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Clouds

Introduction:

  • Clouds provide visible indication of the processes occurring in the atmosphere
    • While there is no real requirement for a pilot to identify every cloud type by name and classification, a basic knowledge can mean the difference between a comfortable flight, and a dangerous one
    • Much like a river, as you approach you can get a feel for currents and stability by how it looks - Clouds are no different
  • Solid clouds usually indicate frontal movement while broken clouds suggest turbulence
  • Clouds are grouped by families according to their shape, behavior, and altitudes:
  • Rain clouds contain the prefix or suffix nimbus
  • Nimbus: heavy or violent precipitation
  • Cumulonimbus: thunderstorm
  • Warmer air is more humid because it can hold more moisture
  • In order to saturate air, you must evaporate or cool it
Air Parcel Lifting Methods, COFT
Figure 1: Air Parcel Lifting Methods, COFT
Air Parcel Lifting Methods, COFT
Figure 1: Air Parcel Lifting Methods, COFT

Cloud Formation:

  • Formation requires moisture to be lifted, where it condenses at its dew-point into visual moisture
  • There are 4 lifting methods which can be remembered using the acronym: COFT [Figure 1]

  • Convergence Lifting:

    • Lifting which occurs when two airmasses converge on one another

  • Orographic Lifting:

    • Lifting which occurs when air is forced up the side of a mountain until it cools and condenses

  • Frontal Lifting:

    • Cold Front: cold air encounters warmer air where it is forced up, causing lifting
    • Warm Front: warm air encounters cooler air where it will rise above it, causing lifting

  • Thermal Lifting:

    • Thermal, also called convective lifting, occurs when the ground is heated, causing the warmer air to rise

Calculating Cloud Bases:

  • To predict general cloud bases you can perform a quick calculation:
    • Temperature minus Dew Point (°C) divided by 2 = Base of clouds
    • Temperature minus Dew Point (°F) divided by 4 = Base of clouds

Reporting of Cloud Bases:

  • Ceiling, by definition in the CFRs and as used in aviation weather reports and forecasts, is the height above ground (or water) level of the lowest layer of clouds or obscuring phenomenon that is reported as "broken," "overcast," or "obscuration"

Advisory Circular (00-45) Aviation Weather Services, Sky Cover Contractions
Figure 2: Advisory Circular (00-45) Aviation Weather Services, Sky Cover Contractions
Advisory Circular (00-45) Aviation Weather Services, Sky Cover Contractions
Figure 2: Advisory Circular (00-45) Aviation Weather Services, Sky Cover Contractions
  • Pilots usually report height values above MSL, since they determine heights by the altimeter
    • This is taken in account when disseminating and otherwise applying information received from pilots
    • "Ceiling" heights are always above ground level
    • In reports disseminated as PIREPs, height references are given the same as received from pilots, that is, above MSL
  • In area forecasts or inflight advisories, ceilings are denoted by the contraction "CIG" when used with sky cover symbols as in "LWRG TO CIG OVC005," or the contraction "AGL" after, the forecast cloud height value
  • When the cloud base is given in height above MSL, it is so indicated by the contraction "MSL" or "ASL" following the height value
  • The heights of clouds tops, freezing level, icing, and turbulence are always given in heights above ASL or MSL

Low Clouds:

  • Low clouds are considered those which extend from the surface up to about 6,500'
  • Contain water, but sometimes may contain super cooled water (icing hazard)
  • Stratus:
    • Layered
    • Form in stable air near the surface due to cooling from below
    • Form when moist stable air is lifted up sloping terrain or when warm rain evaporates as it falls through cool air
    • Restrict visibility
  • Nimbostratus:
    • Cause widespread areas of rain or snow
    • Thick
  • Stratocumulus:
    • White puffy clouds that form as stable air is lifted
    • Often form as a stratus layer breaks up or as cumulus clouds spread out
    • Generally lack enough moisture to cause rain

Middle Clouds:

  • Middle clouds range from 6,500' to ~25,000'
  • Composed of water crystals, ice crystals, or super cooled water
  • May contain turbulence and potential severe icing
  • Altostratus:
    • Flat, dense clouds that cover a wide area
    • Minimal turbulence
  • Altocumulus:
    • Form when altostratus clouds start to break up
    • Light turbulence

Advisory Circular (00-45) Aviation Weather Services, Towering Cumulus
Figure 3: Advisory Circular (00-45) Aviation Weather Services, Altocumulus

High Clouds:

  • High clouds range from 20,000' to ~50,000'
  • White or light gray color composed mainly of ice crystals
  • Seldom pose a serious turbulence or icing hazard
  • Cirrus:
    • Thin and wispy
    • Usually form above 20,000' AGL
  • Cirrostratus:
    • Form long bands or sheets
    • Moisture content is low and pose no icing hazard
  • Cirrocumulus:
    • Look like cotton
    • May produce light turbulence

Clouds With Vertical Development:

  • Very turbulent and unstable although typically fluffy in appearance giving you the false sense of "fair weather cumulus"
  • Cumulus:
    • Form in convective currents resulting from the heating of the Earth's surface
    • Flat bottoms with dome-shaped tops
    • Widely spaced cumulus clouds are called fair weather cumulus
    • Fair weather cumulus indicate some turbulence, but little icing or precipitation
  • Towering Cumulus:
    • Towering Cumulus clouds form from deep areas of unstable air which can produce moderate to heavy turbulence with icing conditions
    • Often form prior and eventually result in thunderstorms

Advisory Circular (00-45) Aviation Weather Services, Towering Cumulus
Figure 4: Advisory Circular (00-45) Aviation Weather Services, Towering Cumulus
  • Cumulonimbus:
    • More commonly called thunderstorms
    • Form in unstable air with large amounts of moisture
    • Very hazardous to flying

Advisory Circular (00-45) Aviation Weather Services, Cumulonimbus
Figure 5: Advisory Circular (00-45) Aviation Weather Services, Cumulonimbus

Others:

  • Lenticular:
    • Form on the leeward side of mountains
  • Cap Clouds
    • Form during rising air
  • Rotor Clouds:
    • Turbulence

Advisory Circular (00-45) Aviation Weather Services, Lenticular and Rotor Clouds
Figure 6: Advisory Circular (00-45) Aviation Weather Services, Lenticular and Rotor Clouds
  • Roll and Wall Clouds:
    • Occur in severe and fast moving thunderstorms

Reporting of Cloud Heights:

  • Ceiling, by definition in the CFRs and as used in aviation weather reports and forecasts, is the height above ground (or water) level of the lowest layer of clouds or obscuring phenomenon that is reported as "broken," "overcast," or "obscuration," e.g., an aerodrome forecast (TAF) which reads "BKN030" refers to height above ground level. An area forecast which reads "BKN030" indicates that the height is above mean sea level
    • AIM, Paragraph 7−1−30 , Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR), defines "broken," "overcast," and "obscuration"
  • Pilots usually report height values above MSL, since they determine heights by the altimeter. This is taken in account when disseminating and otherwise applying information received from pilots. (“Ceiling” heights are always above ground level.) In reports disseminated as PIREPs, height references are given the same as received from pilots, that is, above MSL
  • In area forecasts or inflight advisories, ceilings are denoted by the contraction “CIG” when used with sky cover symbols as in “LWRG TO CIG OVC005,” or the contraction “AGL” after, the forecast cloud height value. When the cloud base is given in height above MSL, it is so indicated by the contraction “MSL” or “ASL” following the height value. The heights of clouds tops, freezing level, icing, and turbulence are always given in heights above ASL or MSL

Reporting Prevailing Visibility:

  • Surface (horizontal) visibility is reported in METAR reports in terms of statute miles and increments thereof; e.g., 1/16, 1/8, 16, 1/4, 16, 8, 2, 5/8, 3/4, 7/8, 1, 1 1/8, etc. (Visibility reported by an unaugmented automated site is reported differently than in a manual report, i.e., ASOS/AWSS: 0, 1/16, 1/8, 1/4, 1 1/2, 1 3/4, 2, 1 3/4, 1, 1 1/4, 1 1/4, 2, 2 1/2, 3, 4, 5, etc., AWOS: M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, etc.) Visibility is determined through the ability to see and identify preselected and prominent objects at a known distance from the usual point of observation. Visibilities which are determined to be less than 7 miles, identify the obscuring atmospheric condition; e.g., fog, haze, smoke, etc., or combinations thereof
  • Prevailing visibility is the greatest visibility equaled or exceeded throughout at least one half of the horizon circle, not necessarily contiguous. Segments of the horizon circle which may have a significantly different visibility may be reported in the remarks section of the weather report; i.e., the southeastern quadrant of the horizon circle may be determined to be 2 miles in mist while the remaining quadrants are determined to be 3 miles in mist
  • When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles, certificated tower personnel will take visibility observations in addition to those taken at the usual point of observation. The lower of these two values will be used as the prevailing visibility for aircraft operations

Conclusion:

  • A reference chart can be found at the AOPA SkySpotter Major Cloud Types page

References: