Clouds, Fog and Precipitation
Cloud classification and formation
Clouds are grouped by height and shape. High clouds (cirrus, cirrostratus, cirrocumulus) are ice; medium clouds (altostratus, altocumulus) are mixed; low clouds (stratus, stratocumulus, nimbostratus) are water droplets and may contain supercooled water. Cumulus and cumulonimbus are vertically developed. Memorise the family logic rather than individual names, and remember that only cumulonimbus and nimbostratus typically give significant steady or heavy precipitation among the low and vertical families.
Fog types
Fog is cloud at the surface, defined as visibility below 1000 m. Radiation fog forms on clear, calm, long nights, especially in autumn and winter, in moist air, often in valleys, and usually clears after sunrise. Advection fog forms when warm moist air moves over a cold surface, commonly over the sea and in coastal areas; it can persist with wind. Frontal fog occurs with rain falling into cooler air below. Upslope and steaming fog complete the list. The trap is distinguishing radiation (needs calm, clear, land, night) from advection (needs wind, a temperature contrast, often sea).
Precipitation and traps
Rain, drizzle (droplets under 0.5 mm), snow, sleet and hail form by different processes. Freezing rain is a serious hazard: snow falls through a warm layer, melts, then falls into sub-zero air near the surface and freezes on impact. Questions often confuse hail (strong updraughts in cumulonimbus) with sleet or ice pellets.
Rule of thumb
If the temperature-dew point spread is 2°C or less, a cloud base near 800 ft and fog or low stratus are plausible; if the spread is 10°C, expect bases near 4000 ft.