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ENSP101Chapter 15 – Air, Weather, and Climate15.1 The Atmosphere is a Complex System- The atmosphere has 4 distinct zones:o Troposphere: Layer of air immediately adjacent to the earth’s surface Within the troposphere, air circulates in great vertical and horizontal convection currents, constantly redistributing heat and moisture around the globe Depth ranges from about 18 km over the equator to about 8 km over the poles Much more dense than the other layers  It contains about 75% of the total mass of the atmosphere Air temperature drops rapidly with increasing altitude A reversal of this temperature gradient creates a sharp boundary called the tropopause, which limits mixing between the troposphere and upper zoneso Stratosphere: Extends from the tropopause up to about 50 km More dilute than the troposphere but has similar composition- Except it has almost no water vapor and nearly 1,000x more ozone- Stratospheric ozone absorbs certain wavelength of UV solar radiation Relatively calm There is so little mixing in the stratosphere that volcanic ash or human-caused contaminants can remain in suspension there for many yearso Mesosphere  Or middle layer, above the stratosphere where the temperature diminishes againo Thermosphere  Heated layer Begins at about 80 km Aurora borealis and aurora australis- Pressure and density decrease with distance form the earth until they become indistinguishable form the near-vacuum of interstellar space- Incoming solar radiation (insolation) is much stronger near the equator than at high latitudeso Of the solar energy that reaches the outer atmosphere, about ¼ is reflected by clouds and atmospheric gases, and another ¼ is absorbed by CO2, H2O, O3, CH4, and other gaseso Some of this energy is reflected by bright surfaces, such as snow, ice, and sand  The rest is absorbed by the earth’s surface and by watero Surfaces that reflect energy have a high albedo (reflectivity)o Absorbed energy heats the absorbing surface, evaporates water, or provides the energy for photosynthesis in plants- Greenhouse effect – gases in the atmosphere are transparent to visible light but absorb infrared (heat) waves that are reradiated from the earth’s surfaceo Too strong a greenhouse effect, caused by burning fossil fuels and deforestation, may cause harmful environmental change- Greenhouse gases – a general term for gases that are especially effective at capturing the long-wavelength energy from the earth’s surfaceo H2O = most abundant greenhouse gas Always present in the atmosphereo CO2 = most abundant human-caused greenhouse gas Followed by CH4, NO2, and others- Evaporated water stores energy, and winds redistribute ito Without oceans to absorb and store heat, and wind currents to redistribute that heatin the latent energy of water vapor, the earth would undergo extreme temperature fluctuations like those of the moono Latent heat – stored energy in a form that is not sensible (detectable by ordinary senses)o Where air rises in convention currents, air pressure at the surface is low  Where air is sinking, or subsiding, air pressure is higho Rising air tends to cool with altitude, releasing latent heat, which causes further rising15.2 Weather Events Follow General Patterns- The Coriolis effect explains why winds seem to curve:o In the Northern Hemisphere, winds appear to bend clockwise (right)  In the Southern Hemisphere, they appear to bend counterclockwise (left) This curving pattern results from the fact that the earth rotates in an eastward direction as the winds move above the surfaceo The apparent curvature of the winds is known as the Coriolis effect On a global scale, this effect produces steady, reliable wind patterns, such as the trade winds and the mid-latitude Westerlies On a regional scale, the Coriolis effect produces cyclonic winds, or wind movements controlled by the earth’s spin- Cyclonic winds spiral clockwise out of an area of high pressure in theNorthern Hemisphere and counterclockwise into a low-pressure zoneo Ocean currents similarly curve clockwise in the Northern Hemisphere and counterclockwise in the southo At the top of the troposphere are jet streams, hurricane-force winds that circle the earth These powerful winds follow an undulating path approximately where the vertical convection currents known as the Hadley and Ferrell cells meet- They greatly affect weather patterns- Warm and cold ocean currents strongly influence climate conditions on lando Surface ocean currents result from wind pushing on the ocean surface, as well as from the Coriolis effecto As surface water moves, deep water wells up to replace it, creating deeper ocean currentso Differences in water density – depending on the temperature and saltiness of the water – also drive ocean circulationo Huge cycling currents called gyres carry water north and south, redistributing heat from low latitudes to high latitudes Gulf Streamo Ocean circulation patterns were long thought to be unchanging, but now oceanographers believe that currents can shift abruptly Lake Agassiz- Large parts of the world, especially the tropics, receive seasonal winds and rainy seasons that are essential for sustaining both ecosystems and human lifeo Sometimes these seasonal rains are extreme, causing disastrous floodingo Sometimes the rains fail, causing crop failures and famineo The most regular seasonal winds and rains are known as monsoons In India and Bangladesh, monsoon rains come when seasonal winds blow hot, humid air from the Indian Ocean Strong convection currents lift this air, causing heavy rain across the sub-continento Tropical an sub-tropical regions around the world have seasonal rainy and dry seasons The main reason for this variable climate is that the region of most intense solar heating and evaporation shifts through the yearo As the earth orbits the sun, the tilt of its axis creates seasons with varying amounts of wind, rain, heat or coldo As the year shifts from summer to winter, solar heating weakens, the rainy season ends, and little rain may fall for months- The boundary between 2 air masses of different temperature and density are called a front- Cold front – a moving boundary of cooler air displacing warmer airo Cold fronts generate strong convective currents and often are accompanied by violent surface winds and destructive stormso The weather after the cold front passes is


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