Weather BasicsEarth CyclesGreenhouse EffectWeatherSlide 5Water Vapor in the AtmosphereThe Hydrologic CycleRelative HumidityTherefore, relative humidity can be changed by...Adiabatic ProcessesUpward movement of air results from:Slide 12Atmospheric StabilitySlide 14Slide 15WindCoriolis EffectSlide 18Right Hand Rule for Rising or Falling Air in the Northern Hemisphere (Left Hand Rule for the Southern Hemisphere)Slide 20Slide 21Weather BasicsChapter 10Climate and Weather Related to HazardsEXTREME WEATHER PHOTOSEarth CyclesDays: Rotation of the Earth on its axis.Seasons: Tilt of the Earth’s axis with respect to its annual orbit around the sun.(Refer to pg. 249)Greenhouse EffectCarbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFC's) are some greenhouse gases of anthropogenic origin.Left: Greenhouse gases trap heat in the Earth’s atmosphere much as the glass in a greenhouse permits the sun to shine in but prevents most heat from escaping.(Refer to pg. 245)Weather is the condition of the atmosphere at a particular time and place. It refers to such conditions of the local atmosphere as •temperature, •atmospheric pressure, •humidity (the amount of water contained in the atmosphere), •precipitation (rain, snow, sleet, & hail),•wind velocity. WeatherVertical Structure of the AtmosphereGeneral trends with increasing altitude:Air pressure decreases. At any given altitude, the air pressure is caused by the weight of air above. Constituent gases decrease in density. Because air pressure decreases with altitude, the amount of air per unit volume (density) also decreases with altitude. Temperature decreases in the troposphere where weather occurs.Water vapor decreases dramaticallyWater Vapor in the Atmosphere Water can exist in all 3 states at the normal range of earth temperature and pressure.Whenever matter changes from one state to another, energy is either absorbed or released.•From liquid to gas - evaporation - heat energy is absorbed •From gas to liquid - condensation - heat energy is releasedThe Hydrologic CycleWater continuously evaporates from oceans and other water bodies, falls as rain or snow, is transpired by plants, and flows through streams and groundwater back to the oceans.(Refer to pg. 251-252)Relative Humidity(Refer to pg. 251-252)Saturation vapor pressure = Maximum amount of moisture air can hold•cannot be exceeded.•evaporation = condensation•temperature dependent.Relative Humidity = The percentage of moisture in air relative to the maximum amount it can hold (Saturation Vapor Pressure) under its given temperature and pressure.Therefore, relative humidity can be changed by...•Changing the water vapor content.–Add water, increase relative humidity.–subtract water, decrease relative humidity.•Changing the temperature.–Increase temperature, decrease relative humidity.–Decrease temperature, increase relative humidity.Adiabatic ProcessesProcesses that occur without the addition or subtraction of heat from an external source.Because air pressure decreases with increasing altitude, rising air expands and sinking air is compressed.Compressional warming - when air is compressed, the temperature rises.Expansional Cooling - when air expands, the temperature decreases.The adiabatic lapse rate - the way temperature changes with altitude in rising or falling air (top right).Lifting condensation level = altitude at which the rising parcel reaches saturation temperature and cloud forms (bottom right).(Refer to pg. 252)Upward movement of air results from:•Convergence lifting - when flowing air masses of equal density converge and are forced upward.•Convective (Density) lifting - When warm, low-density air rises convectively and displaces cooler, denser air.•Orographic lifting - When flowing air is forced upward over a mountain range.•Frontal lifting - when two flowing air masses of different density meet.(Refer to pg. 253, 258)Warm Front (Left): Warm air mass advances rapidly.Cold Front (Right): Cold air mass advances rapidly.Atmospheric StabilityTwo assumptions:–Lifting processes force air upward.–Rising air does not mix substantially with the surrounding atmosphere.Atmospheric stability is a property of air that describes its tendency to remain in its original position or sink (stable) or to rise (unstable) once the initial lifting force ceases.A parcel of air forced to rise will expand and cool adiabatically.Atmospheric StabilityStable air - if an air parcel that is forced aloft cools faster than the surrounding environment. If the lifting forced ceased, the parcel would have the density to sink.High pressure system – an area characterized by descending cooler dry air and clear skies.Cloud formation may occur at an altitude where the saturation temperature is reached (LCL), but clouds would be layered without much vertical development - fair weather clouds.(Refer to pg. 253-254)Atmospheric StabilityUnstable air - if an air parcel that is forced aloft cools slower than the surrounding environment. If the lifting force ceased, the parcel will continue to rise because it is warmer and more buoyant than its surroundings.Low pressure system – An area characterized by rising warmer and humid air and cloudy skies. If the air parcel rises to an altitude where the saturation temperature is reached (LCL), clouds with vertical development will form as the buoyant air rises on its own. (thunderstorm clouds).(Refer to pg. 253-254)WindHorizontal differences in air pressure between high and low pressure systems create winds.Wind results when air flows from a place of high pressure to one of low pressure.Magnitude is determined from the spacing of the isobars.Isobars - lines connecting places of equal air pressure on a map (Compare to the slope of a hill).The spacing of the isobars indicates the amount of pressure change over a given distance = pressure gradient.(Refer to pg. 254)Coriolis EffectDue to the rotation of the earth on its axis.Deflects all free moving objects to the right of their path in the Northern Hemisphere and to the left in the Southern Hemisphere.Deflection increases with wind speed.Deflection is strongest at the poles and weakens equator ward.(Refer to pg. 254-255)Convergent and Divergent FlowIn the Northern Hemisphere •Around a low pressure cell (rising air), an inward counterclockwise flow develops near the ground surface; Centers of low pressure are called cyclones = convergent flow•Around a high
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