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.