|BACKYARD METEOROLOGY: PRECIPITATION TYPE
METEOROLOGIST JEFF HABY
One of the primary observations that can be made in backyard meteorology is the precipitation type. Watching the precipitation fall and
accumulate can be interesting and mesmerizing. This writing will go over the different types of precipitation, fog, as well as moisture
developing on the ground and how they all formed. Knowing the history of the precipitation and moisture makes it all
the more interesting.
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 to melt the snowflakes 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 snowflakes. 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 whiter 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 earth's surface.
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 earth's surface.
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.
14. Dew- Dew is liquid moisture that condenses on objects at the Earth’s surface when overnight cooling produces saturated air.
15. White frost- Depositional frost is also known as white frost or hoar frost. It occurs when the dewpoint (now called the frost point)
is below freezing. When this frost forms the water vapor goes directly to the solid state. Depositional frost covers the vegetation,
cars, etc. with ice crystal patterns (treelike branching pattern). If the depositional frost is thick enough, it resembles a light snowfall.
16. Frozen dew- Frost that forms due to the freezing of liquid water is best referred to as frozen dew. Initially, both
the dewpoint and temperature are above freezing when dew forms. Longwave radiational cooling gradually lowers the temperature
to at or below freezing during the night. Cold air advection can also do the trick (e.g. Cold front moving through in the middle
of the night after dew has formed). Once the temperature falls to freezing, the condensed dew droplets freeze. Frozen dew looks
different from white frost. Frozen dew does not have the crystal patterns of white frost. White frost tends to looks whiter
while frozen dew tends to look slicker and more difficult to see.
17. Fog- Suspended clouds droplets experienced at ground level (a cloud on the ground).