Last Updated on 4 years by teboo
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050 05 00 00 PRECIPITATION
050 05 01 00 Development of precipitation
050 05 01 01 Process of development of precipitation
Bergeron – Colder than zero. Water smashes into ice then grows depleting the water, the fall and combine with more ice before getting too heavy then falling through the base of the cloud. (If above 0 falls as rain. If sub zero under the clouds may form ice pellets and fall as freezing rain.)
Coalescence – Warmer than zero. Water collides then grows and sinks, get lifted up again until it can’t be supported and falls out.
(02) Summarise the outlines of the ice-crystal process (The Wegener–Bergeron–Findeisen process).
(03) Summarise the outlines of the coalescence process.
(04) Explain the development of snow, rain, drizzle and hail.
- Snow – > -5°C ice crystal bind together – large snow, warmer layers soften the branches helping them bind. The colder, the smaller.
- Rain – 0.5 mm to 5.5 mm 9 m/s
- Drizzle – < 0.5mm 4 m/s
- Hail – Can be layered, small hail <5mm GS. Larger is GR.
050 05 02 00 Types of precipitation
050 05 02 01 Types of precipitation, relationship with cloud types
DZ Drizzle
RA Rain
Snow SN
Snow grains SG
Ice Pellets PL
Hail GR
Small hail GS
Snow pellets GS
Freezing drizzle FZDZ etc… FZRA
(02) State the ICAO/WMO approximate diameters for cloud, drizzle and rain drops.
Cloud 0.02 mm
Drizzle < 0.5mm
Rain > 0.5mm
(03) State that, because of their size, hail stones can cause significant damage to aircraft.
(04) X Explain the mechanism for the formation of freezing precipitation.
Very often, precipitation first falls from a cloud as snow (when it is cold enough high up where the cloud is). If it falls through warmer air before reaching the ground, it can melt and turn to rain droplets.
On rare occasions, if it then falls through cold air again just before hitting the ground, the droplets can become ‘supercooled’ and this means that they are still falling in liquid form, even though their temperature has fallen below zero. When this ‘supercooled up to 500 microns’ droplet hits the ground (which is below zero too) it spreads out a little on landing, and then instantly freezes, encasing the surface in a layer of clear ice. This is why it is called freezing rain.
With no nuclei…
(05) Describe the weather conditions that give rise to freezing precipitation.
- Rain must be supercooled or close to zero.
- Airframe must be cold.
- Drops formed in a relatively warm cloud > 0°C falling through cold air < 0°C. (inversion).
- Warm front over cold land in winter. If rain falls in the warm > o°C air through into the cold sector and then onto the aircraft. Climb to get out.
- Found in North America in winter.
Unexpected extensive ice build up in areas when ice does not normally form.
Build up on the upper surfaces aft of protection systems.
Build up on the lower wing surface – aft.
Build up further aft on the spinner or nacelles then expected.
Build up on probes.
Water on windscreen and visible rain when OAT is below zero
(06) Distinguish between the types of precipitation generated in convective and stratiform clouds.
Convective – Showers, rapid onset and cessation.
Stratiform – Continuous – over an hour, intermittent, less than an hour.
(07) Assign typical precipitation types and intensities to different cloud types.
Ci Cc Cs – Nothing
As Ac – Light RA SN PL
Ns – Mod to heavy RA SN PL
Cu – Mod RA SN GS
Cb – Heavy RA SN GS GR
Sc – Light RA SN GS (Snow pellets not small hail)
St – Light DZ SN SG
(08) Explain the relationship between moisture content and visibility during different types of winter precipitation (e.g. large vs small snowflakes).
Low moisture causes small snowflakes and better vis.
High moisture produces large snowflakes and poor vis