Precipitation
Precipitation
What precipitation is, what it does to cloud base and visibility, and its forms.
- Hail
- Rain
- Freezing rain
- Sleet
- Snow
How precipitation forms
ExamTwo ways cloud particles grow big enough to fall.
- Small droplets collide and make bigger droplets.
- Creates drizzle and light rain.
- Water vapour freezes directly onto the ice crystal, with no liquid stage (MET 003), so the crystal grows larger.
- Creates rain, snow, hail, ice pellets and more.
Rain, freezing rain, sleet and snow
What reaches the ground depends on the air it falls through.
Sleet has no METAR code. The sleet drawn here, refrozen snow, is reported as ice pellets, PL. Rain and snow falling together is RASN.
Stratiform and convective precipitation
ExamThe cloud it falls from decides what falls, how long, and how hard.
- Duration
- Continuous
- What falls
- Drizzle, rain, snow
- Intensity
- Steady, normally light to moderate
- Particles
- Smaller
- Duration
- Intermittent
- What falls
- Showers of rain or snow, and hail
- Intensity
- Varying, moderate to heavy
- Particles
- Larger
Stratiform and cumuliform cloud, and why stable air gives layers and unstable air gives heaps, are in MET 004.
Virga
Precipitation that never arrives.
Drizzle, rain and snow
Drizzle and rain are told apart by the size of the drop: 0.5 mm is the line.
DZDrizzle- Falling drops under 0.5 mm.
- Continuous.
- Only from stratiform cloud, in a stable atmosphere.
RARain- Water droplets over 0.5 mm.
- From stratiform cloud in a stable atmosphere, or cumuliform cloud in an unstable one.
SNSnow- White or translucent ice crystals, six-sided.
- Their size and shape vary a lot with the conditions in the atmosphere.
Hail and small hail
Balls of ice, told apart by size: 5 mm is the line.
GRHail- Ice balls 5 mm or more.
- Only from cumulonimbus, a sign of very unstable air.
GSSmall hail or snow pellets- Under 5 mm.
Snow grains and ice pellets
Two more kinds of ice: one tiny, one a warning of what is above.
SGSnow grains- Under 1 mm.
- The solid equivalent of drizzle.
PLIce pellets- They show a layer of freezing rain above the surface.
Intensity and vicinity
How heavy it is, and how near: the codes a report gives with the precipitation.
| Intensity | In plain language and local reports | In a METAR |
|---|---|---|
| Light | FBL | -RAMinus sign |
| Moderate | MOD | RANo sign |
| Heavy | HVY | +RAPlus sign |
A METAR writes the sign, never FBL, MOD or HVY: -RA is light rain, RA moderate rain and +RA heavy rain.
CAVOK needs, among other things, no significant weather: see MET 001.
FBLLightMinus sign in a METARMODModerateNo sign in a METARHVYHeavyPlus sign in a METARVCVicinityWithin 16 km of the aerodrome, but not on it
DZDrizzleDrops under 0.5 mmRARainDrops over 0.5 mmSNSnowSix-sided ice crystalsSGSnow grainsUnder 1 mmPLIce pelletsFreezing rain aboveGRHail5 mm or moreGSSmall hail or snow pelletsUnder 5 mm
The full table of METAR weather codes is in MET 007.