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David Passmore

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Final exam briefing Thursday April 24th, 13:00-15:00, Gym C. 2 hour duration, 35% of course marks Questions are developed on material spanning lectures 11-22 (i.e. after the mid-term) – but note that earlier concepts, etc may be relevant. Format – same as mid-term: Part 1: Questions 1-3 are compulsory questions (totalling 60 marks) Part 2: Question 4 is a short essay question (from a choice of 6) (totalling 40 marks) Key themes: Clouds, cloud formation, precipitation Winds – forces, motion, circulation Air masses Fronts, cyclones and associated weather Weather forecasting (note – will not involve weather map interpretation) Climate change – forcings and feedbacks Lecture 11 - Chapter 9 Re-cap: atmospheric stability / instability Formation of Cloud Droplets Homogeneous (one) nucleation: results in droplets made up of water only - small droplets <0.01 μm: molecules are weak because of greater saturation vapour pressure on a curved surface - these small droplets require a RH of >112% 1 * droplets from this formation quickly evaporate Curvature effect: As a water droplet gets smaller, its curvature increases which makes it easier to evaporate. The required relative humidity for its equilibrium in its surrounding will increase. This effect INHIBITS growth of small droplets EX: Consider a droplet with a 0.05 μm, it requires a relative humidity of 102 % or saturation of 2% to neither evaporate or grow. *once a droplet reaches a size of 1 μm = it acts like a plane surface with a relative humidity of 100% with its surrounding. Heterogenous nucleation: results in droplets made up of water & substance in which water condensed. 2 - needs CCN to form water droplets - responsible for formation of water droplets in the atmosphere - droplets are likely to grow and survive in the atmosphere Cloud Condensation Nuclei (CCN): To be a CCN, an aerosol must be hydrophilic (wettable), able to allow water to form a film. - Also known as cloud seeds as cloud droplets begin to form (condensate) on these (CCN) aerosol through the process of heterogenous nucleation. Hygroscopic Nuclei: A CCN that attracts and dissolves in water. EX: Sea Salt (NaCl), an abundant atmospheric aerosol. Solute Effect: Difficult to evaporate water in solution because saturation vapour pressure is lower for solutions than for pure water. This effect PROMOTES growth of small droplets because water vapour can condense on solution droplets when the relative humidity is below 100%. - As solution droplets grow in radius = the solution becomes diluted thus requires greater % relative humidities for further growth Sources of CNN Over continents - Combustion - Vegetation Over oceans - Sea salt - Dimethyl sulphide produced by phytoplankton Diffusion: movement of water vapour molecules towards a water droplet or ice crystal in which follows condensation. - In a constant relative humidity condition, a droplet can grow very quickly at first then slows down as the droplet gets bigger * Relative humidity in a cloud decreases as droplets grow because their growth uses up the water available limiting how big it can grow through condensation. Droplet Size - average size is 10 μm but size utimately depends on the amount of water available in cloud and number of CCN available (since water molecules condensate with CCN aerosols) EX: Same amount of water and more CCN = smaller droplets 3 - The size of droplets tends to become uniform: - Droplet formation reduces RH in the cloud - Droplet growth slow down due to lack of water vapour Nucleation below Freezing Homogeneous nucleation: freezes water droplets spontaneously in temperature below -40 ‘C - freezing may vary depending on droplet size: larger droplets may require a temperature slightly higher Heterogeneous nucleation: ice crystals are formed in temperature between 0 and -40’C - needs Ice Nuclei Ice Nuclei: Atmospheric aerosol on which ice crystals can form - size is important - insoluble - closer the temperature is to -40’C = more ice crystals formations EX: Clay minerals, organic materials, combustion products 3 ways ice crystals can form: 1. Direct deposit of water vapour onto ice nuclei 2. Supercooled water droplets containing an ice nucleus and temperature gets cold enough 3. Collision of supercooled droplets with ice nuclei Temperature effect on ice nuclei: - As temperature drops, ice nuclei are more effective… Ex: When the temperature is just below 0’C, there’s only supercooler cooled water droplets, ready to freeze, but needs to freeze onto aircrafts (dangerous) Clouds *Coexistence of water droplets and ice crystals are important to the growth of cloud droplets to form precipitation. Cloud formation - Forms due to cooling produced when air rises - mixing: by jet stream cirrus, jet contrails and some stratocumulus Fogs: Clouds formed near the ground - forms when air becomes saturated and begins to condense 4 5 Mechanisms: by which air rises to form clouds 1. Convection: warm air from the surface rises, cools and forms clouds 2. Orographic Lift: air forced to rises up a slope (explains clouds at mountain peaks) 5 3. Convergence: air streams flowing together (directional) or slows down along the direction of flow (speed). Low pressure areas have air from higher pressure areas flow into them = convergence 4. Frontal Lift: Air rising when it meets colder air at a front (a kind of convergent lift). In mid- latitudes, low pressure systems develop along fronts (frontal system). 5. Divergence: Air can be pulled up from middle to upper troposphere usually in conditions opposite to convergence. Often happens together with surface convergence and frontal lifting. describe the classification system used to name clouds, and apply this system to recognize major cloud types (Chapter 9); Cloud Classification named by Luke Howard in 1802, combination of the following = 7 cloud types Cumulus (“heap”) Stratus (“layer”) Cirrus (“curl of hair”) Nimbus (“rain”) International Cloud Atlas * based on altitude and form - 10 basic cloud type / genera - 3 height categories: Low clouds (< 2 km asl.; water or water/ice mixture) Middle clouds (from 2 km up to 3–6 km; lower at high latitudes) High clouds (extend up to the tropopause; entirely ice crystals - 3 main forms: Cirrus (high clouds, thin, fibrous appearance) Cumulus (isolated heaps of cloud, can span multiple levels) 6 Stratus (sheets or layers of cloud) Low Level Clouds (0-2 km above sea level) Stratus - featureless sheet of cloud often with ragged edges - similar to fog but not at the ground - can be deep enough to produce drizzle or light snow Statocumulus - almost continuous sheet of cloud with a well-defined base that resembles cumuliform(variation in form) - can produce light precipitation Cumulus - detached heaps of cloud with mounded tops and flat bottoms - usually scattered across the sky Cumulonimbus (extends throughout all 3 levels) - large heaped cloud that usually extends from low altitudes to the tropopause, where it forms an anvil top resembling cirriform cloud. - produces heavy precipitation, like hail, combined with thunder and lightning Middle Level Clouds (2-4 km in polar regions, 2-7 in temperate regions, 2-8 in tropical regions) Altocumulus - a sheet of cloud broken up into individual clumps or rolls - individual cloud elements are larger than for cirrocumulus clouds and smaller for stratocumulus clouds 7 Altostratus - featureless sheet of clouds - can make sun appear to be shining through frosted glass - can occasionally produce light precipitation Nimbostratus (can extend through more than one level) - featureless sheet of cloud thick enough to block sun and mooon - usually produces steady precipitation of moderate intensity High Level Clouds (3-8 km in polar regions, 5-14 in temperate regions, 6-18 in tropical regions) Cirrus - isolated fibrous clouds that appear as streaks scattered across the sky Cirrocumulus - sheet of cloud broken up into tiny individual clumps - the individual cloud elements are smaller than altocumulus cloud Cirrostratus - featureless sheet of cloud resembling a veil over the sky, can produce a halo around sun or moon - thin enough to see sun and moon (above are descriptions of clouds in the picture provided) Main Groups of Clouds Cirriform Clouds EX: Cirrus, Cirrocumulus and Cirrostratus - occur in the upper troposphere, first cloud to appear as warm front approaches - they are THIN and WISPY in appearance due to low water vapour content in the air at these heights - composed of ice crystals due to low temperature at these heights Halo: rings of light appearing around the sun or moon due to refraction of light by ice crystals - a sign of cloudy, wet weather is coming Fall Streaks: ice crystals falling from cirrus cloud Cumuliform Clouds EX: Cumulus, Stratocumulus, Cumulonimbus, Cumulus humilis, Cumulus congestus - Atmospheric stability determines whether clouds are Cumuliform or Stratiform 8 - Cumuliform clouds are form in unstable air as air parcels rise from a heated (by the sun) surface. - Also associated with cold fronts: can form as air is forced up the slopes of mountains (a species of cumuliform can also form downwind of a mountain) Lenticular: think of a lens-shaped, forms as wind pushes them over mountains *takes an hour or so to reach tropopause Formations Cumulus Humilis Temperature of the surround = ELR (blue) Air rising from the ground = DALR (red) When the air (DALR) reaches the LCL, it will start to condense. Conditions under the ELR inversion is FAVOURABLE for formations The air continues to cool along the SALR line, forming the clouds * height of cloud depends on the difference between the height of LCL and height at which air stops rising due to inversion Cumulus Humilis Clouds - individual heaps of clouds - created by updrafts of few metres per second - do not create precipitation 9 Cumulus Congestus - in order to form, air must rise past inversion (from greater surface heating) - can produce precipitation (heavy & short) Cumulonimbums - in order to form, air must rise past inversion (from greater surface heating) - created by strong updrafts of over 10 m per sec - produces precipitation (heavy & short) - can reach up to 11 km above surface, marking tropopause layer Anvil Tops: horizontal spread because ice crystals form if top reaches the temperature of -10’C Stratiform Clouds EX: featureless: Cirrostratus, Altostratus, Stratus lumpier (instability): Cirrocumulus, Altocumulus, Stratocumulus, Nimbostratus (produce light rain) - Form in stable air as a result of forced lifting - as air rises up fronts, gently sloping topography or a large region of air is lifted due to convergence in low pressure areas. - forms as air rises slowly and continuously therefore longer lasting than cumulus clouds - blocks moon and sun - takes a day or more to reach tropopause Clouds Formation by Mixing 10 Stratocumulus clouds form when the layer of air is mixed by wind. - Can produce clouds at any level of the atmosphere 1. Initial Conditions: When the air temperature and dew- point temperature are close to each other and slightly decreasing with height. (air is close to saturation) 2. Mixing: Brings the temperature closer to the dry adiabatic lapse rate meaning the lower layer warms while the upper portion cools. The water vapour content becomes uniform with height = less water vapour at the bottom and more near the top. Combination makes the air layer reach its dew-point temperature which begins condensation (forming of clouds). Mixing Condensation Level: Height at which water vapour condenses as a result of mixing Jet Contrail: Long thin clouds left behind by jets, created by condensation of water vapour in aircraft exhaust. (happens in upper troposphere) Lecture 12 - Chapter 9 - 10 Fogs: Similar to clouds they form closer to the surface rather than higher in the atmosphere. Most are formed by cooling. - it indicates the air temperature and dew-point is nearly identical at ground level Upslope Fogs: form as air rises and cools along slopes Radiation Fogs: When a layer of moist air cools to its dew-point temperature (emits radiation rather than absorbing it). - Commonly occurs overnight. - More common in the fall - 3 conditions needed for radiative cooling: 1. Clear skies: more longwave radiation leaves than return - helps when there’s more dry air than moist air since dry air absorbs and emits less longwave radiation. 2. Light wind: - Strong wind tends to mix warm air downward from above (slowing cooling near the surface). It also mixes in drier air which increases dew-point depression. - No wind = fog layer will be very shallow thus only dew. - Without mixing radiative cooling alone cannot produce a layer of cold air deep enough for fog to form in. 11 3. Long nights: (like in the winter) allows more time for the air to cool to its dew- point temperature Advection Fog: Forms through contact cooling (conductive), when warm air is cooled as it is advected by wind over a cold surface = moist air cools to its dew-point temperature as it contacts with the cold surface - can be up to 1 km deep - forms during the day or night - common in Newfoundland and San Francisco Newfoundland fog - form year-round (more than 200 days a year) San Francisco fog 12 - Warm moist air blows towards the shore off the coast of S.F where cold water is upwelling from below = advection fog formations - forms mostly in the summer, spring and fall temperatures Adding Vapour and Mixing Fog: unlike advection and radiation fogs who form through cooling of the air, this one forms by adding water vapor to the air and mixing it. Steam Fogs: forms when water vapor evaporates from a warm, moist surface and mixes into the colder air above. Mixing of the air may create a saturation point, causing condensation thus fog. EX: When cold air from the land moves over to a warm lake Precipitation Fogs: forms when water vapour results from the evaporation of raindrops causes saturation as it mixes with the cold air. Precipitation: Any liquid or solid water particles that falls from the atmosphere and reaches the ground. Drizzle: water droplets smaller than typical raindrops Snow: precipitation in form of ice crystals Hail: frozen precipitation made up of concentric layers, alternating between clear and opaque ice - forms only in very strong updrafts of cumulonimbus clouds Sleet: frozen precipitation without the crystal structure. Falls as snow, melts into rain as it falls through a warm layer of air, and refreezes as small ice pellets as it falls through a freezing layer of at least 250 m deep at the surface. 13 Freezing rain: precipitation melting as it falls and refreezes upon contact with object surface. Forms the same way as sleet but surface layer is shallower thus allowing it to refreeze only upon contact. Cloud Droplets vs Raindrops Size: - tiny drizzle = < 100 μm in radius - typical raindrops = 1000 - 2000 μm (1-2 mm) - Hailstones are > 3000 μm - clouds are usually 10 μm * Droplets need to grow before precipitation can occur Terminal Velocity: in order for objects to fall freely, like water drops in the atmosphere, it must reach a constant speed which the force of gravity equals the opposing resistance force (object) in the air.
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