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Condensation(Fogs and Clouds)
Geronimo R. Rosario
Condensation is the process of a substance in a gaseous state transforming into a liquid state. This change is caused by a change in pressure and temperature of the substance.
Condensation
Condensation can form dew, fog, or clouds and they all need saturated air to develop.Condensation is the direct cause of precipitation.It is the reverse of evaporation.Water vapor is changed from the vapor state and becomes droplets of water.
The necessary condition: cooling of air to below its dew point until
it is saturated [individual / combined changes in air volume, pressure,
temperature / R.H.] The sufficient condition:
presence of condensation nuclei [hygroscopic particles – wettable substances]
Condensation conditions
The necessary condition: cooling of air to below its dew point until
it is saturated - radiation cooling
- advective cooling- orographic and frontal uplifting & cooling- convective or adiabatic cooling
Condensation conditions
The sufficient condition:presence of condensation nuclei- Condensation does not take place easily in clear or pure air which can be cooled below its dew point without condensation occurring.hygroscopic particles – wettable substancesThey attract water-vapour molecules when the moisture content is near saturation point.
e.g. dust, sea salt from evaporated spray, sulphur acid from combustion, volcanism
Condensation conditions
Hygroscopic- water - seeking nuclei . Ex. Ocean salt, dust and smoke
Hydrophobic- water-repelling such as oils, gasoline, and paraffin waxes
Hygroscopic vs Hydrophobic Nuclei
Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water vapour condenses.
Water requires a non-gaseous surface to make the transition from a vapour to a liquid; this process is called condensation.
Condensation Nuclei
Aitken nuclei- radius less than 0.2 m Large nuclei- Particles ranging in size from 0.2 to 1
μm Giant nuclei,- are much larger and have radii
exceeding 1 μm. ◦ The condensation nuclei most favorable for producing
clouds (called cloud condensation nuclei) have radii of 0.1 μm or more.
◦ Usually, between 100 and 1000 nuclei of this size exist in a cubic centimeter of air.
◦ Condensation nuclei are extremely light (many have a mass less than one-trillionth of a gram), so they can remain suspended in the air for many days.
CCN Sizes
Characteristic Sizes and Concentration of Condensation Nuclei and Cloud Droplets
defined when visibility < 1 km Condensation on less active
nuclei Fogs form when air saturates (its
relative humidity reaches around 100 percent), and the water vapor within the air mass condenses on small particles in the air to form liquid cloud droplets.
Fogis a visible mass consisting of cloud water droplets or ice crystals suspended in the air at or near the Earth's surface.
Mist is defined as 'when there is such obscurity and the associated visibility is equal to or exceeds 1000 m'.
Mist and fog are often used interchangeably - and they are closely related - but there is a key difference which depends on how far you can see through them.
The defining difference between mist and fog is visibility; if it is less than 1,000 metres we call it 'fog' and if visibility is greater than 1,000 metres we call it 'mist'.
Fog vs Mist
Haze is also another term associated with fog but technically are different from each other.
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky.
Dry haze is when there are dust or salt particles in the air that hinder visibility.
Wet haze occurs when water vapor condenses on the particles, which happens at a relative humidity of around 75%.
Haze
Fog• Type of CN affects fog• Over the ocean
– Fewer, larger drops• Over urban areas
– More, smaller drops– Lower visibility– London Fog
• Chemical reactions can cause fog to become acidic
• If temperatures drop belowfreezing, freezing fogmay result
by cooling the air (air is cooled below its saturation point (dew point)
by evaporation and mixing - water vapor is added to the air by evaporation, and the moist air mixes with relatively dry air.
Fog formation
Radiation fog Advection fog Upslope fog Evaporation or
steam fog Precipitation or
frontal fog
Types of Fog
Results from radiation cooling of the ground and the surrounding air.
It occurs at night It requires clear skies and fairly high relative
humidity. Under these circumstances, the ground and the
air just above it will cool rapidly. Because the relative humidity is so high, just a
small amount of cooling will lower the temperature to the dew point.
Radiation fog
Radiation fog• Form upward from the
ground– Deepest around sunrise– May intensify after
sunrise (dew evaporation)
• “Burns Off” with more insolation– Dissipates from bottom up– Dissipates easily around
edges (thin, mixing
• type of fog where warm, moist air moves over cooler land and/or water, cooling the air moving over it to its dew point
• Breeze required• May combine with radiation
fog• radiational fog tends to form
in calm conditions over inland areas, advection fog tends to form in breezier conditions along coastlines
Advection fog
Advection fog
A type of fog where it forms when winds blow air up a slope (called orographic lift), adiabatically cooling it as it rises, and causing the moisture in it to condense.
Air expands adiabatically (cooling or warming that results when air expands or contracts, but not because heat was added or removed.)
This is the only type of fog that forms adiabatically.
If the dew point is reached, an extensive layer of fog may form.
Upslope fog
Upslope fog
forms over bodies of water overlain by much colder air. The most common form, which occurs when colder air overlies
Warmer water, is often called steam fog since the condensing water vapor looks like steam rising from the water
This commonly occurs over lakes in autumn, particularly in the early morning hours, when cold air moves over water that is still warm from summer
Evaporation or Steam fog
Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores.
Evaporation or Steam fog
forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor.
As the rain falls through the layer of cold air, some of the water from the rain drops evaporates
If enough water vapor is added to the air it can become saturated, resulting in condensation into very tiny water droplets (fog)
This most commonly is associated with warm fronts in winter when warm air rises over cold air which is why this type of fog is sometimes called frontal fog
Precipitation or frontal fog
Precipitation or frontal fog
Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is sometimes used to refer to radiation fog.
Valley fog forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into a valley, with warmer air passing over the mountains above.
Other types of fog
Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime. This is very common on mountain tops which are exposed to low clouds. It is equivalent to freezing rain, and essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type.
Ice fog is any kind of fog where the droplets have frozen into extremely tiny crystals or ice in midair. Generally this requires temperatures at or below −35 °C (−30 °F), making it common only in and near the Arctic and Antarctic regions.
Other types of fog
As air moves onshore, it crosses the coastline at nearly a right angle. This causes the air to flow together or converge in the vicinity of the headlands. This area of weak convergence causes the surface air to rise and cool just a little. If the rising air is close to being saturated, it will cool to its dew point, and fog will form. Meanwhile, near the beach area, the surface air spreads apart or diverges as it crosses the coastline. This area of weak divergence creates sinking and slightly warmer air. Because the sinking of air increases the separation between air temperature and dew point, fog is less likely to form in this region..
Why Are Headlands Usually Foggier Than Beaches?
Fog dissipates usually in an hour through solar heating
It takes several hours in the valley and coastal regions particularly during cold season.
Fog clears at outer edges first.
Dissipation maybe affected by wind, fog thickness, overlying cloud layers and underlying surfaces ( snow cover, cool lakes or ocean and soil condition)
Dissipation of fog
Fog Visibility Index
Clouds
Clouds A cloud is a visible
aggregate of tiny water droplets and/or ice crystals suspended in the atmosphere and can exist in a variety of shapes and sizes.
In 1802 an Englishman by the name of Luke Howard invented the cloud naming system that is still in use today. Howard used Latin names to describe clouds.
• part of a cloud's name describes height• part tell us something about the cloud’s shape The prefixes denoting heights are:
cirro, high clouds above 20,000 feet;alto are mid level clouds between 6,000 – 20,000 ftThere is no prefix for low level clouds.
The names denoting shapes are:cirrus mean curly or fibrous
stratus means layered cumulus means lumpy or piled. Nimbo or nimbus is added to indicate that a cloud can produce
precipitation.
Clouds
• Properties:– Variety of sizes– Contain water drops
and/or ice crystals– Various heights– Stratospheric clouds
rare, but important for creation of the ozone hole
Clouds
Cloud Classification
Clouds that grow high up into the atmosphere rather than spreading across the sky.
They span all levels of the troposphere and can even rise up into the stratosphere.
Clouds with vertical growth develop by warm air rising from the surface
Types of CVD Cumulus Cumulonimbus
Clouds with Vertical Development
Cumulus humilis◦ Develop primarily from
convection◦ Associated with fair
weather◦ Usually evaporate shortly
after formation and are vertically limited
Cumulus fractus◦ Ragged-edge cumulus
clouds that are smaller than cumulus humilis and scattered across the sky.
◦ strong tattered edges; ◦ rapidly changing contours.
Cumulus
Cumulus congestus◦ More organized development
as cloud towers appear◦ Each tower is indicative of
uplift cells◦ Cells are short lived but are
constantly replaced ◦ Each tower progresses higher◦ Large height extension; like
cauliflower-looking forms.
Cumulus mediocris◦ Moderate development in
height.◦ Not so wide, not so tall
Cumulus
Cumulus virga◦ look like a rod or stripe◦ responsible for cloud seeding. ◦ This is because it carries the
small particles to different cloud formation.
Cumulus praecipitatio◦ look like puffy cotton balls in the
sky◦ With precipitation◦ common in most parts of the
world although they are more frequently visible in tropical climates where there is a higher level of humidity in the area
Cumulus
Cumulus velum◦ With a veil (from ice) on the
upper part of the subsequent swelling - sometimes broken
◦ sail-shaped clouds and they appear as a thin sheet of cloud
◦ argely formed during fair weath
Cumulus pileus◦ with flat cap or hood (ice
crystals).◦ buffer between cumulus or
cumulonimbus formations
Cumulus
Cumulus arcus◦ horizontally formed cloud and it
is a low level cloud which usually produces a drizzle or rain.
◦ Shield cloud and looks like a wall cloud
Cumulus tuba◦ white clouds seen on sunny
days and usually against the blue sky. This heap of cloud has a flat basis that is seen in the middle and its vertical development produces a tower-like or even a cauliflower's shape.
◦ with funnel cloud or tornado
Cumulus
Cumulus pannus◦ shreds or patches of clouds
which are large, cottony or even puffy in nature
◦ These clouds can be seen around the globe except in Antarctica since it is very cold there
Cumulus radiatus◦ also known as cloud streets
because they appear to form as parallel lines that run across the sky
◦ Less common
Cumulus
Cumulus mamma Common white and puffy,
these clouds look like cotton balls or bubbles that are hanging downwards from the sky.
The cumulus mamma clouds tend to have a flat base and appear lumpy. They do not necessarily form in a large formation and it is quite common to see small, free flying formations of the cumulus mamma cloud
Cumulus
Cumulonimbus
Cumulonimbus mamma Cumulonimbus incus
Cumulonimbus pannus Cumulonimbus calvus
Cumulonimbus
Cumulonimbus arcus Cumulonimbus capillatus
Cumulonimbus pileus Cumulonimbus praecipitatio
Cumulonimbus
Cumulonimbus tuba Cumulonimbus velum
Cumulonimbus virga
Low clouds, with their bases lying below 2000 m (6500 ft), are almost always composed of water droplets; however, in cold weather, they may contain ice particles and snow.
Types◦ Stratus◦ Stratocumulus◦ Nimbostratus
Low Clouds
Stratus fractus appear during a cloudy day
having a dark gray to almost white color as they are made up of water droplets and generally formed in precipitation.
look like ragged sheets which became separated from a large stratus clouds because of the wind.
Stratus nebulosus uniformly layered, foggish-
looking and monotonous. most common forms of stratus
clouds
Stratus
Stratus opacus look like any other stratus clouds
that are flat and have no features at all
usually comes in different forms and sizes sometimes resembling different animals or creatures.
they almost entirely cover the sky and blocks out the sun in the area.
Stratus praecipitatio normally relatively featureless
and will partially or fully cover the sky.
Initially they may be light grey but progressively they will darken as they absorb more moisture
Rain cloud
Stratus
Stratus translucidus characterized by its veil-
like features and are thinly spread out in the sky such that the moon or the sun is still recognizable.
Stratus undulatus typically seen as slabs of
cloud that can actually cover the whole sky; the only distinguishing feature can be the waves as the base of the clouds are ruffled by the wind.
Stratus
Stratocumulus lenticularis Stratocumulus cumulogenitus Stratocumulus radiatus Stratocumulus castellanus Stratocumulus translucidus Stratocumulus undulatus Stratocumulus lacunosus Stratocumulus stratiformis Stratocumulus duplicatus Stratocumulus praecipitatio Stratocumulus perlucidus Stratocumulus virga Stratocumulus mamma Stratocumulus opacus
Types of stratocumulus clouds
Stratocumulus
Stratocumulus castellanus Stratocumulus cumulogenitus
Stratocumulus lacunosusStratocumulus duplicatos
Stratocumulus
Stratocumulus lenticularis Stratocumulus mamma
Stratocumulus mamma Stratocumulus opacus
Stratocumulus
Stratocumulus perlucidus Stratocumulus praecipitatio
Stratocumulus radiatus Stratocumulus stratiformis
Stratocumulus
Stratocumulus translucidus Stratocumulus undulatus
Stratocumulus virga
Nimbostratus pannus look like tubes essentially flat featureless and
essentially boring. cover the sky with a grey and
depressing clouds which signifies that something is developing and the weather may be changing for the better
Nimbostratus praecipitatio generally bring heavier precipitation They are not accompanied by
thunder or lightning Will generally last longer and are
constant
Nimbostratus
Nimbostratus Nimbostratus virga have shoots of rain
falling like triangular wedges that are wider at the top, near the base of the cloud that reach a point somewhere above the ground without touching it.
Rain bearing cloud
oBases between 2000 and 6000 m (6-19,000 ft)oLargely composed of liquid dropsoCarry the “alto” prefixoAltostratus is typically thick enough to almost
fully obscure the sun or moon and blanket the sky from horizon to horizon
oAltocumulus, typically typified by a banded arrangement of billowy clouds
Middle Clouds
Altocumulus castellanus Altocumulus undulatus Altocumulus mamma Altocumulus stratiformis Altocumulus virga Altocumulus lacunosus Altocumulus perlucidus Altocumulus radiatus Altocumulus lenticularis Altocumulus duplicatus Altocumulus undulatus Altocumulus floccus Altocumulus opacus Altocumulus translucidus
Types of altocumulus clouds
Altocumulus
Altocumulus castellanus
Altocumulus duplicatus
Altocumulus floccus
Altocumulus lacunosus
Altocumulus
Altocumulus lenticularis Altocumulus mamma
Altocumulus opacus Altocumulus perlucidus
Altocumulus
Altocumulus radiatus Altocumulus stratiformis
Altocumulus translucidus
Altocumulus
Altocumulus virga Altocumulus undulatus
Altostratus
Altostratus duplicatus Altostratus mamma
Altostratus opacus Altostratus pannus
Altostratus
Altostratus praecipitatio Altostratus radiatus
Altostratus translucidus
Altostratus
Altostratus undulatus
Altostratus virga
◦ Bases above 6000 m (19,000 ft)◦ Composed of ice◦ Cirrus is the most common
Wispy appearance due to low water content and cold temperatures
Fall streaks may appear below as ice crystals descend Mares’ tails - horizontal swirls, occur in turbulent conditions Cirrostratus occurs when cirrus thickens and stretch across
the sky May form a halo about the sun or moon as entering light is
refracted 22o by cloud ice crystals Cirrocumulus occurs due to thickening causing a billowy
appearance which resembles fish scales - a mackerel sky
High clouds
Cirrus
Cirrus castellanusCirrus aviaticus
Cirrus duplicatus Cirrus fibratus
Cirrus
Cirrus floccus Cirrus intortus
Cirrus radiatus Cirrus spissatus
Cirrus
Cirrus uncinus Cirrus undulatus
Cirrus vertebratus
Cirrocumulus virga Cirrocumulus castellanus Cirrocumulus lacunosus Cirrocumulus floccus Cirrocumulus undulatus Cirrocumulus mamma Cirrocumulus lenticularis Cirrocumulus stratiformis
Types of cirrocumulus clouds
Cirrocumulus
Cirrocumulus castellanus
Cirrocumulus lenticularisCirrocumulus lacunosus
Cirrocumulus floccus
Cirrocumulus
Cirrocumulus mamma Cirrocumulus stratiformis
Cirrocumulus undulatus Cirrocumulus virga
Cirrostratus
Cirrostratus duplicatus Cirrostratus fibratus
Cirrostratus nebulosus Cirrostratus undulatus
formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust.
Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft.
The exhaust of an aircraft contains both gas (vapor) and solid particles.
Contrail clouds
Lenticular clouds
Form along the crest of waves caused by air flowing over mountains
They are frequently lens shaped which is why they are called “lenticular” clouds
They can form one above another like a stack of pancakes and, given their unusual appearance, “UFO sightings” are common when these clouds are present
Unusual Clouds
Wall Cloud A localized lowering from the
rain-free base of a strong thunderstorm.
The lowering denotes a storm's updraft where rapidly rising air causes lower pressure just below the main updraft, which enhances condensation and cloud formation just under the primary cloud base.
Wall clouds take on many shapes and sizes.
Some exhibit strong upward motion and cyclonic rotation, leading to tornado formation, while others do not rotate and essentially are harmless
Shelf Cloud A low, horizontal, sometimes
wedge-shaped cloud associated with the thunderstorm
Although often appearing ominous, shelf clouds normally do not produce tornadoes
Billow Clouds created from instability
associated with air flows having marked vertical shear and weak thermal stratification.
The common name for this instability is Kelvin-Helmholtz instability.
These instabilities are often visualized as a row of horizontal eddies aligned within this layer of vertical shear.
Mammatus clouds pouch-like cloud
structures and a rare example of clouds in sinking air.
Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form; a commonly held misconception.
mammatus are usually seen after the worst of a thunderstorm has passed
Noctilucent clouds (NLC’s) or polar mesospheric clouds (PMC’s)
are found very high in the Earth's atmosphere. They are called Polar Mesospheric Clouds when they are viewed from space, and are referred to as noctilucent clouds when viewed by observers on Earth.
Unlike lower clouds that are associated with weather, these clouds form at the very edge of space in the atmospheric layer called the mesophere.
scientists think these clouds are made of frozen water or ice crystals.
Most clouds form as air parcels are lifted and cooled to saturation The main mechanism for cooling air is to force it to rise. Air expands
as it rises, because the pressure decreases through the atmosphere, and therefore cools. Eventually it may become saturated and the water vapour then condenses into water droplets to form cloud. If the temperature reaches below about -20 deg C many of the water droplets will have frozen so the cloud is mainly composed of ice crystals.
Mechanisms that Lift Air◦ Orographic Lift occurs as air is displaced over topographic
barriers such as mountains and hills◦ On the windward side of the barrier, air is displaced toward higher
altitudes and undergoes adiabatic cooling, possibly to saturation◦ On the leeward side, descending air warms through compression
leading to a dry rainshadow
Cloud Development
Orographic lifting
Orographic uplift (left) and orographically induced clouds(below)
◦ When boundaries between air of unlike temperatures (fronts) migrate, warmer air is pushed aloft
◦ This results in adiabatic cooling and cloud formation
◦ Cold fronts occur when warm air is displaced by cooler air
◦ Warm fronts occur when warm air rises over and displaces cold
Frontal lifting
A cold front (a) anda warm front (b)
Convergence◦ Atmospheric mass is non-uniformly distributed over Earth◦ Air advects from areas of more abundant mass to areas of
less mass◦ Air moving into these low pressure regions converges◦ Stimulates rising motions and adiabatic cooling
Localized Convection◦ Localized surface heating may lead to spatially limited free
convection◦ Vertical motions are stimulated from the surface upward
resulting in towering clouds and a chance for intense precipitation over small spatial scales
Convergence
Prerequisites for cloud formation water low T supersaturation Cloud Condensation Nuclei (CCN) or Ice Nuclei
(IN)
Sky opacity- the amount of sky visible in relation to the presence of cloud layers—is described on the basis eighths or oktas.
Sky Opacity
Why are clouds white?Since light travels as waves of different lengths, each color has its very own unique wavelength. Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light.
Why do clouds turn gray?Clouds are made up of tiny water droplets or ice crystals, usually a mixture of both. The water and ice scatter all light, making clouds appear white. If the clouds get thick enough or high enough all the light above does not make it through, hence the gray or dark look. Also, if there are lots of other clouds around, their shadow can add to the gray or multicolored gray appearance. Why do clouds float?A cloud is made up of liquid water droplets. A cloud forms when air is heated by the sun. As it rises, it slowly cools it reaches the saturation point and water condenses, forming a cloud. As long as the cloud and the air that its made of is warmer than the outside air around it, it floats!
How do clouds move?Clouds move with the wind. High cirrus clouds are pushed along by the jet stream, sometimes traveling at more than 100 miles-per-hour. When clouds are part of a thunderstorm they usually travel at 30 to 40 mph.
Why do clouds form at different heights in the atmosphere?The characteristics of clouds are dictated by the elements available, including the amount of water vapor, the temperatures at that height, the wind, and the interplay of other air masses.
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