Reading a cloud report
Written from
Joshi 8, Cloud Amount, Cloud Ceiling, Reporting
Oxford Vol 9, 12, Cloud Amount to Measurement
Clouds are the signposts in the sky. Before naming them, the exam wants you to read the numbers in a report: how much cloud, how low it sits and how that height is measured.
Cloud amount, in oktas
The sky is split into eight parts and the cover is counted in oktas, eighths.
| Oktas | Code | Meaning |
|---|---|---|
| 0 | SKC | Sky clear |
| 1 to 2 | FEW | Few |
| 3 to 4 | SCT | Scattered |
| 5 to 7 | BKN | Broken |
| 8 | OVC | Overcast |
Base and ceiling
- Cloud base is the lowest level where the sky stops looking like clear air haze and starts looking like water droplets or ice crystals. It is reported above aerodrome level in a METAR or TAF, but above mean sea level on an area forecast chart.
- Cloud ceiling is the height of the lowest layer of more than 4 oktas, 5 eighths or more, below 20 000 ft. It is also called the main cloud base.
- A cloud of operational significance has its base below 1500 m, about 5000 ft, or below the highest minimum sector altitude, whichever is greater.
How the base is measured
By day, a balloon of known rate of ascent is timed to the point it enters cloud. By night, an alidade reads the angle of a searchlight spot on the cloud from a known distance. The modern instrument is the laser ceilometer, which times a light pulse to the base and back and is the most accurate. Cloud movement is read with a nephoscope, and cloud tops are estimated from the lapse rates and, more reliably, from pilot reports.
The three forms and the three levels
Written from
Joshi 8, Classification, Altitude of the Cloud Genera
Oxford Vol 9, 12, Cloud Classification, Cloud Height Bands
There are ten genera of cloud. They are sorted first by form and then by the level they form at, and the two prefixes and one suffix tell you most of what you need.
Form
- Stratiform, layered. Wide, shallow, comes from stable air.
- Cumuliform, heaped. Marked vertical extent, comes from unstable air.
- Cirriform, fibrous and wispy. Ice only, high level only.
Level, and the naming
| Level | Temperate band | Prefix | Genera |
|---|---|---|---|
| High | 16 500 ft to the tropopause | cirro | Cirrus, cirrostratus, cirrocumulus |
| Medium | 6500 to 23 000 ft | alto | Altostratus, altocumulus, and nimbostratus |
| Low | Surface to 6500 ft | none | Stratus, stratocumulus, cumulus, cumulonimbus |
The band is higher in the tropics, high cloud to 18 km, and lower over the poles. Nimbo at the start or nimbus at the end means rain bearing.
Clouds that break the level rule
Cumulus and cumulonimbus build from low level right up to the tropopause. Nimbostratus spans the low and medium levels. Everything else stays in its band.
Interactive Where each cloud sits
FormStratiform
LevelLow
Made ofWater droplets
IcingLight to moderate
The low and medium clouds
Written from
Joshi 8, Low Clouds, Medium Clouds, Clouds with Vertical Development
Oxford Vol 9, 12, Stratus to Altostratus
| Cloud | What it is | Main hazard |
|---|---|---|
| Stratus, St | Grey layer with a uniform base, lowest of all clouds. On the ground it is fog | Poor slant visibility, hides high ground, drizzle |
| Stratocumulus, Sc | Grey or white rolls and patches, usually 1000 to 4500 ft base | Light turbulence and icing, drizzle |
| Nimbostratus, Ns | Dark, dense, rain bearing sheet, spans low and medium levels. Found at the warm front | Continuous rain or snow, poor visibility, moderate to severe icing |
| Cumulus, Cu | Heaped, sharp edged, cauliflower tops. Marks thermals and vertical motion. Fair weather Cu peaks about 1500 | Turbulence below the base, smooth above the tops |
| Cumulonimbus, Cb | Vast convective tower to the tropopause, often with an anvil. Made of water droplets, supercooled water and ice crystals | Everything: severe turbulence and icing, hail, lightning, squalls, microbursts. Avoid, and stay 10 to 20 km clear |
| Altocumulus, Ac | Speckled white or grey patches at medium level. Lenticularis marks mountain waves, smooth inside but turbulent nearby. Castellanus, turret shaped, marks medium level instability | Light to moderate turbulence and icing |
| Altostratus, As | Grey or bluish medium level sheet, gives a ground glass sun, produces the corona | Continuous rain or snow, icing below freezing. Flight in it is usually smooth |
Where students lose the mark
Hail can fall in clear air below and downwind of a cumulonimbus anvil, not just directly under the cloud. Fair weather cumulus is benign to look at but the air below its base is turbulent, which is the answer to what it indicates.
Interactive A cumulus growing into a cumulonimbus
StageCumulus humilis
Also calledFair weather cumulus
Vertical extentShallow
HazardTurbulence below the base
The high clouds, contrails and the hazard summary
Written from
Joshi 8, High Clouds, Condensation Trails, Mintra Level
Oxford Vol 9, 12, Cirrus to Cirrocumulus and the hazard table
The high clouds
| Cloud | What it is | Notes |
|---|---|---|
| Cirrus, Ci | White fibrous filaments and hooks, ice crystals only, the highest cloud | No icing, no precipitation. Often heralds a warm front. Wind blown streaks mark a jet stream |
| Cirrostratus, Cs | Thin whitish veil of ice crystals over the whole sky | Produces a halo round the sun or moon. Heralds a warm front, or a tropical revolving storm in the tropics |
| Cirrocumulus, Cc | Thin white rippled patches of ice crystals, rarely seen | Light turbulence, no icing |
Optical clues
A halo means cirrostratus, ice crystals, no icing. A corona means altostratus or altocumulus, water droplets. To dodge icing in cloud, aim for the one showing a halo.
Contrails
- Wingtip trails, thin and short lived, from the pressure drop over the wing.
- Exhaust trails, long and persistent, from about 9 km up, revealing an aircraft's track.
- Distrails, a clear lane cut through thin cloud by the heat of the exhaust.
- The Mintra level is the lowest level at which contrails form, roughly where the temperature is about −45°C at 100 per cent relative humidity for a jet.
Cumulus development stages
Fractus, then humilis, fair weather cumulus, then mediocris, then congestus, also called towering cumulus, then cumulonimbus calvus, rounded but not fibrous, then cumulonimbus capillatus, the one with the anvil. By convention a cumulus is reported as cumulonimbus the moment it produces thunder, lightning, hail or showery precipitation.
Stable against unstable, at a glance
Layer clouds, St, Sc, As, Ns, Ci, Cs, mean stable air. Heap clouds, Cu, Cb, Ac castellanus, mean unstable air. To dissipate cloud you need subsidence, the sinking air of an anticyclone.