Weather Basics Chapter 10 Climate and Weather Related to Hazards EXTREME WEATHER PHOTOS Earth Cycles Days Rotation of the Earth on its axis Season s Tilt of the Earth s axis with respect to its annual orbit around the sun Refer to pg 249 Greenhouse Effect Carbon dioxide methane nitrous oxide and chlorofluorocarbons CFC s are some greenhouse gases of anthropogenic origin Refer to pg 245 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 Weather 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 Vertical Structure of the Atmosphere General 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 dramatically Water 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 released The Hydrologic Cycle Water 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 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 Refer to pg 251 252 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 Processes Processes 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 Stability 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 Two assumptions Lifting processes force air upward Rising air does not mix substantially with the surrounding atmosphere A parcel of air forced to rise will expand and cool adiabatically Atmospheric Stability Stable 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 Stability Unstable 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 Refer to pg 253 254 Wind Horizontal 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 Refer to pg 254 The spacing of the isobars indicates the amount of pressure change over a given distance pressure gradient Coriolis Effect Due 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 Flow In 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 pressure cell sinking air an outward clockwise flow develops near the ground surface Centers of high pressure are called anticyclones divergent flow Refer to pg 253 255 Right Hand Rule for Rising or Falling Air in the Northern Hemisphere Left Hand Rule for the Southern Hemisphere Refer to pg 253 256 Vertical Flow Net downward movement of air and fair weather Low pressure center generally related to unstable conditions and stormy weather Net upward movement of air often resulting in cloud formation and precipitation Around a surface high air is spiraling outward which leads to a downward flow of air at the center of the high and convergence aloft Around a surface
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