Wind and visibility done, the rest of the observation follows: the weather itself, the clouds counted in eighths and measured in feet, the sky that sometimes disappears altogether — and, twice a day from stations around the world, the balloon that gives the atmosphere its X-ray.
Watching the weather itself
Present weather (the rain, drizzle, fog and thunder of the reports) is observed by human observers, supported by automatic present-weather sensors that discriminate precipitation type and intensity, and by lightning-detection networks for thunderstorms. The coded weather groups of the METAR are this observation written down.
Watching the clouds
Each cloud property has its own instrument:
Type — the human observer's job; automatic stations can barely tell genera apart.
Amount — from the observer, or from a ceilometer sampling the sky over time.
The height of the base — the ceilometer's specialty: a laser pulse fired upward, timed to the echo.
Tops — not routinely measured from the ground at all; they come from aircraft reports and satellites.
Which clouds make the reports
The reports are selective about names: the only cloud types identified in METAR, TAF and SIGMET are cumulonimbus (CB) and towering cumulus (TCU), the significant convective clouds. Every other genus appears anonymously, as an amount and a height. The thunderstorms lessons explained why those two earn the exception.
Oktas
Cloud amount is counted in oktas, eighths of the sky covered: FEW is 1-2 oktas, SCT (scattered) 3-4, BKN (broken) 5-7, and OVC (overcast) 8.