5 Cloud Development Flashcards
Adiabatic Process
•the adiabatic process is important very near earths surface
• moving higher into the atmosphere, the adiabatic process
becomes much more important
• recall that an unsaturated parcel of air cools by 1 °C for every 100 m it
rises – the dry adiabatic lapse rate
• at the same time, the dew point of the air parcel is decreasing, at a rate
of 0.2 °C per100 m – the dew point lapse rate
• the air parcel temperature is approaching the dew point by 0.8 °C
for every 100 m that it rises – eventually the air parcel temperature
reaches the dew point, and saturation is achieved
Level of condensation
the point at which the dry adiabatic lapse rate and dew point lapse rate meet and saturation occurs
-the air parcel then moves at the wet adiabatic lapse rate
Rising to the level of condensation, the dry adiabatic lapse rate is __ degrees/1000m and the dew point lapse rate is __ degrees/1000m
wet adiabatic rate is __degrees/1000m
10
2
5
flat cloud bottoms are cause by the …
level of condensation seperating the dry and wet adiabatic lapse rate
cloud droplets
• saturation is achieved at the lifting condensation level (LCL), and the air
parcel will continue to rise and cool at the saturated adiabatic lapse rate
• condensation onto condensation nuclei begins immediately above the
LCL in the form of small cloud droplets
• as the air continues to rise, small droplets continue to form until all of
the condensation nuclei are consumed – > 50m above the LCL
• since there are no more condensation nuclei, water vapour condenses
onto already existing droplets, making them bigger
• recall the importance of size and curvature, in terms of evaporation
rates
•eventually, the air parcel will stop rising, and condensation will cease
• the adiabatic process is completely reversible
• if the air parcel begins to descend it will do so at the SALR, but evaporation of the cloud droplets will occur
• eventually, the parcel reaches the LCL, evaporation is complete, and
the air parcel continues to sink along the DALR
• if the air parcel reaches the surface again, it will have the same
temperature and dew point as when it started to rise
• this is slightly idealized, since there is a chance that some of the moisture
is lost through precipitation, but this is usually only a minor component
4 types of cloud lifting
- orographic lifting
- frontal lifting
- convergence
- localized convection
orographic lifting
• air flowing across the landscape will be forcibly deflected by
topographic barriers – mountains
• typically, air is forced to rise up and over a mountain range, cooling as it
does so
• condensation occurs and clouds begin to form – orographic clouds
• the height of the clouds is
not restricted by the height
of the barrier –orographic clouds sometimes penetrate into the stratosphere
.this produces the rain shadow effect
• once the air has passed the barrier and begins descending down the
leeward side, it warms adiabatically, and evaporation occurs
•this produces the rain shadow effect – limited cloud cover and little
precipitation occurs on the leeward side of mountains
Frontal Lifting
• cold and warm fronts are generated by the horizontal movement of air and
displacement of the lower density air parcel upward
• in a cold front, colder air pushes into a warmer air mass – the cold air stays near the surface and the warm air is forced to rise • in a warm front, warmer air blows into colder air – the warm air rides up and over the cold, dense air
Convergence
• when air flows into a low pressure centre, it must also be expelled
somehow – the centre cannot hold an infinite amount of air
• the air is forced upwards into the atmosphere
•this is why cyclones are typically accompanied by stormy weather
Localized Convection
- free convection is generated by heating of Earth’s surface
* lifting is generated by the relative buoyancy of warmer air parcels
Atmospheric Cloud Stability
• what happens once lift is initiated?
• this depends on the static stability of the air
• statically unstable air becomes buoyant when lifted and will
continue to rise even after the lifting mechanism has stopped
• statically stable air resists upward movement, and will sink back to its
original position if the lifting mechanism is stopped
• statically neutral air rises by a lifting mechanism, but stops rising
once the mechanism stops – it will not fall back down
•.stability is strongly related to buoyancy
• if the air parcel is warmer than the surrounding air, it has positive
buoyancy and will be statically unstable
• if the air parcel is colder than the surrounding air, it has negative
buoyancy and will be statically stable
Temperature and atmospheric cloud stability
• rates of temperature change are also important
• if a rising air parcel is cooling faster than the air around it, lift will be
suppressed (DALR/SALR > ELR)
• if a rising air parcel is cooling slower than the air around it, lift will
continue (DALR/SALR < ELR)
• the stability of air is therefore dependent on the relative temperature
and density differences between the parcel and the surrounding
environment
the relative density of a rising air parcel depends on:
•.whetehr or not is it saturated
•.the environmental lapse rate
• the relative density of a rising air parcel depends on:
- .whetehr or not is it saturated
* .the environmental lapse rate
Absolute instability of a cloud
.this is known as absolute Instability, and the parcel will continue to rise and t an ever increasing rate
-if the air parcel is saturated it will rise faster and higher because it is even more unstable
If DALR>ELR and Once the lifting mechanisms stops, this air parcel will
sink back down to the ground, this is absolute stability
-the same holds for a saturated air parcel, but in this case the density difference becomes greater in the unsaturated air, so sinking will be faster for unsaturated vs saturated air
conditional instability
• the issue of stability becomes complicated when we recall that the
lapse rate of an air parcel will change once saturation is achieved – the
DALR turns into the SALR
• sometimes, the DALR > ELR but the SALR < ELR
• this is known as conditional instability, and whether or not the air parcel
will rise or sink depends on whether saturation is achieved and how much
lifting has already occurred
level of free convection
marks the altitude to which the air must rise
under force, but above which it will rise due to buoyancy
potential instability
• these examples refer to idealized parcels of air, but what if a large mass
of air was forced to lift off the ground
• in this case, the ELR is forced to change, and is therefore no longer static
• this is called potential instability, and is a particularly important
mechanism for severe storm generation
• this often occurs when warm and dry air sits above warm and wet air
• as these rise, the situation is statically stable, but eventually, they will
rise high enough that the top layer will become colder (and denser)
than the lower layer
• this is because the lower layer will cool slower, at the SALR, than
the top layer which cools quickly at the DALR
• since this is uplift dependent, we can say that the situation is potentially
unstable
Environmental Lapse Rate
its changes due to: 3
• it changes due to:
- heating or cooling of Earth’s surface
- advection of warm and cold air at different levels
- advection of a new air mass
• during the day, insolation warms the surface which warms the lower
atmosphere
• early in the day, the lower troposphere will have a steeper ELR than
higher up, and that steep section will continue to rise as the day
progresses
• at night, cooling of the surface cools the lower troposphere faster producing a night-time inversion
Uplift and Stability
• so, air is forced to rise by several mechanisms
• once that mechanism stops, whether or not the air continues to rise
depends on the stability of the atmosphere
• stability is determined by the relationship of the air parcel lapse rate
(DALR or SALR) and the ambient air lapse rate (ELR)
• if the air parcel is warmer, and hence less dense, than the surrounding
air,It will rise
• if the air parcel is colder, and hence more dense, than the surrounding
air,it will sink
• the speed of rising and falling air will be determined by the difference
between the lapse rates
will
stop rising air?
• eventually, a rising air parcel will encounter a layer of stable air
• even if this does not happen in the troposphere, the stratosphere is extremely stable, and therefore will stop the rising air
entrainment
• alternatively, mixing along the boundary
between the air parcel and the surrounding
air will erode away the air parcel
• this process is known as entrainment –the surrounding air gradually takes over the air parcel, reducing the difference in temperature
• entrainment is important on the outer edges
of clouds
• unsaturated air outside the cloud is
mixed into it, which promotes evaporation
at the cloud margin, effectively shrinking
the cloud
• at the same time, latent heat is
absorbed, cooling the cloud and
reducing the temperature and making
the cloud less buoyant
- Inside the cloud is saturated(condensation dominates), outside of the cloud it is unsaturated and evaporation dominates
- entrainment sucks in dry air
______ is what gives clouds there really firm boundary, looks like solid feature
entrainment
Inversions
• under normal conditions, air rises because temperature decreases with
altitude
• this promotes instability because it encourages positive buoyancy
• in an inversion, the ELR is reversed, so a rising and cooling air parcel will
encounter a region of increasing temperature
•.this strongly encourages negative buoyancy and stops air from rising
• for this reason, inversions are extremely stable and resist the vertical
movement of air
3 reasons why inversions form?
1•Radiation inversion: diabatic cooling at night produces a cold
surface layer while higher air is warmer
2•Frontal inversion: the forced rising of warm air over cold air due to
frontal lifting promotes the development of an inversion
3•subsidence inversion: as air sinks, it contracts, but the top of the air
parcel will contract more than the bottom, so warming tends to be
greater at the top than the bottom
weather implications of inversions
• inversions have many important implications:
• rain falling through warm air then cold air can produce freezing rain
• radiation inversions promote frost, and have substantial impacts on
agriculture
•rising air helps remove pollution from the surface, but inversions restrict this motion
Howars Classification scheme: 4
- cirrus – thin, whispy clouds of ice
- stratus – layered clouds
- cumulus – clouds having vertical development
- nimbus – rain-producing clouds
Modern classification of clouds is based on cloud _____
1.
2.
3.
4.
height
- HIGH CLOUDS (– cirrus, cirrostratus, cirrocumulus)
- MIDDLE CLOUDS(altostratus, altocumulus)
- LOW CLOUDS(stratus, stratocumulus, nimbostratus)
- VERTICAL FORMS(cumulus, cumulonimbus)
- cross these boundaries
High Clouds
•top half of troposphere
• at this height, temperature is usually -35°C, so vapour condenses
into ice crystals and not supercooled water
• but if surface temperature is low (eg, in the Arctic), high clouds may
form as low as 3000 m – there’s a temperature dependency
.generally form above 6000m high
.made of ice, not water
.not as much entrainment so borders aren’t as distinct
cirrus clouds
high
• cirrus clouds (Ci) – thin, white, whispy clouds resembling mares tails
• thin because they contain very little ice overall, but the ice crystals are
large
• these can overcome gravity and fall, but they sublimate into vapour
long before they reach the surface
•whispy because high winds redistribute the crystals
• contrails are a type of
artificially produced cirrus
cloud
-sunlight penetrates through easily
cirrostratus clouds
high
• cirrostratus clouds (Cs) – extensive, shallow clouds somewhat
transparent to sunlight, producing a halo around the Sun or Moon
•-are more horizontally extensive than cirrus but also contain less ice, therefore they are very thin
• the halo is produced by refraction of light by
the ice crystals
• the angle of refraction is 22°, so a halo will
always be 44° across
cirrocumulus clouds
high
• cirrocumulus clouds (Cc) – high, layered cloud with billows or parallel
rolls
• also called mackerel sky, since they resemble fish scales
• produced by wind shear, where wind speed and direction is changing
with height
• wind shear typically forms in
advance of storm systems, therefore these clouds are considered a precursor to rain
Middle Clouds
• typically occur between 2000 and 6000m high and are composed of
liquid droplets,often being supercooled water
• altostratus clouds
Middle Clouds
(As) – extensive, watery layered cloud; allows some penetration of sunlight but Moon or Sun appears as a bright spot within the cloud • more extensive than high clouds, and more effective at scattering radiation back into space • any radiation reaching the ground is diffuse, and shadows are rare
altocumulus clouds
middle
• altocumulus clouds (Ac) – shallow, mid-level cloud containing patches
or rolls; generally more opaque and having less distinct margins than
cirrocumulus
• typically produce a more extensive cloud cover, so they reduce
insolation more – this gives them a greyish colour
Low clouds
• the cloud base is below 2000m, and so are composed of liquid water
droplets
.tend to produce rainy conditions
Stratus clouds
low cloud
• stratus clouds (St) – uniform layer of low cloud ranging from whitish to
grey
•form where large air masses are lifted
• their vertical extent
may only be ½ km, but horizontally can cover 100s of km
•overcast conditions
- very massive/widespread clouds that are very low in sky and block out sun
- warm, so composed of a lot of moisture
- brought in by weather fronts
nimbostratus clouds
low
low cloud producing light rain; produces
darker skies than altocumulus
• stratus and nimbostratus clouds experience very slow rates of uplift, and
often contain low water contents, so they usually only yeild light rain
• visually indistinguishable
from stratus, but these
produce rain
Stratocumulus clouds
low
– low-level equivalent to altocumulus
• there is slight vertical development here, which can be seen by
changes in colour –bright and dark spots where clouds are thicker, complete coverage
vetically formed clouds
• vertical development is produced when the air is absolutely or conditionally unstable
• uplift speeds can regularly exceed 150 km h-1
(faster than hurricane
wind speeds)
• the water content is also much greater than in other cloud forms – thus
promoting larger clouds
-can be up to 10 km thick
-forms in unstable conditions
-extremely fast uplifting air, faster then a hurricane
-have a tremendous amount of moisture
cumulus clouds (3 types)
vertical
–detached, billowy clouds with flat bases and moderate vertical development; sharply defined cloud boundaries
• these are typically subdivided into cloud species, depending on the extent of vertical development
1• cumulus humilis clouds (fair-weather cumulus)are composed of a single plume of rising air, usually generated by localized surface heating • vertical development is not as extensive • the clouds represent rising air, while the cloud-free areas represent falling air • these clouds may evaporate quickly
2.cumulus congestus clouds
-form under more intense conditions, where
multiple plumes of rising air create pillars of clouds
• the large vertical extent of these clouds suggests that the tops must be
much colder than the bottoms, so they will often contain a mix of water,
supercooled water and ice crystals
• glaciation of water into ice
at the cloud tops produce
less defined cloud
boundaries, which is easily
visible
- much more water content and are massive
- because they are so thick, the bottom of the cloud is warm and water dominates, top is cold and ice dominates; perfect for hail conditions
- glaciation causes cloud boundaries at top to look less definitive
3.Cumulonimbus clouds
-clouds with intense vertical development
with a characteristic anvil top; may be 10 km thick and appear very dark
when viewed from below
• a large cumulonimbus can occupy the entire vertical extent of the
troposphere, and the anvil is produced by the strong horizontal winds of
the stratosphere pushing ice crystals away from the cloud
.strong vertical motions can propel hailstones vertically through the top of the cloud
- produces A LOT of rain, with big rain drops and high intensity
- lifespan of about 45 mins because of evaporation
- when they group up they last much longer
lenticular clouds
form above mountain peaks, as the barrier creates
waves in the wind
• as the wind rises to the wave crest, cooling and condensation
occurs; as the wind falls to the wave trough, warming and
evaporation occurs
• lenticular clouds appear to be stationary, but are
actually controlled by a balance between condensation and evaporation
Pileus cloud
similar to lenticular clouds except lifted by rising air parcel rather than a mountain
banner cloud
are similar to lenticular clouds, but are typically individual streams of cloud forming immediately downwind of a mountain peak
mammatus clouds
s form under cumulonimbus clouds, where strong downdrafts force water droplets below the cloud base, or below the anvil • since these clouds have very high water contents, the water does not evaporate quickly in the unsaturated air below the cloud
nacreous clouds
• nacreous clouds form in the stratosphere, and are only visible at twighlight hours, during the winter at high latitude
• they are composed of supercooled water and/or ice crystals, and often
appear white and soft
• also known as mother of
pearl clouds
-incredibly thin clouds
Mother of pearl cloud
nacreous cloud
noctillucent cloud
form even higher, in the mesosphere
• due to their height, they are illuminated by the Sun well after nightfall,
allowing them to “glow”
• surface water vapour
does not reach the
mesosphere –they are produced by the oxidation of methane high in the atmos
cellometers
-used to measure cloud coverage
• of course, at any given moment, multiple cloud types may occur
• weather reports often list cloud coverage
• eg, scattered cumulus at 1000m, broken altostratus at 4000m,
overcast cirrostratus at 7000m…
•cellometers use lasers to detect and measure the height of clouds
• these are often employed at
airports, where cloud cover properties are a safety issue
• satellites are also very effective at tracking cloud development
• visible images are good at observing clouds, but are ineffective at and of course cannot see night clouds
judging cloud height,
• infrared images show the radiation emitted by clouds – high clouds
are typically colder than low clouds
