Unit 2 Notes (Study Guide)
Unit 2 Notes (Study Guide) AERN-25250
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This 7 page Study Guide was uploaded by Cedric Ossey on Monday October 26, 2015. The Study Guide belongs to AERN-25250 at Kent State University taught by Dr. Richard Mangrum in Fall 2015. Since its upload, it has received 298 views. For similar materials see Elements of Aviation Weather in Engineering and Tech at Kent State University.
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Date Created: 10/26/15
Aviation Weather Unit 2 Exam on Tuesday 27th October 2015 Bring Laptop Chapter 5 Vertical Motion and Stability An air parcel when moving from one location to another has a horizontal component as well as a vertical component vertical motion Causes The most frequent causes are Convergence Divergence Orography Fronts Convchon 1 Convergence corresponds to a net in ow of air into a given area It may occur when wind speed slows down in the direction of ow andor when opposing airstreams meet Convergence of surface wind in the low pressure area causes upward motion 2 Divergence is the net out ow from a given area Winds may diverge when the wind speed increases in the direction of the ow andor when an airstream spreads out in the downstream direction Divergence of surface winds in a highpressure area causes downward motions 3 Orography Air can be forced upward or downward when it encounters a barrier An example is orographic lifting When wind intersects a hill or a mountain it is simply pushed upward then downward on the downhill side 4 Fronts When the atmosphere creates an obstacle to the wind a barrier similar to a mountain can be produced Eg When a cold air mass is next to a warm air mass a narrow sloping boundary is created between the two called a front If either airmass moves toward the other the warm air moves upward over the cold dense airmass in a process called frontal lifting 5 Convection As bubbles of warm air rise in the convective lifting process the surrounding air sinks Convection occurs under unstable atmospheric conditions The rapid increase of wind velocity near the earth s surface causes mechanical turbulence Turbulent eddies produce gusty winds and vertical motions as they are swept along Gravity waves are disturbances in which stable air that was displaced vertically oscillates due to the restoring force of gravity Stable system tends to return to its original state when disturbed Unstable system tends to move away from its original state when disturbed Neutral system tends to stay in its new position when disturbed Atmospheric stability condition that makes it difficult for air parcels to move upward or downward Atmospheric instability condition that promotes vertical motions Buoyancy property of an object that Allows it to oat on the surface of a liquid Allows it to ascend through and remain freely suspended in a compressible uid atmosphere Archimedes Principle when an object is placed in a uid liquid or gas it will be subjected to positive or negative force Positively buoyant when a parcel of air is displaced upward and it Becomes warmer than surroundings The parcel accelerates upward It is unstable continues away from equilibrium Negatively buoyant when a parcel of air is displaced upward and it Becomes colder than surroundings The parcel accelerates downward It is stable returns to equilibrium Meteorologists use three methods to determine atmospheric stability Dry Adiabatic Process Atmospheric Soundings Lapse Rates Dry adiabatic process cooling of an unsaturated parcel of air by expansion rising air and the warming of a parcel of air by compression sinking air Adiabatic Cooling cooling of gas by expansion Always with upward motion Adiabatic Heating warming of gas by compression Always with downward motion Dry adiabatic process temperature change is always 3 C per 1000 feet 54 degrees F Lapse Rate rate of decrease of temperature with height Surfacebased inversions when the temperature increases with height very stable Isothermal layer no change in temperature with height Stability Evaluation LR gt DALR Absolutely Unstable LR DALR Neutral LRlt DALR Stable Main difference between stable and unstable environment is in cloud formation Both stable and unstable environments can be clear Stratiform stable Cumuliform unstable The presence of fog smoke or haze indicates that the atmosphere is stable Conditions favorable to instability are afternoons when ground temperatures are warmer than air temperatures Highpressure areas usually have good weather and lowpressure areas do not The lifted index Ll is a common measure of atmospheric stability The Lifted Index Analysis Chart depicts a number associated with the stability of a surface parcel of air lifted to 500 mb Lifted index values range from positive to negative A positive lifted index indicates stable air Larger positive numbers imply greater stability A negative lifted index indicates unstable air The K index is a measure of thunderstorm potential based on vertical temperature lapse rate moisture content of the lower atmosphere and vertical extent of the moist layer With the K index the higher the positive number the likelihood of thunderstorm development is greater Charts You will be shown a portion of a chart and will be expected to answer questions about the information presented to you There will not be a legend You do need to know the basic weather symbols used on the charts 1 Weather Depiction Chart a ACOO4SG 54 b Know how VFR IFR MVFR are depicted on chart c Be familiar with basic symbols rain drizzle showers fog snow 2 Radar Summary Chart a ACOO45G 56 b 6 levels of intensity c Cell movement speed direction 3 Surface Analysis Chart a ACOO45G 51 4 Signi cant Weather Prognostic Chart a ACOO45G 82 5 Current and Forecast Icing Charts a ACOO45G 9394 High Level Sig WX Charts ACOO4SG 84 Chapter 6 Atmospheric Moisture One of the trace gases found in the atmosphere is water vapor Water Vapor colorless odorless tasteless gas in which the molecules are free to move about It can exist in three different states in the atmosphere solid liquid gas in the normal temperature and pressure range of the atmosphere Change of state transition from one form of H20 to another Water transferring to higher state of energy Melting ice to water Evaporation water to vapor Sublimation ice directly to vapor Water transferring to lower state of energy Condensation vapor to water Freezing water to ice Deposition vapor directly to ice Understand the relationship of Relative Humidity and Dew Point to saturation of the atmosphere be able to interpret what the TemDew Point spread given to you means Relative Humidity amount of water vapor actually in the air as a percentage of the amount required for saturation Changes in RH are helpful for anticipating clouds fog and low visibilities Remember RH is relative to SVP SVP depends on temperature Tells us nothing about the actual water vapor in the air Eg Saturated air in Alaska at 4 F has 120th of the water vapor in it that saturated air in Florida at 68 F Both places have RH of 100 In fair weather near the ground RH is usually highest at the time of minimum temperature and lowest at the time of maximum temperature Dewpoint Refers to the temperature to which air must be cooled at a given pressure to become saturated Always lower than air temp except when air is saturated RH100 then the temperature and dew point are equal Useful in predicting precipitation amounts thunderstorms and icmg TemperatureDeWDoint Spread Difference between air temperature and Dewpoint When the spread is small the RH is high When it is very large the RH is low Understand what Vapor Pressure is and what Saturation Vapor Pressure is Vapor Pressure is the partial pressure exerted by water vapor It is the force per unit area exerted by the molecules of water vapor and is proportional to the amount of water vapor in the atmosphere For example if water vapor is added to the atmosphere then vapor pressure is increased An important condition with respect to the presence of water vapor is saturation It occurs when the same amount of molecules are leaving a water surface as are returning The vapor pressure exerted in this equilibrium is called saturation vapor pressure SVP is dependent primarily on temperature SVP increases as temperature increases At sea level standard conditions SVP is 05 in Hg When SVP equals total air pressure water m Understand what the atmospheric condensation level is ie where you would nd the base of a cloud Clouds A suspension of water droplets or ice crystals in the atmosphere If there is a cloud there is atmospheric saturation Three requirements for clouds to form Water vapor Condensation Nuclei and Cooling 1 Water Vapor Require enough water vapor so that a reasonable amount of cooling will cause change of state from vapor to water droplets or ice More likely to form when Relative Humidity RH is high Tempdew point spread is low RH must be brought to 100 2 Condensation nuclei microscopic particles that serve as surfaces on which water vapor condenses or deposes Hydroscopic having tendency to draw water from atmosphere Dust and salt Fog in industrial areas 3 Cooling Remember RH must be brought to 100 Add water vapor to raise saturation level Cool the air As air temperature decreases amount of water vapor required for saturation decreases Cooling is more effective than adding water vapor Cooling usually result of Contact of air with cold surface Adiabatic expansion Contact cooling process by which heat is conducted away from warmer air to a colder surface Advection fog results from the transport of warm humid air over a cold surface California coast notorious for low clouds and fog as NW winds bring warm moist air across colder California Current Radiation fog occurs when radiational cooling of the earth s surface lowers the air temperature near the ground to or below its initial dewpoint on calm clear nights Ground fog radiation fog that is less than 20 ft deep Adiabatic Cooling cooing of gas by expansion Saturated adiabatic process process by which saturated air cools by expansion or warms by compression Saturated Adiabatic Lapse Rate SALR variable vice the constant DALR of 3deg C per 1000 Amount of latent heat released or absorbed depends on the amount of water vapor present which in turn depends upon the temperature Condensation Level level at which cloud forms in rising air Below this level air cools at DALR Above this level air cools at SALR Latent heat amount of heat absorbed or released during a change of state Example On a hot day you step out of a swimming pool Water on your skin evaporates changes to higher energy state Your skin feels cooler because the water is taking the heat from your skin to supply the necessary heat to change state The release is of latent heat and is not measurable in the surrounding air Clouds Know the 10 basic types of clouds the altitude they form and the type of precipitation associated with each Classi ed according to appearance Height Shape Behavior Classi ed as either Low Middle High Extensive vertical development Low Clouds bases below 6500 AGL Stratus ST Stratocumulus SC Nimbostratus NS Middle Clouds bases 6500 to 20000 AGL Altocumulus AC Altostratus AS High Clouds bases above 20000ft Cirrocumulus CC Cirrostratus CS Cirrus Cl Clouds with Vertical Development occur in all categories Cumulus CU Cumulonimbus CB Cumulus clouds form in air that is rising from the ground It is possible to determine bases from temperature and dewpoint H T DP 44 Fahrenheit H T DP 22 Celsius Base heights are AGL Cumulus heap Stratus layer Nimbus rain Cirrus ringlet high Alto middle High clouds are mostly made of ice crystals CU and CB indicative of great instability
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