomena

• Rainbows: Formed when sunlight is refracted then reflected by raindrops, breaking white light into the spectrum.

  • Structure: Usually consists of a primary and secondary rainbow. The secondary rainbow features a reversed color order and is often fainter.

• Aurora Borealis (Northern Lights): Shimmering, pulsating lights in the upper atmosphere.

  • Cause: Collision between fast-moving particles from the sun and gases in the upper atmosphere. Generally seen on clear winter nights.

• Parhelia (Sundogs): Luminous spots of colored light on either side of the sun.

  • Cause: Sunlight shining through ice crystals floating in the air. Most common in cold winter months.