A glance at the sky is a weather briefing in itself, if you can read it. The clouds overhead are the visible result of what the air is doing, and the same naming scheme used by every meteorologist tells you whether the air is stable or unstable, wet or drying, and whether the weather ahead is settling or building.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The ten genera and the naming scheme
The WMO International Cloud Atlas classifies clouds into ten basic types, called genera, grouped by the height of their base. The names are built from a handful of Latin roots that, once learned, decode themselves:
- cirro or cirrus: high and wispy, made of ice crystals;
- alto: middle level;
- stratus or strato: a flat, layered sheet;
- cumulus: a heaped, lumpy cloud;
- nimbus or nimbo: rain-bearing.
So altostratus is a middle-level layer, cumulonimbus is a heaped rain cloud, and nimbostratus is a layered rain cloud.
High, middle and low
The base-height bands overlap and shift with latitude and season, so the UK Met Office and the FAA Aviation Weather Handbook give them as typical temperate-latitude figures rather than hard limits:
- High (bases roughly above 20,000 feet): cirrus, cirrocumulus, cirrostratus, all ice-crystal cloud.
- Middle (bases roughly 6500 to 20,000 feet): altocumulus, altostratus, and nimbostratus, the thick grey rain layer.
- Low (bases roughly below 6500 feet): stratus, stratocumulus, and the bases of cumulus and cumulonimbus.
Two of these, cumulus and cumulonimbus, have large vertical extent and can tower through all three layers from a low base to a high top.
The one distinction that matters most
For a pilot the most useful split is not the height band but the shape, because the shape reveals what the air is doing:
- Cumuliform (heaped) cloud forms in unstable air rising in convective currents. It brings showers, gusty winds, turbulence and good visibility between the showers.
- Stratiform (layered) cloud forms in stable air lifted gently over a wide area. It brings steady, widespread cloud, often continuous rain or drizzle, and a smoother ride.
This ties straight back to stability and inversions: heaps mean instability, layers mean stability.
A worked example
You walk out for an afternoon flight and see, high up, thin wisps of cirrus thickening into a milky cirrostratus sheet, with altostratus greying the middle level behind it. To the west, the sky is lowering. That sequence, high ice cloud thickening and lowering into middle-level layers, is the classic signature of an approaching warm front: stratiform cloud, gentle lift, and steady rain on the way rather than showers.
Contrast a summer morning where the sky starts clear, fair-weather cumulus pop up by late morning, and by early afternoon one cell has grown into towering cumulus with a hard, cauliflower top. That is unstable air and active convection, and if the top glaciates into an anvil it has become cumulonimbus, the thunderstorm cloud the FAA handbook warns carries severe turbulence, icing, hail, lightning, downbursts and wind shear. That one you route well around.
Common pitfalls
- Height bands are typical, not fixed. The base figures shift with latitude and season, so use them as a guide, not a measurement.
- A growing cumulus is a warning. Towering cumulus is the stage immediately before cumulonimbus; treat rapid vertical growth as a reason to stay clear.
- In reports, cloud base is above the aerodrome. The cloud heights in a METAR are heights above the field, not above sea level.
In Pilot EFB
Pilot EFB decodes the cloud groups in the METAR and TAF, so the amounts and bases are in plain language alongside the raw report, and it keeps the thunderstorm and convective warnings you have pulled in one place. A briefing you have already pulled stays readable with no signal; fetching a fresh observation needs a connection. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.