METEOROLOGIST JEFF HABY
This webpage describes the different types of precipitation and explains how they form. METAR and other
frequently used abbreviations for each precipitation type are given.
1. Rain (R, RA)- Rain is liquid precipitation that reaches the surface in
the form of drops that are
greater than 0.5 millimeters in diameter. The intensity of rain is determined by the accumulation
over a given time. Categories of rain are light, moderate and heavy.
2. Snow (SN, SNW, S)- Snow is an aggregate of ice crystals that form into flakes. Snow forms at temperatures below
freezing. For snow to reach the earth's surface the entire temperature profile in the troposphere needs to
be at or below freezing. It can be slightly above freezing in some layers if the layer is
not warm or deep enough the melt the snow flakes much. The intensity of snow is determined by the accumulation
over a given time. Categories of snow are light, moderate and heavy.
3. Snow Pellets (GS)- A snow pellet is precipitation that grows by supercooled water accreting on ice crystals
or snow flakes. Snow pellets can also occur when a snowflake melts about
half way then refreezes as it falls. Snow pellets have
characteristics of hail, sleet and snow. With sleet (ice pellets), the snowflake almost completely melts before
refreezing thus sleet has a hard ice appearance. Soft hail grows in the same way snow pellets can grow and that
is ice crystals and supercooled water accreting on the surface. Snow pellets
will crush and break apart when pressed. They can bounce off objects like sleet does. Snow
pellets have a whiter appearance
than sleet. Snow pellets have small air pockets embedded within their structure and have visual remnants of
ice crystals unlike sleet. Snow pellets are typically a couple to several millimeters in size.
4. Snow Grains (SG)- Snow grains are small grains of ice. They do not produce much accumulation and are the
solid equivalent to drizzle.
5. Ice Crystals (IC)- Also called diamond dust. They are small ice crystals that float with the wind.
6. Sleet / Ice Pellets (PE, PL, IP, SLT)- Sleet (Ice Pellets) are frozen raindrops that
strike the earth's surface. In a sleet situation
the precipitation aloft when it is first generated will be snow. The snow falls through a layer that is a
little above freezing and the snow partially melts. If the snow completely melts it will be more
likely to reach the earth's surface as supercooled water instead of sleet. If the snow partially melts there
will still be ice within the falling drop for water to freeze on when the drop falls into a subfreezing layer. The
lowest layer of the troposphere will be below freezing in a sleet situation and deep enough to freeze drops
completely. The lower boundary layer can be above freezing and sleet occur if the sleet does not have
time to melt before reaching the surface.
7. Hail (GR, A)- Hail is dense precipitation ice that is that least 5 millimeters in diameter. It forms due to ice
crystals and supercooled water that freeze or stick to the embryo hail stone. Soft hail is more white
and less dense since it has air bubbles. Soft hail occurs when hail grows at a temperature below freezing
by ice crystals and small supercooled water and cloud droplets merging onto the hail. Hard hail occurs
when liquid water drops freeze on the outer edges of the hailstone after the outer edge is above
freezing. The freezing of supercooled water releases latent heat and this can result in the outer
edge of the hail stone warming above freezing. Then the water refreezes creating solid ice.
Hail will commonly have soft ice and hard ice layers when it is sliced open.
8. Graupel (GS)- Graupel forms in the same way as hail except the diameter is less than 5 millimeters. It usually
grows by soft hail processes.
9. Drizzle (DZ, L)- Drizzle is liquid precipitation that reaches the surface in the form of drops that are
less than 0.5 millimeters in diameter.
10. Freezing Drizzle (FZDZ, ZL)- Freezing Drizzle is liquid precipitation that reaches the
surface in the form of drops that are less than 0.5 millimeters in diameter. The drops then freeze on the
11. Freezing Rain (FZRA, ZR)- Freezing Rain is liquid precipitation that reaches the
surface in the form of drops that are greater than 0.5 millimeters in diameter. The drops then freeze on the
12. Freezing Fog (FZFG)- Freezing fog is a fog composed of supercooled
water drops. These drops freeze just after they wet the earth's surface.
13. Mixed Precipitation (MXD PCPN)- The combination of two or more winter precipitation
types occurring at the same time or over a period of time at the same place.