Clouds and Precipitation
This domain tests cloud classification, the link between formation mechanism and cloud form, the precipitation types each cloud produces, and the operational hazards associated with particular genera.
Formation mechanisms
- Convection (surface heating, unstable air) gives cumuliform cloud.
- Orographic lifting gives cap cloud, lenticular cloud and heavy orographic precipitation.
- Frontal and convergence lifting gives extensive stratiform cloud and organised rain belts.
- Cooling by radiation or advection at the surface gives fog and stratus.
- Turbulence mixing gives stratocumulus.
Classification
- Low (base below about 6,500 ft): stratus, stratocumulus, nimbostratus, cumulus, towering cumulus, cumulonimbus.
- Medium (about 6,500-20,000 ft): altostratus, altocumulus including the lenticular form.
- High (above about 16,500 ft, all ice): cirrus, cirrostratus, cirrocumulus.
- Cumulonimbus spans all levels and may reach the tropopause with an anvil.
Precipitation
- Forms by the Bergeron ice-crystal process above the freezing level and by coalescence of droplets below it.
- Drizzle and steady rain come from stratus and nimbostratus; showers from cumulus and cumulonimbus; snow, sleet, hail and virga are further types. Continuous precipitation indicates a deep, stable layer; showery precipitation indicates instability.
Hazards by cloud type
- Cumulonimbus: severe icing, hail, turbulence, lightning, microburst and low-level wind shear, and a hazard to aircraft on the ground.
- Nimbostratus: low base, poor visibility, moderate icing and continuous precipitation.
- Stratus and stratocumulus: low base, obscured terrain, and often an inversion on top.
- Altocumulus lenticularis: mountain wave, moderate to severe turbulence, strong downdrafts.
- Freezing rain from any layer above 0°C overlying sub-zero air is the worst icing scenario.
Worked example
Surface temperature 20°C, dew point 12°C, air forced over a ridge 3,000 ft high. The spread is 8°C, so the condensation level is 8 × 400 = 3,200 ft above the surface over the plain. Over the ridge, the same lifting is achieved at about 200 ft above the ridge line, so the summit is enveloped in cloud and the valley windward is clear - terrain clearance must be planned on the higher figure.
Traps that cost marks
- Assuming high cloud produces precipitation: cirrostratus signals an approaching warm front but yields no significant rain itself.
- Forgetting that cloud base is quoted above ground level, not above sea level.
- Treating altocumulus as harmless when lenticular forms indicate mountain wave.