Synoptic Meteorology 1 Lecture 8

Sahraei

Physics Department Razi University

http://www.razi.ac.ir/sahraei 1 Cloud Type by Rain

Finally, we can classify them based on the presence of rain Nimbus: any cloud that rains

Cumulonimbus: taller, towering versions Nimbostratus: low, flat clouds that are often of cumulus clouds. Their height can be associated with steady precipitation and from two to five miles. These clouds often occur in thick, continuous layers and are form thunderstorms. often dark gray in color.

2 Cumulonimbus Clouds As seen from Apollo 8

3 Lenticular or Mountain Wave Clouds

Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. 4 Lenticular clouds sometimes form at the crests of these waves. Under certain conditions, long strings of lenticular clouds can form, creating a formation known as a wave cloud.

5 6 Mountain Wave Clouds

7 Influences of Clouds

Reflect and absorb solar radiation

Reflect and absorb terrestrial radiation

Latent heat release  atmospheric heating

8 Cloud Radiative Effects

Clouds play an important controlling role in the global radiation budget.

Reflection of incoming solar (short-wave) radiation

Absorption of both solar and thermal infra red (long-wave) radiation (incoming & outgoing)

Emission of infra red radiation (up and down)

The altitude, type, and thickness of cloud, along with that of clouds above & below determines whether the local net effect is to warm or cool the air & surface below.

9 The ten main cloud types

Cirrostratus Cirrus Cumulonimbus Nimbostratus Cirrocumulus

Altocumulus Altostratus

Cumulus Stratocumulus Stratus

Ci. Cc. Cs. Ac. As. Ns. Sc. St. Cu. Cb. Cirrus

11 Cirrocumulus

These clouds look like tiny lumps. They have clear gaps between them.

They are ice crystals high in the sky. 12 Cirrostratus

These clouds are featureless sheets at high levels These can signal approaching bad weather

13 Altocumulus

These clouds are at mid-level in the sky They are formed from clear lumps with gaps between them

14 Altostratus

These are made of sheets of featureless clouds at a medium level in the sky

15 Nimbostratus

These dark grey clouds, found at middle levels, often also extend lower down They can bring heavy rain The rain can be seen falling in this picture 16 Stratocumulus

This is a mixture of both lumps and layers There can be some gaps in the clouds

It is a low level cloud 17 Stratus

This featureless, grey cloud can be found at low levels If it was any lower it would be fog

18 Cumulus

These low level clouds are made of fluffy white rounded heaps

19 Cumulonimbus

These are very large towering clouds They extend to great heights They often bring heavy precipitation It can be a giant storm cloud 20 Fog Clouds at ground level

Fog is caused by tiny water droplets suspended in the air.

The thickest fogs tend to occur in industrial areas where there are many pollution particles on which water droplets can grow.

Types of fog

Fogs which are composed entirely or mainly of water droplets are generally classified according to the physical process which produces saturation or near-saturation of the air.

21 Radiation fog

Radiation fog usually occurs in the winter, aided by clear skies and calm conditions.

The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually dissipate soon after sunrise as the ground warms.

An exception to this can be in high elevation areas where the sun has little influence in heating the surface.

22 Radiation Fog For the development of this fog, warm water is evaporating into cool air The cool air becomes saturated (its relative humidity becomes 100%) and condensation creates the fog

Cold Air

Condensation

Evaporation Warm Water

23 Advection fog

Advection fog occurs when moist air passes over a cool surface and is cooled. A common example of this is when a warm front passes over an area with snow cover.

It is also common at sea when moist tropical air moves over cooler waters.

If the wind blows in the right direction then sea fog can become transported over coastal land areas.

24 Valley fog

Valley fog forms where cold dense air settles into the lower parts of a valley condensing and forming fog.

It is often the result of a temperature inversion with warmer air passing above the valley.

Valley fog is confined by local topography and can last for several days in calm conditions during the winter

25 Evaporation fog

Evaporation fog is caused by cold air passing over warmer water or moist land. It often causes freezing fog, or sometimes types of frost.

When some of the relatively warm water evaporates into low air layers, it warms the air causing it to rise and mix with the cooler air that has passed over the surface. The warm, moist air cools as it mixes with the colder air, allowing condensation and fog to occur. Evaporation fog can be one of the most localised forms of fog.

26 Precipitation

Cloud droplets require a condensation nuclei on which to form; growth then occurs by deposition of water molecules from vapour.

Hydroscopic nuclei Hydrophobic nuclei

Cloud droplets are typically 10 to 30 m in diameter. Growth/evaporation can occur within a few 10s of seconds

27 Rain drops are typically 0.5 to 5 mm in diameter, growth from the vapour would take several hours, longer than the lifetime of typical convective clouds.

Convection clouds are ones that form and grow by the process known as convection. Cumulus and Cumulonimbus are examples of this type of cloud.

Convection in the atmosphere is the way air floats upwards on account28 of being warmer than the surrounding air. In order to grow into rain drops, cloud droplets must grow by coalescence

Ice crystals provide a more efficient process

Saturation vapour pressure over ice is less than that over water  ice crystals grow at expense of water droplets

29 Ice Nucleation Mechanisms in the Atmosphere Homogeneous freezing

Homogeneous freezing is the process by which a supercooled liquid drop freezes without the assistance of an ice nuclei. Homogeneous freezing becomes statistically more likely as temperature decreases such that below -38oC all drops will freeze. The temperature at which homogeneous freezing occurs is affected by the presence of dissolved material in the droplet, especially when the droplet is a highly concentrated solution such as is the case for haze droplets.

30 Heterogeneous Freezing

Heterogeneous freezing is the process by which a supercooled liquid drop freezes with the assistance of a solid aerosol particle which is able to act as an ice nuclei. Heterogeneous freezing is thought to operate via several different modes:

1-Condensation Nucleation

condensation nucleation, here water vapour condenses onto a solid particle to form a droplet, the particle then acts as an immersion nuclei.

31 Immersion Nucleation

Immersion nucleation, here a solid particle within an existing drop acts as a nuclei for ice formation and the droplet freezes;

32 Contact Nucleation

Contact nucleation, here a solid particle is in collision with an existing droplet and initiates freezing of that drop.

33 At all temperatures below 0◦C the saturation pressure over ice is lower than over water.

Thus, if cloud droplets and ice crystals occur together in the same cloud (mixed-phase cloud), the ice crystals will grow at the expense of the water droplets until either the droplets are all gone or the crystals become large and start to fall out of the cloud.

34 IN are thought of having an unique structure or surface properties which allow water molecules to cluster into small ice embryos on their surface. Ice crystals subsequently grow starting from these ice embryos.

Most IN become active in the temperature range of -15 C to -35 C.

Some serve as immersion freezing nuclei, others as deposition nuclei or in any other nucleation mode depending upon the condition of the atmosphere.

Air contains a certain amount of oating microscopic and submicroscopic particles. While some particles are fairly large, especially those made up of dust bears and other light objects (e.g. pollen), the majority are compact solids or tiny liquid droplets.

They originate from many sources ranging from plants, volcanoes, forest res, sea spray, and desert dust storms 35 Ice nucleation occurs either homogeneously at temperatures below -38 C (where cirrus clouds form) or heterogeneously at temperatures colder than 0 C on the surface of aerosol particles present in the air.

36 Schematic diagram of the effect of ice nuclei from various possible aerosol sources on midlevel precipitating clouds and cirrus ice clouds. 38 Chemistry

Clouds provide an environment within which aqueous phase chemical reactions can take place within the atmosphere

Aerosol particles can be substantially modified within clouds

Aqueous phase reactions with gases dissolved in droplets Coalescence of droplets brings multiple aerosol particles together

Chemically different aerosol may react

On evaporation of droplet, a single aerosol particle is formed, containing material from all contributing droplets

39 High-Level Clouds

Cirrus (Ci): White, delicate, fibrous in Cirrus clouds are formed entirely of appearance. Forms in patches or narrow ice crystals. These grow and bands. May for comma-shaped streaks or evaporate slowly, leading to soft “mare’s tails” (cirrus uncinus) edges to clouds.

40 Cirrostratus (Cs): Thin, transparent sheet or veil; sun clearly visible & casting shadows at surface. A halo may be seen around the sun (or moon). Sheets of cirrostratus may cover entire sky, and be up to several 1000m deep.

41 Cirrocumulus (Cc): Thin white patch or sheet of cloud; appears dappled or rippled. Dappling results from convective overturning within Aircraft contrails: condensation the cloud, ripples from gravity from aircraft exhaust. May dissipate waves. quickly, or be very long-lived depending on conditions.

42 Medium-Level Clouds

Altostratus (As): A greyish sheet of cloud, may be fibrous or uniform in appearance. Thin enough in parts to make out the sun, but no halo.

43 Altocumulus (Ac): white or grey patches arranged in sheets. Shape and texture are variable. There are several distinct sub-classes of altocumulus

44 Altocumulus lenticularis (Ac len): white or grey lenticular (lens shaped) clouds formed by the lifting of air over a topographic barrier.

45 Altocumulus castellanus (Ac cas): white or grey, broken cumulus-like clouds; upper part appearing castle-like. Sometimes arranged in lines.

46 Altocumulus undulatus (Ac und): white or grey patches or sheets of cloud with an undulating or rippled appearance.

47 Low-Level Clouds

Cumulus (Cu): Brilliant white to grey, dense detached clouds. Forms clumped or heaped (cauliflower-like) shapes, usually with sharp outlines and flat base. Field of Cu often have bases all at same (lifting condensation) level.

48 Cumulus humilis (Cu hum): small cumulus, of limited vertical extent, may have a flattened appearance. Also called fair- weather cumulus

49 Cumulus mediocris : cumulus, of moderate vertical extent.

50 Cumulus congestus: crowded (congested) field of cumulus or greater vertical extent. May produce rain.

51 Pileus : cap clouds that form above large cumulus as the upward motion of the convective cloud distorts the layer of air above (pileus is latin for skull-cap) 52 53 Cumulonimbus (Cb) : huge towering cloud, dark base and white sides. Associated with heavy rain, thunderstorms, and hail. Frequently has an anvil shaped top. 54 mammatus : smooth, rounded shapes sometimes formed on the underside of cumulonimbus; they result from downdrafts within the cloud. 55 Stratocumulus (Sc) : white or grey sheet of cloud, usually formed in mounds or rolls.

56 57 58 Stratocumulus with virga – hair-like strands of falling rain, which evaporate below cloud before reaching the surface. 59 Stratus (St) : grey featureless layer of cloud with a uniform base. Often associated with drizzle or snow.

60 Nimbostratus (Ns) : Dark grey, featureless, thick layer of cloud. Associated with prolonged precipitation. Commonly forms in frontal systems

61 Cloud Classification

• In 1803, Luke Howard devised the basic system of cloud classification • Still used today • Based on Latin names • Two parts to a cloud’s name: - Shape (ex: cirrus, stratus, cumulus) - Height (cloud base & vertical extent)

62 Shapes & Heights

• Shapes - Cirrus = curly and wispy - Stratus = layered or stratified - Cumulus = lumpy or piled up • Heights - Cirro = high (bases above 20,000 ft) - Alto = mid level (bases 7,000-20,000 ft)

- Nimbo = producing precipitation 63 What do clouds tell us?

• Clouds just don’t happen - there’s always a reason • A particular cloud’s shape and location depend on (and can therefore tell us about): - the movement of the air - amount of water vapor in air - stability (flat clouds = stable air while puffy clouds = unstable air) 64 Cirrus Clouds

• Cirrus = high altitude wispy clouds • Quite thin and often have a hairlike or filament type of appearance. • Made up of ice particles • The curled up ends (called mares’ tales) as depicted in the following picture are very common features.

65 Cirrus Photo

66 Stratus Clouds

• Stratus clouds are usually the lowest of the low clouds. • Often appear as an overcast deck (as shown in next slide), but can be scattered. • The individual cloud elements have very ill-defined edges compared to cumulus • Fog is just stratus clouds on the surface

67 Stratus Photo

68 Fog (Stratus on Ground) Photo

69 Cumulus Clouds

• Cumulus clouds are puffy (like popcorn) • Often have noticeable vertical development • Cells can be rather isolated or they can be grouped together in clusters as shown • The base of a cumulus cloud can look like a stratus cloud if it is overhead. • Thick cumulus can make skies dark (filters out sun’s rays)

70 Combining Shapes & Heights

• Many different names of clouds combine: - a height (cirro-, alto-, nimbo) - a shape (cirrus, stratus, cumulus) • Let’s look at examples of these word combinations to describe different cloud types.

71 Cirrocumulus

• high cumulus clouds • Can see individual “puffy” features

72 Cirrostratus • High-level stratus clouds • Not as thin as cirrus and less defined than cumulus

73 Altocumulus • Mid level cumulus clouds

74 Altostratus • Mid level stratus clouds

75 Stratocumulus • Cross between stratus and cumulus

76 Nimbostratus

• Stratus clouds that are precipitating

77 Cumulonimbus

• Cumulus cloud with precipitation

78 Orographic Clouds

• Clouds can also be caused by mountains or hills • Result to air flowing up and over mountains which causes condensation to occur and clouds to form • Different types of clouds caused by orographic lift follow

79 Cap Clouds • Air containing water vapor lifted until it is saturated, producing liquid water cloud droplets which can "cap" the summit. (cap cloud over Mt. Ranier)

80 Lenticular Clouds • Lenticular means “like a lens” • Looks like flying saucers • Forms from air rising up a mountain

81 Kelvin-Helmholtz Billows • Occurs in regions of strong density and velocity changes. (dynamics class)

82 Contrails • Contrail is short for “condensation trails” • Formed from vapor contained in the exhaust of a jet engine when it condenses in cold air aloft

83 84