The module's last hazard is the simplest of all: not being able to see. Everything the course has grown (precipitation, fog, dust, ice, even the sun itself) takes its turn at the windshield in this lesson, along with the geometry question the exam loves best: why a runway plainly visible from overhead can vanish completely on final approach.
Rain, drizzle, snow
The three falling reducers rank in a perhaps surprising order:
Drizzle — many tiny drops packed close together, dense out of proportion to its gentleness: thick drizzle can cut visibility below 1,000 m, into fog territory;
Rain — usually reduces visibility only moderately, a few kilometres, though heavy rain and downpours can be severe, and the water film on the windshield degrades whatever remains;
Snow — the worst of the three: heavy snow can take visibility below 200 m, and wet snow plasters the windshield as it goes.
The obscurations: fog to volcanic ash
The suspended reducers are old acquaintances from the clouds module, plus two newcomers:
fog — visibility below 1,000 m by definition, the aviation obscuration;
mist — visibility 1,000 to 5,000 m at high humidity, fog's outer apron;
haze — dry particles, a milkiness that thickens with distance;
smoke — combustion particles, dense near fires and industry, and packed into sharp layers under inversions;
volcanic ash — an obscuration whose visibility effect is the least of it: ash sandblasts windshields opaque and destroys engines, so avoidance is absolute.
Sand and dust in suspension
Two more codes from the reports: SA (sand) and DU (dust) — fine particles raised elsewhere and carried in suspension over vast areas and to considerable heights. The climatology module's Harmattan is the standing example: dust hazing half a continent for months, with no storm anywhere in sight.
Ground, flight, slant: which visibility
The words matter, because they measure different lines of sight through the same air. Ground visibility is measured horizontally at the surface; flight visibility is what the pilot sees ahead from the cockpit — and above a thin layer of fog or haze the two part company spectacularly. Looking straight down from overhead (the vertical visibility), the sight line crosses only the layer's shallow depth: the runway is visible. On the approach, the slant sight line runs a long distance inside the layer, and the runway disappears, while the ground visibility inside the layer may be the worst number of the three. The operational moral: "field in sight" from above promises nothing about final approach, and in shallow fog the runway can vanish in the flare.