Cloud Morphology

Mechanism of cloud formationTypes of clouds & NomenclaturePrimary ClassificationSecondary ClassificationLow level clouds (Troposphere)Mid level clouds (Stratosphere)High level clouds (Mesosphere)

Noctilucent Clouds

What are clouds?Clouds are formed of tiny droplets of water or ice.Clouds form when water vapour cools and condenses. The temperature at which condensation occurs is called dew point.Condensation also requires nuclei (Aerosol particles) such as dust or soot.

Have you ever wondered why clouds have such unusual names? 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. (The first part of a cloud's name describes height, the second part shape.) The prefixes denoting heights are: cirro, high clouds above 20,000 feet (6,250 meters), alto and mid level clouds between 6,000 - 20,000 feet (1,875 - 6,250 meters). There is no prefix for low level clouds. The names denoting shapes are:cirrus mean curly or fibrous, stratus means layered, while cumulus means lumpy or piled. Nimbo or nimbus is added to indicate that a cloud can produce precipitation. Given that information, describe what each of the following clouds would look and act like? CumulonimbusNimbostartusCirrocumulusAltostratus

Types of Ascent of a cloud => formation of different kind of clouds.Local ascent of warm, buoyant air parcels in a conditionally unstable environment will result in the production of convective cloudsDiameter: 0.1 to 10 kmVertical velocity: few meters per secWater content: 1g/cubic meter

Forced lifting of stable air will result in the formation of layer clouds.Occurrence between 0 km to tropopause.Spread over hundreds of sq.kmVertical velocity: few cm to 10 cm/secWater content: few tenths of a gm/cubic meter

Forced lifting of air as it passes over hills or mountains => orographic clouds

Cooling of air below dew point after coming in contact with a cold surface => fog clouds

Cloud classificationDepending on the region of occurrenceLow-level Clouds: < 6,500 ft.Mid-level Clouds: 6,500 to 23,000 ft.High-level Clouds: 16,500 to 45,000 ft.

The primary classification of clouds mainly uses four Latin names coined by Luke Howard and Quaker in 1802

Cumulus(White and puffy clouds)Stratus(Sheets of low and grey clouds)Cirrus(Thin feather like clouds)Cumulonimbus(Rain clouds)

The secondary classification of the clouds will be based on their height of occurrence.

Cumulus Clouds

Low altitude cloud.Typical base below 7,000 feet.Clearly identified by distinct edges.Usually white and puffy.Noticeable vertical development.can be seen as isolated or grouped.Mostly composed of water droplets.In colder climates have ice crystals.Formation: Thermal convection currentsSeen : Worldwide except Antarctica because it is too cold. Precipitation: NONE

Stratus CloudsThese are lowest of the low lying cloudsMainly seen as layered clouds.The base below 7,000 feet.They appear as a grey overcast or can also be found scattered.Mainly water droplets.Individual stratus clouds have ill defined edges.Seen: Worldwide mainly at coasts and near mountains.Precipitation: LITTLE DRIZZLE

Cirrus CloudsThey are seen at high altitudes.Seen as very thin feather like clouds.Highest of all the clouds.Occurrence: 16,500-45,000 ft.Mostly made of ice crystal.Generally occur in fair weather and point in the direction of the air movement.Seen: WorldwideTypical bases above 18,000 feet.Precipitation: None that reaches Earth.

Cumulonimbus CloudsTallest of all the cloudsDark towering clouds produce Rain, Thunderstorms, Lightening, Strong winds, Tornados etc.They can span all cloud layers Can reach upto a maximum altitude of 60,000 feet.Formation: Upwardly mobile cumulus clouds.Seen: Mainly in tropics rare in poles.Cumulonimbus clouds usually have large anvil-shaped tops because of the stronger winds at those higher levels of the atmosphere. Most magnificent cloud of all.Precipitation: H E A V Y

Secondary cloud classificationBased on altitude of occurrence.

Low Clouds (

Mid level clouds:

AltocumulusAltostratusNimbostratus

Altocumulus:Form due to Mid-level atmospheric and wave propagationSeen: 6,500-18,000 feetWorldwide occurrence Precipitation; light rain.Composition: Liquid water

AltostratusBelow 6,500 ft is stratusBetween 6,500 to 23,000 ft is altostratus cloudsUsually formed from the thickening and lowering of a cirrostratus cloud on its way to becoming a nimbostratus cloudWorldwide occurrence, mainly at mid latitudesPrecipitation: Occassional rain and snow.AltocumulusAltostratus

Nimbostratus:Mainly rain clouds.Usually formed from the thickening and lowering of a altostratus cloud.Seen at 2,000 to 18,000 feet.World wide occurrence and common in mid latitudes.Precipitation: Moderate to heavy rain or snow, which is generally steady and prolonged

High CloudsCirrusCirrocumulusCirrostratus

CirrocumulusCirrocumulus clouds are usually a transitional phase between cirrus and cirrostratus clouds.Worldwide occurrence at 16,500 to 45,000 feetMainly composes of ice crystalsCloudlets formed by choppy winds and high moisture levels in upper troposphere.

Cirrostratus:Formation: Spreading and joining of cirrus cloudsThey are mainly delicate cloud streaksWorld wide occurrence at 20,000-42,000 ft.Composition: Ice crystalsPrecipitation: noneProduce a variety of optical effects in the upper atmosphere.

Noctilucent CloudsThe most brightest and magnificent clouds on Earth.Highest clouds in Earths atmosphere (75 85 km).Contains mainly water ice.Seen only in deep twilights.Very difficult to spot.Only between 50 70 deg of north and south of equator.Formation is not totally understood.

* Phase changes of watervapor liquidliquid solidvapor solidThe phase transitions from left to right are in increased molecular order. They must overcome a strong free energy barrierThese phase changes do not occur at thermodynamic equilibriumEx: water droplets are characterized by strong surface tension forces. For a droplet to grow by condensation from vapor, the surface tension has to be exceeded by a strong gradient in the vapor pressure.Saturation is equilbirum situation when rates of evaporation is equal to the rate of condensation. Since there is free energy barrier, phase transition does not occur at equilibrium saturation of bulk water. Vapor (Pure) when cooled adiabatically to equilibrium saturation should not give droplets. This can happen only when the saturation or RH reaches several hundred percent.

SO HOW DOES DROPLETS FORM IMMEDIATELY WHEN RH=100 @ LCL

Nucleation: Random formation of a new embyonic phase within the supersaturated parent phase. First order phase transitions, associated with latent heat.

Homogeneous nucleation: Phase transition occurs at random location within the parent phase, without the presence of a foreign surface. E.g. cloud droplets forming directly from the vapor phase.

Homogeneous cloud droplet nucleation requires several hundred percent supersaturation

In the presence of soluble CCN, no real nucleation process is requiredto form cloud droplets. Cloud droplets can form close to equilibrium saturation with CCN.

Heterogeneous nucleation: Formation of the new phase on a foreign surface (e.g. insoluble aerosol particles)

Many different types of condensation nuclei are present in the atmosphereSome becomes wetted at relative humidities less than 100% and account for haze that impedes visibility.The relatively large condensation nuclei are those which may grow to cloud droplet sizes.When the moist air rises (cooled) in adiabatic ascent, the relative humidity reaches 100% the hygroscopic nuclei then begin to serve as centers of condensation.

Precipitation is developed when the cloud becomes unstable. When the cloud becomes unstable the droplets may grow at the expense of other droplets. There are two mechanism by which the cloud becomes unstable. 1. Collision and Coalescence2. Interaction between droplets and ice crystals.

When a large drops falls through small drops, it will collide with a limited number of droplets and it may result in the growth of the droplet. Clouds with larger droplets tend to become unstable and result in precipitation.When an ice crystal exists in the presence of large number of supercooled water droplets The situation is immediately unstable. Equilibrium vapor pressure over ice is less than that over water at the same temperatureThis implies that the ice crystals grow by diffusion of vapor and drops evaporate to compensate for this. When the ice crystal becomes sufficiently larger, it will fall through the other droplets, Now if the collision is with ice crsytals, then it will result in snow flakes, or if the ice crystal collects water it will result in hail.Once the temperature reaches more than 00 C. melting might happen and as a result we would see water emerge from the base of the cloud.

Rain Drops, Cloud Droplets, and CCN

relative sizes of rain drops, cloud drops, and CCN:raindrops - 2000 mm = 2 mmfall at a speed of 4-5 ms-1 cloud drops - 20 mm = 0.02 mmremain suspended in the air CCN - 0.2 mm = 0.0002 mmremain suspended in the air To get a droplet (20 mm) to grow to raindrop size (2000mm) it must increase in size by a factor of 100 (two orders of magnitude):2000mm/20mm = 100 this occurs in about 30 minutes in a thunderstorm!!!

How does this happen??

Processes for Cloud Droplet Growth How does this happen??By:condensationcollision/coalescenceice-crystal process

Water Droplet GrowthCondensation & CollisionCondensational growth: diffusion of vapor to droplet

Collisional growth: collision and coalescence (accretion, coagulation) between droplets

Water Droplet Growth - CondensationFlux of vapor to droplet (schematic shows net flux of vapor towards droplet, i.e., droplet grows)Need to consider:

Vapor flux due to gradient between saturation vapor pressure at droplet surface and environment (at ).

Effect of Latent heat effecting droplet saturation vapor pressure (equilibrium temperature accounting for heat flux away from droplet).

Cloud Droplet Growth by Condensation Consider pure water in equilibrium with air above it

C-C equation to calculate es

Cloud Droplet Growth by Condensation Consider pure water in equilibrium with air above it: then the RH = 100% evaporation = condensation vapor pressure (e) = saturation vapor pressure (es) if evaporation > condensation, water is _________if evaporation < condensation, water is ________Now, a droplet surface is not flat, instead, it has curvature.....Q: how does curvature affect the evaporation/condensation process??

Equilibrium

Surface tensionWork per unit area necessary to increase the surface area.Process stores potential energy in the surface.Units: J/m2 or N/m.For water ~ 7.5x10-2 N/m at meteorological temps.

Vapor pressureThe pressure on a liquid or solid surface due to the partial pressure of the molecules of that substance in the gas phase which surrounds the surface.

The Curvature Effecte

Flat versus Curved Water Surfaces

Flat versus Curved Water Surfaces: curvature effect more energy is required to maintain the "curvature" of the drop therefore, the water molecules on the surface of the drop have more energy therefore, they evaporate more readily that from the flat water surface (compare the length of the red arrows) therefore: evaporation rate off curved surface > evaporation rate off of flat surface since air above both surfaces is saturated, then evaporation rate = condensation rate therefore, condensation rate onto droplet > condensation rate onto flat water surface therefore, esdrop > esflat therefore:if RHflat = 100%, then RHdrop > 100% the air surrounding the drop must be supersaturated!! This is called the curvature effect

Curvature Effect Curvature effect -->

notice that for the droplet to be in equilibrium (evaporation off drop = condensation onto drop), the environment must be supersaturated

also notice that the curvature effect is larger for smaller dropsthis makes sense since smaller drops have more curvature that larger drops

Class activity-Curvature Effect Q: what will happen to a drop 1.9 mm in size that is in a cloud where the RH is 100.05%?Q: what will happen to a drop 1.9 mm in size that is in a cloud where the RH is 100.15%?

KELVINS EQUATION

We will then make a suitable transformation to deduce the vapour pressure over a spherical drop of radius r.

We begin with a sketch of the meniscus in a capillary tube that is inserted into the plane surface of a bulk liquid:

*********************From Bill Olsen

**Wang and Olsen slide.**Answer 1: Being added toAnswer2: Being removed from*****