Unit 3 Cloud and Precipitation 8 Hrs

Condensation and Precipitation Phenomen, Formation of clouds, the different types of clouds, Development and classification of clouds as per height, Cloud atlas; Drizzle, Hail, snow, sleet; dew. Precipitation : Processes, forms and types.

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UNIT 3(Cloud & Precipitation)

Explain the following terms ?

(a) Condensation : Air can hold a certain quantity of watervapour at a given temperature and no more. This temperature is called Dew Point Temperature. If the air is cooled below the dew point temperature, the excess watervapour settles down as ‘Dew’ as very small water droplets. The conversion of watervapour into liquid is called Condensation. The condensation depends on temperature and cooling. The watervapour in the air forms into cloud, rain, snow, frost, dew, etc., on account of condensation process. When condensation takes place, latent heat is also released which heats up the surrounding atmosphere.

(b) Sublimation/Deposition: Is the process by which, in sub freezing air, water vapour changes directly to ice without becoming liquid. This is how snow forms in clouds, as well as frost and hoar frost on ground.

What is meant by LCL ?

Lifting Condensation Level(LCL):The level in the upper atmosphere at which saturation is reached and condensation occurs due to adiabatic ascent is known as Lifting Condensation Level(LCL), which can be found out from Tephigram where the cloud base is formed .

What is meant by Condensation Nuclei ?

Condensation Nuclei: Condensation Nuclei are required for the vapour to condense. The Nuclei are provided by the dust particles, sea spray, combustion products, pollutants etc., when the cloud formation takes place.

Briefly explain each form of the condensation ?

· Dew: When the water vapour condenses into droplets of water & gets deposited on exposed surfaces on or near the ground, it is dew.

· Frost: Is the name given to ice crystals deposited on exposed surfaces on or near the ground, when the ground temperature is much lower than freezing point. In this case, the water vapour directly turns into ice without becoming water.

· Mist & Fog: Due to the presence of very high quantity of watervapour, mist and Fog are caused. When visibility is <1 Km is called Fog, with high relative humidity. When visibility >1 Km is called Mist. In both the cases, R.H must be high and more than 90 %.

· Haze: If visibility is reduced by solid particles such as dust, sand, volcanic ash, etc. in suspension in the air, haze is said to exist. Haze can, in rare cases, reduce visibility to 200 m or less.

· Cloud: When air is cooled below its dew point temperature, the excess watervapour condenses into minute(very small) particles of water, which remain suspended in air. Millions of such particles, close together, become visible as cloud.

What is meant by Precipitation Phenomena ?

Is the name given to water drops from a cloud, which fall towards the ground. Precipitation occurs when a local portion of the atmosphere becomes saturated with watervapour, so that the water condenses and "precipitates". Whilst falling through different layers of atmosphere, the water drops may freeze into soft ice(snow) or hail . Mist & Fog not called precipitation because the water droplets remain suspended in air and do not fall. Sometime precipitation evaporates completely, and does not reach ground. It may then visible as vertical streamers below clouds and is called ‘Virga’.

Briefly explain different forms of Precipitation ?

Liquid Precipitation:

· Drizzle: Is a type of precipitation from clouds consisting of minute water deoplets50n the nature of spray whose diameter size is less than 0.5 mm. Termed heavy or light depending on intensity of precipitation.

· Rain:Is a precipitation of medium size water drops falling mainly from layer type of clouds with diameter larger than 0.5 mm. Termed heavy or light depending on intensity of precipitation.

· Clouds: Same as for rain/drizzle.

Freezing Precipitation

Freezing Rain/Drizzle: The water drops freeze on impact with cold ground.

Frozen Precipitation:

· Snow or Snow Flakes: Frozen rain in the form of small ice.

· Snow Pellets: White opaque grains of ice, very soft and spherical or conical in shape, diameter 2 & 5 mm.

· Ice Pellets: Transparent pellets of ice, frozen hard, less than 0.5 mm Diameter

· Hail : Solid ice pellets falling from thunder clouds with diameter size> 0.5 mm.

Sleet: Is a mixture of rain and snow.

Cloud and Precipitation:

Cloud

Precipitation

Stratus

Drizzle

Altostratus

Drizzle, snowflakes

Nimbostratus

Rain, snowflakes

Cumulus

Showers: rain, snow

Cumulonimbus

Showers, rain, snow, hail

Stability of the Air, Cloud Form and Associated Precipitation

STABLE AIR

UNSTABLE AIR

Stratiform clouds

Cumuliform clouds

Layer cloud

Discrete clouds

Slow upwards motion

Fast upwards motion

Cloud forms slowly

Cloud forms quickly

Air forced upwards

Air rises spontaneously

Orographic, frontal & turbulence clouds

Orographic, frontal & convection clouds

Drizzle, rain & snow

Rain, snow & hail

Write short notes on various types of clouds/Rainfall ?

Convectional : Clouds are formed by insolation. The air which becomes warmer than surroundings will rise and cool adiabatically leading to the formation of heap type clouds and consequent convectional thundershowers.

Orogaphic : When the moist air stream moves upwards against the hill top, due to adiabatic cooling, orographic clouds and rainfall occur. Indian South West monsoon is a typical example of an orographical rainfall.

Cyclonic or frontal : When different types of airmasses like warm airmass and cold airmass meet and due to Frontgenesis, frontal type of rainfall occurs.

Picture: 1. Convection rainfall 2. Oragraphic rain fall 3. Cyclonic or frontal rainfall

Turbulent type

When the terrain or the surface is rough, the air stream is lifted upwards and turbulent clouds like stratocumulus cloud form.

Describe Cloud formation processes ?

Deposition of water above the Earth's surface creates clouds. In general, clouds develop in any air mass that becomes saturated (relative humidity becomes 100%). Saturation can occur by way of atmospheric mechanisms that cause the temperature of an air mass to be cooled to its dew point or frost point. The following mechanisms or processes can achieve this outcome causing clouds to develop:

1. Orographic uplift: occurs when air is forced to rise because of the physical presence of elevated land. As the parcel rises it cools as a result of adiabatic expansion at a rate of approximately 10° Celsius per 1000 meters until saturation. The development of clouds and resulting heavy quantities of precipitation along the west coast of Canada are mainly due to this process.

2. Convectional lifting: is associated with surface heating of the air at the ground surface. If enough heating occurs, the mass of air becomes warmer and lighter than the air in the surrounding environment, and just like a hot air balloon it begins to rise, expand, and cool. When sufficient cooling has taken place saturation occurs forming clouds. This process is active in the interior of continents and near the equator forming cumulus clouds and or cumulonimbus clouds (thunderstorms). The rain that is associated with the development of thunderstorm clouds is delivered in large amounts over short periods of time in extremely localized areas.

3. Convergence or frontal lifting: takes place when two masses of air come together. In most cases, the two air masses have different temperature and moisture characteristics. Oneof the air masses is usually warm and moist, while the other is cold and dry. The leading edge of the latter air mass acts as an inclined wall or front causing the moist warm air to be lifted. Of course the lifting causes the warm moist air mass to cool due to expansion resulting in saturation. This cloud formation mechanism is common at the mid-latitudes where cyclones form along the polar front and near the equator where the trade winds meet at the intertropical convergence zone

4. Radiative cooling: occurs when the Sun is no longer supplying the ground and overlying air with energy derived from solar insolation (e.g., night). Instead, the surface of the Earth now begins to lose energy in the form of long wave radiation which causes the ground and air above it to cool. The clouds that result from this type of cooling take the form of surface fog.Of course these causes of cloud development do not always act in a singular fashion. It is possible to get combinations of all four types, such as when convection and orographic uplift cause summer afternoon cloud development and showers in the mountains.

How the atmosphere is cooled to dew point temperature ?

(a) Outgoing radiation of heat from the ground resulting in cooling of the air close to it by conduction.

(b) Incursion of moist air stream from elsewhere over the cold ground.

(c) Adiabatic ascent of air to high levels.

* In case of (a) & (b) above, if the dew point temperature is reached near the groundmist fog or Low clouds can from.

Which cloud develop thunder storm and what are the conditions favourable for formation of Thunder Showers of Thunderstorm Clouds ?

Cumulonimbus is a cumulus cloud which develops into great heights due to instability. The release of energy due to over turning of air in unstable layer gives rise to a Thunder storm.

Condition favourable for formation of Thunder storm are:

1. Steep lapse rate. That is stepper than SALR (more than 50 C/ km), upto 5 to 6 km of depth.

1. Adequate supply of moisture from below.

1. Trigger action.

1. Trigger action can be provided by : Local Convection, Orgraphic Lifting, Convergence,Frontal lifting and Radiational or Katabatic cooling.

Briefly explain the Life cycle of Thunder Storm ?

a. Developing Stage or Cumulus stage. : At this stage the cloud keeps on growing at a very rapid rate to a height of more than 40000 feet or so Up-draught prevails throughout the cloud with an extreme velocity of 100 feet/Second. Strongest up draughts are found near the middle part of the cloud.

b.Matured stage: This stage begins with the fall of rain from the clouds. Due to the fall of rain, down- draughts are caused. The down draughts are quite cold with a maximum velocity of about 40 feet/Sec. At this stage, both up and down draughts are found side by side.

· The down draughts can cause microburst, squall, Gust, etc., Matured stage is the 2nd stage. At this stage both up and down draughts co exist. Once this stage is attained, the growth of Cumulonimbus cloud stops. This is the severe phase of a Thunderstorm.

c.Dissipating stage: At this stage, only downdraughts are present. Entire lower portion exhibits down draughts. In the upper part, the cloud spreads out laterally giving an appearance of cirrus cloud.

How clouds are formed ?

· Cloud formation is associated with rising air

· Rising air cools adiabatically

· The Air reaches its dewpoint

. Cloud is formed(Shape) depends upon the stability of rising air.

What is the type of weather expected in an anti cyclone ?

(a) Cirrus clouds in bands or filaments aligned towards the direction of the storm centre.

(b) Unusually clear visibility may occur.

(c) Sometimes peculiar dark red/copper color of sky is seen at sunset before a TRS

(d) Increase of wind force as the pressure falls

(e) Threatening appearance of dense, heavy clouds on the horizon

(f) Frequent lightning may be seen

(g) Succession of squalls, with or without rain.

Brieflyexplain the following Theory/Formation of Rain Drops ?

(a) Bergeron Ice crystal theory: Inside a cloud, both ice crystal and water droplets coexist side by side. The saturation vapour pressure over ice crystal is much less compared to that of over water droplets because of which the watervapour evaporates from the water surface and deposits over the ice crystal which keeps on growing bigger and increases in weight. Due to gravity it starts falling out of the cloud and due to lower level warming up, ice crystal melts and reaches the ground as raindrops. This was found out by a Norwegian meteorologist Bergeron and hence it is called Bergeron ice crystal theory.

(b)Coalescence theory : The bigger drops inside the cloud collides with the smaller drops and eventually they become very big and come out of the cloud as raindrops.

(c)Wake theory: In the cloud, when big or giant drops move, leavening a vacuum behind into which the small drops are sucked in and join with big drops. The big drops fall out of the cloud as raindrops.

Give a detail account of ten(Low/Medium/High & Special) fundamental clouds ?

Low Clouds( Bases between sea level and 2 Km above sea level

· Stratus(St): A Low even layer of dark grey cloud with light and dark patches. It has a dry look and does not cause precipitation. It resembles for, but is not experienced at sea level. It can obscure the sun completely and can greatly weaken the daylight.

· Nimbostratus (NS) : Grey colour, often dark grey colour which gives continuous heavy rain. Its base can be from surface to 2.5 Km. It is a thick cloud. Thickness is (450-1500 m). Visibility in this cloud can be zero. Severe Icing can occur. It is associated with a warm front.

· Strato Cumulus (SC): They are soft and dark clouds, grey or whitish rounded masses regularly arranged. Light Intermittent rain or shower is likely. Base of the cloud is from (1500-4000 ft). Its thickness is about 500 to 3000 ft.

Medium Clouds(Bases between 2 Km & 6 Km above sea level

Altocumulus (AC):They are white or gray patch or layer clouds composed of rounded masses, rolls etc, which are sometimes partly fibrous or diffuse. The elements are arranged in the form of waves. AC cloud sometimes gives rain, which does not reach the ground. Base is from 2 – 6 Km). Icing is not likely. It is a thin cloud.

Altostratus (AS): It is a grayish or bluish cloud sheet. It is a layer cloud. Sun or Moon can be seen vaguely like through a ground glass. It gives continuous light rain or drizzle. Its base is from(2-6 Km). Its thickness is about (4 - 5 Km)

High Clouds(Bases between 6 Km above sea level & Tropopause:

Cirrus (C I):This cloud is composed of ice crystals white in colour having feather like appearance and fibrous structure. It is a sign of near approach of bad weather. Its base is higher than 8 Km. No icing, No rain. It is a thin cloud. Sometimes, it gives condensation trails also known as contrails, behind an aeroplane

Cirrocumulus (CC): It is composed of globular masses white in colour. It resembles sand ripples on sea shore. It is a very thin cloud. No rain occurs. Base is more than 6 Km.

Cirrostratus (CS): A thin whitish veil cloud which does not blur the outlines of Sun or Moon but gives rise to HALO. No Icing, No rain. Base is more than 2000ft. It is associated with warm front.

Special Cloud( With vertical Development Bases at low level but Tops may extend well into high

Cumulus (CU):Detached clouds, generally dense and with sharp outlines developing vertically in the form of rising mounts, domes or towers. Base is 3000 to 5000ft. Its thickness can be upto 15000ft.

Cumulonimbus (CB):Heavy masses of clouds with great vertical development. Its summit rises in the form of mountains or towers. The upper parts have fibrous texture and often spread out in the form of anvil. This cloud gives thunderstorms. Base of the cloud is 1500 to 5000ft. Its thickness is upto 40000ft or sometimes even more. Severe Turbulence, Icing squalls are likely in the cloud. Vertical clouds generally form in an atmosphere which is unstable with adequate moisture

OR(Short form)

· High Clouds-Above 6 Km(Cirri From,): Cirrus(Ci), Cirrostratus(Cs), Cirrocumulus(Cc) - High Cloud, Tenuous(Thin),Ice crystals & Associated with depressions.

· Heap Clouds(Cumuliform): Cumulus(Cu), Cumulonimbus(Cb) - Unstable air, Discrete (partly) clouds, Vigorous(strong) upward motion & Showers.

· Low Clouds-0-2 Km(Strati Form): Stratus(St), Stratocumulus(Sc), Nimbostratus(Ns) - Stable air, widespread, slow upward motion & Continuous precipitation.

Medium Clouds-2 – 6 KM): Altostratus(As), Altocumulus(Ac)

Name the type of cloud where the rainbow forms ?

Cumulonimbus covers the sky completely, its top cannot be seen and hence it cannot be visually distinguished from Nimbostratus. However if precipitation is experienced, they can be distinguished Cumulonimbus gives showers, whereas Nimbostratus gives continuous precipitation with little variation in intensity. However, in both cases, the precipitation may sometimes evaporate before reaching the ground and is then visible streamers called ‘Virga”/Rainbow below the base of the cloud.