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MET 2204 METEOROLOGY Presentation 5: Moisture, Cloud Formation, & Precipitation. Presented by Mohd Amirul for AMC 1 Recapitulate • Lets recap what we have done last week: – – – – – – – – What is the Wind? Concept of Convection How the air move? Use of Isobar The force acting on the wind: PGF, GF, Friction Major Wind: Geostrophic and Gradient Wind Jet Stream Local and Minor Surface Wind: mountain and Valley; Katabatic; Land and Sea. – Wind Shear Presented by Mohd Amirul for AMC 2 Presentation Outline • • • • • • • • Introduction Water Vapor Change of State Cloud Formation Cloud Identification. Precipitation Land and Water Effects Conclusion Presented by Mohd Amirul for AMC Part 1 Part 2 3 Learning Outcomes • At the end of this session, student should be able to: – Identify the physical processes related to the transfer of heat and moisture – Explain change of state – Understand the background of formation of cloud and its classification. Presented by Mohd Amirul for AMC 4 Introduction • Moisture in atmosphere creates a variety of hazards. • Water can be solid, liquid, and gaseous states. • What is the difference between moisture and humidity? – Moisture: caused by water(usually can be seem) – Humidity: the amount of water vapor inside atmosphere. (usually can’t be seem) Presented by Mohd Amirul for AMC 5 Water Vapor Presented by Mohd Amirul for AMC 6 Water Vapor • Invisible • Can be measured in TWO ways: i. Relative Humidity ii. Dew Points Presented by Mohd Amirul for AMC 7 i. Relative Humidity • Express in percent • Relative humidity: express the degree of saturation. – 100% - Saturated. – Below 100% - unsaturated. • ‘Relative’ means relates the actual water vapor present that which could be present. – E.g. 50% humidity means the air could hold twice as much water vapor as it actually present. Presented by Mohd Amirul for AMC 8 Cont. • In fact, warm air can hold more water than cool air. – Temperature determine the maximum amount of water vapor air can hold – Temperature increase, water vapor increase, relative humidity decrease. • When air is cooled, can’t hold more water vapor (water becomes liquid/solid) • Condensation/sublimation begins to occur - SATURATED (100% Rel. Humidity) Presented by Mohd Amirul for AMC 9 Cont. Presented by Mohd Amirul for AMC 10 ii. Dew Points • Dew Points: Temperature to which air must be cooled to become saturated by the water vapor already present in the air. (Temperature at which water vapor becomes saturated and condensation begins.) Presented by Mohd Amirul for AMC 11 Cont. Presented by Mohd Amirul for AMC 12 Cont. • Temperature-dew point spread. – Aviation weather reports normally include air temperature and dew point temperature. – Temperature-dew point spread: difference between temperature and dew point. – As SPREAD less, relative humidity increase. – Spread will be 100% when temperature and dew point are same. Presented by Mohd Amirul for AMC 13 Cont. • Has a little bearing about precipitation but important in anticipating (predicting) fog. *saturate: cause chemical compound, vapour, solution to unite with the greatest amount of another substance. Presented by Mohd Amirul for AMC 14 Change of State Presented by Mohd Amirul for AMC 15 Change of State • Change of State: a change from one state (solid or liquid or gas) to another without a change in chemical composition. Presented by Mohd Amirul for AMC 16 Cont. • Involving: – Evaporation: The process of becomes a vapor from liquid. – Condensation: The process of change from gas to liquid – Sublimation: The process of change directly from solid to gas or vice versa. – Freezing: The process of change from liquid to solid. – Melting: The process of change from solid to liquid. *Example of Sublimation: Snow/Ice crystals result from sublimation of water vapor directly to solid state. Presented by Mohd Amirul for AMC 17 Cont. Presented by Mohd Amirul for AMC 18 Cont. Latent Heat • Any change of state involves heat transaction. • Latent heat: Heat absorbed or radiated during a change of phase at a constant temperature and pressure • Gas have the high energy than solid. It can be prove by its rapid movement of molecules. Presented by Mohd Amirul for AMC 19 Cont. • Types of latent heat: – Latent heat of vaporization • Involves during evaporation and condensation process • The heat energy restored in the water vapor and release during condensation. – Latent heat of fusion • Involves when melting and freezing • Heat energy restored during melting and released during freezing. – Latent heat of sublimation: • Involve sublimation Presented by Mohd Amirul for AMC 20 Cont. Presented by Mohd Amirul for AMC 21 Condensation Nuclei • As air become saturated water vapor begins to condensed (water vapor decrease) on the nearest available surface. • What surface in the atmosphere on which water vapor may condense?? Presented by Mohd Amirul for AMC 22 Cont. • Surface in the atmosphere on which water vapor may condense Condensation Surface. • Condensation Surface: the surface where abundance microscopic solid particles suspended in the air. • Theses microscopic solid particles we call as CONDENSATION NUCLEI – E.g. salt, dust, combustion by product. Presented by Mohd Amirul for AMC 23 Cont. • As the water vapor condense or sublimates on condensation nuclei, liquid or ice particles begin to grow. • The particle ice or liquid doesn’t depend entirely on temperature – E.g.: Liquid Water may be present at temperature well below freezing. Presented by Mohd Amirul for AMC 24 Supercooled Water • Supercooled: Water droplet colder than 0˚C (liquid water below freezing point) • When these water droplets strike on expose objectthe impact induce FREEZING!! – May result aircraft icing. Presented by Mohd Amirul for AMC 25 Cont. Presented by Mohd Amirul for AMC 26 Cont. • Some of the supercooled water left in clouds at temperature between 0˚C to -15 ˚C – Amount of supercooled water become less. • At temperature colder than -15 ˚C sublimation commonly occur. • Sublimation will form ice crystals/clouds/fog. Presented by Mohd Amirul for AMC 27 Cont. • Strong vertical current may carry supercooled water to great height where temperature are much colder than -15 ˚C Presented by Mohd Amirul for AMC 28 Dew and Frost • Dew: water that has condensed on a cool surface overnight from water vapor in the air. • Formation of Dew: – During night (with wind or no wind), Flora (vegetation) cools by radiation (terrestrial radiation) to a temperature at or below the dew point of air. *moisture form on the leaves ambiguous to moisture form in warm room. Presented by Mohd Amirul for AMC 29 Cont. • Frost: The formation of ice on the surface. • Formation of Frost: – Form in much same way with dew. – The difference is dew point of surrounding air must be colder than freezing. In this case, water vapor sublimates directly as ice crystals frost rather than condensing as dew. Presented by Mohd Amirul for AMC 30 Cont. • But sometimes dew forms and later freezes Frozen Dew • However, frozen dew can easily be distinguish from frost. – Frozen dew is hard and transparent. – Frost is white and opaque. Presented by Mohd Amirul for AMC 31 Cont. • What is the difference between frost and snow? – Snow-can form/fall all day long – Frost-form in the cold of winter morning Presented by Mohd Amirul for AMC 32 Clouds Presented by Mohd Amirul for AMC 33 The Clouds • Clouds: A visible mass of water or ice particles suspended at a considerable altitude. (A visible manifestation of condensation or deposition in the atmosphere) • Clouds can indicate following problem: – Turbulence – Visibility – Precipitation – Icing Presented by Mohd Amirul for AMC 34 Cont. • Turbulence: Unstable flow of a liquid or gas in atmosphere. • Fog: ‘cloud’ on the ground. Presented by Mohd Amirul for AMC 35 Cont. • Cloud Base – The height of the lowest part of the cloud above the surface. – Similar to condensation surface/level. – Can be estimated using ceilometer. • Ceilometer: Laser beam pointing vertically and the receiver put at the same location. Presented by Mohd Amirul for AMC 36 Cloud Formation • Clouds are formed by air being lifted and cooled adiabatically (without heat transfer) until condensation occur. Presented by Mohd Amirul for AMC 37 Cont. • Remember back Condensation Nuclei – atmosphere contains particles that have an affinity for water ---These serve as centers for condensation. Cloud Condensation Nuclei (CCN) Presented by Mohd Amirul for AMC 38 • The atmosphere has plenty of CCN Dust Salt Spray from Oceans Volcanoes Sulfate Particles from Phytoplankton Forest Fires Trees Anthropogenic Origins Presented by Mohd Amirul for AMC 39 • CCN are more plentiful near the surface of the earth. • CCN are more plentiful over land rather than the ocean. • The formation of cloud droplets using CCN is called: Heterogeneous Nucleation. Presented by Mohd Amirul for AMC 40 Cont. • The air must become saturated for condensation/sublimation to occur. • Saturation can result from – cooling temperature, – increasing dew point – or both Presented by Mohd Amirul for AMC 41 Cont. • But the cooling is more predominant (frequent) for the air to become saturated. • Cooling Process: – 3 basics process may cool the air to saturation: 1. Air moving over a colder surface 2. Stagnant air overlying a cooling surface 3. Expansion cooling in upward moving air Presented by Mohd Amirul for AMC 42 Cont. • Expansion cooling is a major cause of cloud formation. Presented by Mohd Amirul for AMC 43 Cloud Classification • Clouds are classify as follows: a. Heap Clouds b. Layer Clouds Presented by Mohd Amirul for AMC 44 Cont. a. Heap Clouds • General Characteristics: – Unstable – Isolated with large vertical and small horizontal extent. – Associated with changeable weathers, showers, thunderstorm and turbulence. Presented by Mohd Amirul for AMC 45 Cont. • 3 main types: i. Cumulus (Cu) ii. Comulonimbus (Cb) iii. Altocumulus Castellanus (Acc) Presented by Mohd Amirul for AMC 46 Cont. i. Cumulus (Cu) • Characteristic: – Give heavy showers and moderate to heavy clear ice and turbulence. – Brilliant (striking) white to grey, dense isolated clouds. – Clumped or heaped shapes. – Field of Cu often have bases all at same (Condensation Surface) level – Instability cloud at cold front. *Cold Front: zone where cold air replaces Presented by Mohd Amirul for AMC warm air 47 Cont. ii. Comulonimbus (Cb) • Characteristics: – Thunderstorm Clouds • Severe turbulence and heavy clear icing., heavy showers, hail, thunderstorm and lightning. – Occur at cold front and major tropical storms – Huge towering cloud, dark base and white sides. Presented by Mohd Amirul for AMC 48 Cont. iii. Altocumulus (Acc) • Characteristics: – Warning of unstable air and thunderstorms within 24 hours. – white or grey, broken cumulus-like clouds; – Sometimes arranged in lines. Presented by Mohd Amirul for AMC 49 Cont. b. Layer Clouds • General Characteristics: – Occur in stable condition – Sheets of cloud cover a wide area – The weather can be dull (darkened with overcast(cover by cloud)) for lengthy periods with continuous light to moderate precipitation. Presented by Mohd Amirul for AMC 50 Cont. • Main types: i. High level • • • Cirrus (Ci), Cirro Stratus (Cs), Cirro Cumulus (Cc) ii. Medium Level • • Alto cumulus (ac) alto stratus (As) iii. Low Level • • • Nimbo Stratus (Ns), Strato Cumulus (Sc), Stratus(St) Presented by Mohd Amirul for AMC 51 Cont. i. High Level • Cirrus (Ci) – Characteristics: • White, delicate, fibrous in appearance. Forms in patches(contrast) or narrow bands. • Cirrus clouds are formed entirely of ice crystals. These grow and evaporate slowly, leading to soft edges to clouds. Presented by Mohd Amirul for AMC 52 Cont. • Cirro Stratus (Cs) – Characteristics: • Thin, transparent sheet or veil(hide); • sun clearly visible & casting shadows at surface. • A halo(circle of light) may be seen around the sun (or moon). • Sheets of cirrostratus may cover entire sky, and be up to several 1000m deep. Presented by Mohd Amirul for AMC 53 Cont. • Cirro Cumulus – Characteristics: • Thin white patch or sheet of cloud; • Appears dappled or rippled. Presented by Mohd Amirul for AMC 54 Cont. ii. Medium Level • Alto Cumulus (ac) – Characteristics: • white or grey patches arranged in sheets. • Shape and texture are variable. Presented by Mohd Amirul for AMC 55 Cont. • Alto Stratus (As) – Characteristics: • A greyish sheet of cloud, may be fibrous or uniform in appearance. • Thin enough in parts to make out the sun, but no halo. Presented by Mohd Amirul for AMC 56 Cont. iii. Low Level • Nimbo Stratus (Ns) – Characteristics: • Dark grey, featureless, thick layer of cloud. • Associated with prolonged precipitation. • Commonly forms in frontal systems *Front: zone/surface of interaction between two air masses of different temperature. Presented by Mohd Amirul for AMC 57 Cont. • Strato Cumulus (Sc) – Characteristics: • white or grey sheet of cloud • usually formed in mounds (laid on top of each others) or rolls. Presented by Mohd Amirul for AMC 58 Cont. • Stratus – Characteristics: • grey featureless layer of cloud with a uniform base. • Often associated with drizzle or snow. Presented by Mohd Amirul for AMC 59 Cont. Four latin terms form the basis for the naming of clouds: – – – – Cirrus : fibrous or hair-like Cumulus : a heap or pile Stratus : a horizontal sheet or layer Nimbus : rain-bearing The prefix Alto is used to indicate medium altitude clouds. Presented by Mohd Amirul for AMC 60 Precipitation Presented by Mohd Amirul for AMC 61 Cont. • Precipitation occurs when the particles (e.g. : water droplets or ice crystals) grow in size and weight until the atmosphere no longer can suspend them and they fall. • Precipitation versus Cloud Thickness: – To produce significant precipitation, clouds usually are 4000 ft thick or more – Thicker Cloud Heavier Precipitation Presented by Mohd Amirul for AMC 62 Cont. • There is two ways of particles grows: i. Growth of water droplet/ice crystal • Once water droplet or ice crystals forms, it continue to grow by added condensation or sublimation directly into particle ii. Collision between cloud particle/droplets • • • • • • Rapid growth process Cloud particles collide and merge into larger drop Produce larger precipitation particles Upward currents enhance the growth rate and support larger droplet. Light rain and snow - produce by merging drops with mild upward current. Heavy rain, heavy snow and hail - produce by strong upward current support larger drops and built clouds to great height. Presented by Mohd Amirul for AMC 63 Cont. Growth of raindrops by collision of cloud droplets Presented by Mohd Amirul for AMC 64 Cont. • Precipitation types are: a) Rain • Water droplets are supported by upcurrents but if the weight of water becomes too great rain or drizzle will occur b) Snow • Precipitation in the form of ice crystals c) Hail • Solid balls or piece of ice Presented by Mohd Amirul for AMC 65 Examples of Precipitation • Rain – Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. The intensity of rain is based on rate of fall. • The rate of fall with respect to the intensity is as follows: – Slight - <0.5mm/hr – Moderate - 0.5 – 4.0mm/hr – Heavy - >4.0mm/hr Presented by Mohd Amirul for AMC 66 Cont. • Drizzle • Very small, numerous, and uniformly distributed water drops that may appear to float while following air currents. • Unlike fog droplets, drizzle falls to the ground. • It usually falls from low stratus clouds and is frequently accompanied by low visibility and fog Presented by Mohd Amirul for AMC 67 Cont. • Hail – Precipitation in the form of balls or irregular lumps of ice, always produced by cumulonimbus cloud. – An individual unit of hail is called a hailstone. – By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin, formerly called small hail, may be classed as either ice pellets or snow pellets – The destructive effects of hailstorms- plant and animal life, buildings and property, and aircraft Presented by Mohd Amirul for AMC 68 Cont. •Shower • Precipitation from a cloud that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE). It is reported as "SH" in an observation and on the METAR. •The rate of fall with respect to intensity is as follows; Slight - < 2mm/hr Moderate - 2-10mm/hr Heavy - > 10-50mm/hr Violent - > 50mm/hr *METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing Presented by Mohd Amirul for AMC 69 Land and Water Effects Presented by Mohd Amirul for AMC 70 Cont. • Land and water can frequently affect cloud and precipitation. How? • Large body of water (e.g. oceans/large lakes) can add water vapor to the air. Presented by Mohd Amirul for AMC 71 Cont. • Variation of season to cloudiness – In winter: • Cold air frequently moves over warm lakes/oceans warm water adds heat and water vapor to the air causing shower to the lake side/oceans side. – In other seasons: • Air may be warmer than lakes air become saturated by evaporation from water while also becoming cooler in the low levels by contact with cool water form a fog - dense side to the lake/oceans. Presented by Mohd Amirul for AMC 72 Cont. Presented by Mohd Amirul for AMC 73 Cont. • Variation of diurnal to cloudiness – Day: • Cool air over the lake/oceans blows toward the land • Clouds form cloud over the land – Night • Reverse pattern. • Clouds tend to form over the lake Presented by Mohd Amirul for AMC 74 Conclusion • Water can be solid, liquid, and gaseous states. • Water Vapor can be measured in TWO ways: Relative Humidity and Dew Points. • Change of State: a change from one state (solid or liquid or gas) to another without a change in chemical composition. • Condensation Surface: the surface where abundance microscopic solid particles suspended in the air. Presented by Mohd Amirul for AMC 75 • Clouds: A visible mass of water or ice particles suspended at a considerable altitude. • Clouds are formed by air being lifted and cooled adiabatically (without heat transfer) until condensation occur. • Clouds are classify as follows:Heap Clouds and Layer Clouds. • Precipitation occurs when the particles (e.g. : water droplets or ice crystals) grow in size and weight until the atmosphere no longer can suspend them and they fall. Presented by Mohd Amirul for AMC 76 Key Points • • • • • • • • • • • • • • Water vapor Measurement of Water Vapor Change of State Evaporation, Condensation, Sublimation, Freezing, Melting Latent Heat Condensation Surface and Condensation Nuclei Supercooled Water Dew and Frost Clouds Clouds Base Clouds Classification: Heap and Layer Clouds Precipitation Types of Precipitation Land and Water Effect. Presented by Mohd Amirul for AMC 77 End of Presentation #5 5 Minutes for Q/A session Presented by Mohd Amirul for AMC 78 Quote of the Day • I'm a dreamer. I have to dream and reach for the stars, and if I miss a star then I grab a handful of clouds. ~Mike Tyson Presented by Mohd Amirul for AMC 79