L5 Clouds and Precipitation

Flashcards about clouds and precipitation based on lecture notes.

How is a cloud formed?

When air cools adiabatically below its dew point temperature and water vapor condenses into water droplets or ice.

What are condensation nuclei (hygroscopic nuclei)?

Microscopic particles of dust, smoke, or salt that water vapor condenses onto.

What are the principal factors in cloud formation?

Humidity and cooling process

What are the different vertical lifting actions that can cause cloud formation?

Free convection, frontal or convergence, and orographic lifting.

What is advection in relation to cloud formation?

Horizontal moving action

How does turbulence contribute to cloud formation?

The mixing of air, which can result in cooling and cloud formation.

How do lifting actions lead to cloud formation?

Rising air expands and cools.

What typically causes free convection?

Generated by an area of the surface heating more quickly than its surroundings.

What type of cloud is known as cumulus?

Heaped and lumpy cloud

What causes 'fair-weather' cumulus?

Unequal surface heating in a stable atmosphere, don't grow to great vertical extent

What is cloud base?

the lowest level of a cloud

How can you estimate the height at which convective cumulus cloud will form?

Multiply the difference (in °C) between the surface temperature and the dew point by 400.

What type of clouds form when the slope between differing air masses is gentle?

Clouds of great horizontal extent (often stratus) form, indicating a stable atmosphere.

What type of clouds form when the slope between differing air masses is steeper?

Large cumulus clouds of great vertical extent form, indicating an unstable atmosphere.

What is orographic ascent/uplift?

Air is forced to rise when it meets an obstruction such as a mountain range.

What type of clouds will form in a stable atmosphere during orographic lifting?

Layered clouds

What type of clouds are more likely to form in an unstable atmosphere during orographic lifting?

Cumulus clouds

Where does convergence cloud form?

Two differing air masses meet.

What is a 'street' of cumulus cloud?

A long line of cumulus clouds that can stretch for hundreds of miles downwind.

What happens to temperature in temperature inversion regions?

The atmospheric temperature increases with an increase in height.

What effect does a temperature inversion have on the atmosphere?

Stabilizes the atmosphere and reduces the effect of lifting processes and cloud formation.

What happens to cumuliform clouds when there is an inversion?

Cumuliform clouds can spread out under the inversion layer.

What is cloud spread out over the sea by a temperature inversion called?

Marine cumulus

What happens when warm air moves horizontally over cooler water?

Warm air is cooled below the dew point temperature and the water vapor condenses into visible moisture.

How does turbulent mixing lead to cloud formation?

It mixes different layers, resulting in some parcels of the warmer air being raised to a level where their temperature falls below the dew point.

What causes turbulent mixing?

Rough terrain or a deepening of the friction layer caused by surface heating.

What form does the cloud produced by turbulent mixing take?

Layered, broken, or fractured and may be like stratocumulus.

What weather conditions on summer mornings cause turbulence clouds to form?

Rising sun heats the surface and with a light wind the atmosphere is "stirred up".

What causes winter "anticyclonic gloom"?

Turbulence cloud around the level of a temperature inversion caused by a persistent anticyclone in winter.

What are the two basic cloud classifications?

Vertical cumulus-type and horizontal stratus-type.

What is the altitude for low level clouds?

6500 ft and below

What is the altitude for medium level clouds?

6500 ft to 23 000 ft

What prefix do medium level clouds have?

alto

What is the altitude for high level clouds?

16 500 ft to 45 000 ft

What is the cloud type for high level clouds?

cirriform

Which prefix/suffix indicates rain-bearing clouds?

Nimbo or nimbus

How far can clouds of large vertical extent reach through the atmosphere?

Extend from the low level through the medium level and sometimes into the high level.

What is stratus cloud?

A grey sheet-like layer of cloud often covering the whole sky.

How low can the base of stratus clouds be?

Very low, down to ground level.

How much turbulence is in stratus clouds?

Little or no turbulence.

What is cumulus cloud?

A well-defined cloud with varying vertical extent.

How strong are the vertical currents within cumulus clouds?

Very strong.

What do large cumulus clouds indicate?

Turbulence and instability in the atmosphere.

How can stratocumulus clouds be produced?

Several forms: cumulus collapsing and spreading, turbulent mixing, orographic lifting.

What color are stratocumulus clouds?

White or greyish.

What is 'stratocumulus' used to indicate?

A cloud that is neither strictly cumuliform nor stratiform.

What is nimbostratus cloud?

Thick, dark-grey layered cloud with an indistinct base, producing precipitation over a wide area.

With what are nimbostratus clouds often associated?

Warm fronts.

What thickness does nimbostratus cloud indicate?

A thickness of 4000 ft.

What is altostratus cloud?

Flat, layer clouds which are usually light grey or white.

How is altostratus distinguishable from nimbostratus?

Clearly defined base and less thick.

What is altocumulus cloud?

Grey or white patches of rolled or bubble cloud, possibly producing some precipitation.

What is virga?

Rain or snow falling from cloud but evaporating before reaching the ground.

What is cirrus cloud?

Thin, wispy clouds composed almost entirely of ice crystals.

How much moisture or turbulence is in cirrus clouds?

Little moisture or turbulence.

What is cirrostratus cloud?

A transparent veil of high cloud.

What visual effect does cirrostratus cloud produce?

A halo effect around the sun or moon.

What is cirrostratus cloud indicative of?

An approaching "front".

What causes cirrocumulus cloud?

Caused by shallow convection currents at high altitude.

What does cirrocumulus consist of?

Regular flakes or ripples, and sometimes produces a visual effect known as a "mackerel sky".

What is cumulonimbus cloud associated with?

Associated with thunderstorms.

What hazards can a cumulonimbus contain?

Turbulence, windshear, intense precipitation, lightning, etc.

What generates large cumulus-type clouds?

Strong vertical currents (often convective).

What does taller the cumulus clouds mean?

The greater the atmospheric instability.

What do stratus-type clouds indicate?

Settled weather and a more stable atmosphere.

What wind direction do clouds indicate?

The wind direction at altitude.

What area forecasts indicates cloud type?

UK METFORM 215 low-level forecast.

What aerodrome observations indicate cloud type?

Reports of actual weather (METARs) and aerodrome forecasts (TAFS).

What cloud types are recorded in METARS and TAFS?

Towering cumulus (TCU) or cumulonimbus (CB).

In what units are cloud cover expressed?

Eighths.

What do eight oktas of cloud mean?

The whole sky is completely covered by cloud.

How many oktas are in few clouds?

1 to 2 oktas.

How many oktas are in scattered clouds?

3 to 4 oktas.

How many oktas are in broken clouds?

5 to 7 oktas.

How many oktas are in overcast clouds?

8 oktas.

What does BKN/OVC SC AC AS 060-080 / XXX mean?

Broken/Overcast Stratocumulus, Altocumulus, and/or Altostratus base 6000-8000 ft.

What does ISOL EMBD CB 015-025 / XXX mean?

Isolated embedded Cumulonimbus base 1500 - 2500 ft.

What does SCT/BKN ST 006-012/030 mean?

Scattered/Broken Stratus base 600 - 1200 ft, tops 3000 ft.

What does OCNL BKN CU 012-014/025 mean?

Occasional Broken Cumulus, base 1200-1400 ft, tops 2500 ft.

What is EGPK 251450Z?

A METAR (actual report) from EGPK (Prestwick) at 1450 UTC.

What is precipitation?

Particles falling from cloud.

What does anything other than the lightest precipitation imply?

Cloud at least 4000 ft thick.

What type of precipitation tends to come from stratiform-type clouds?

Light: drizzle, light rain, light snow, etc.

What type of precipitation can come from cumulus clouds?

Moderate or heavy.

What type of cloud is associated with prolonged rain?

Nimbostratus.

What size are drizzle drops?

Water drops between 0.2 mm and 1.0 mm.

What is drizzle?

Light liquid precipitation whose drops are too small to constitute rain.

What size are rain drops?

Water drops between 1.0 mm and 5.8 mm.

How does rain form?

Larger clouds with internal updrafts circulating smaller water droplets and allowing them to merge to form larger water drops.

What is the process where smaller water droplets merge to form larger water drops?

Coalescence

What is snow?

Precipitation in the form of ice crystals.

What surface temperature is needed for snow to reach the ground?

+4°C or below.

With what temperatures are the heaviest falls of snow usually associated?

Around 0°C (freezing).

What are snow grains?

Small ice particles, the frozen equivalent of drizzle.

What is hail?

Hard ice stones generated in well-developed cumulonimbus clouds.

When is hail most likely in the UK and Western Europe?

Spring or summer.

What are ice pellets?

Pellets of clear ice, which tend to fall from thick layered clouds such as nimbostratus.

What is "diamond dust"?

Tiny ice crystals that appear to be suspended in the air and which fall from clear skies in calm arctic conditions.

What characterizes virga?

Streaks below the cloud.

What is 'sleet' in the UK is described as internationally?

Rain and snow (RASN).