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. 

FAQs