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:
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"
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
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/AWOS: 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
