Precipitation Types

Main Precipitation Types in Aviation

• Drizzle (DZ): Fine, light water droplets (0.2–1.0 mm) that appear to float, typically originating from stratus or stratocumulus clouds. Drizzle reduces visibility and is associated with stable, humid conditions.
• Rain (RA): Larger water droplets (1.0–5.8 mm) formed through coalescence in clouds with moderate vertical development. Rain can be slight, moderate, or heavy, with intensity linked to cloud thickness and updraft strength. Showers (SH) are sudden, intense bursts of rain from convective clouds, especially cumulonimbus.
• Snow (SN): Ice crystals or snowflakes, falling when the temperature from cloud base to surface is cold enough to prevent melting. The largest, fluffiest flakes occur near 0°C, while colder conditions produce smaller, denser crystals. Snow grains (SG) are the solid equivalent of drizzle—tiny, non-bouncing ice particles.
• Hail (GR) and Small Hail (GS): Hard, layered ice balls formed in strong cumulonimbus clouds through repeated cycling in updrafts. Hailstones can become very large and are hazardous to aircraft.
• Ice Pellets (PL), Snow Pellets (GS), and Ice Crystals (IC): Ice pellets are small, hard, transparent balls of ice, often resulting from melting and refreezing processes. Snow pellets are soft, white, and compressible, while ice crystals are minute, needle-like forms seen in very cold air.
• Freezing Rain (FZRA) and Freezing Drizzle (FZDZ): Supercooled liquid droplets that freeze on contact with surfaces, forming a glaze of ice. These occur when a warm layer aloft causes snow to melt into rain, which then becomes supercooled as it passes through a sub-zero layer near the surface.

Cloud Types and Precipitation

• Stratiform Clouds (Stratus, Nimbostratus): Produce continuous, steady precipitation such as drizzle, light rain, or snow.
• Convective Clouds (Cumulonimbus, Towering Cumulus): Generate showery, often intense precipitation, including heavy rain, hail, and sometimes snow showers.

Precipitation and Visibility

• Large, wet snowflakes (near 0°C) reduce visibility more than small, dry flakes (colder conditions).
• Drizzle and freezing precipitation can cause significant visibility loss and are especially hazardous for aircraft due to icing risk.

METAR/TAF Codes

• Common codes include DZ (drizzle), RA (rain), SN (snow), SG (snow grains), PL (ice pellets), GR (hail), GS (small hail/snow pellets), IC (ice crystals), FZDZ (freezing drizzle), and FZRA (freezing rain).

Formation of Freezing Precipitation

• Occurs when snow melts in a warm layer aloft, then supercools in a sub-zero layer before reaching the ground, freezing on contact with surfaces. This is a major hazard for aircraft operations.