ENSP101 Chapter 15 Air Weather and Climate 15 1 The Atmosphere is a Complex System The atmosphere has 4 distinct zones o Troposphere Within the troposphere air circulates in great vertical and horizontal Layer of air immediately adjacent to the earth s surface 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 Much more dense than the other layers It contains about 75 of the total poles 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 zones o Stratosphere More dilute than the troposphere but has similar composition Extends from the tropopause up to about 50 km 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 years o Mesosphere Or middle layer above the stratosphere where the temperature diminishes again o 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 latitudes o Of the solar energy that reaches the outer atmosphere about is reflected by clouds and atmospheric gases and another is absorbed by CO2 H2O O3 CH4 and other gases o 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 water o 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 surface o 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 surface o H2O most abundant greenhouse gas Always present in the atmosphere o CO2 most abundant human caused greenhouse gas Followed by CH4 NO2 and others Evaporated water stores energy and winds redistribute it o Without oceans to absorb and store heat and wind currents to redistribute that heat in the latent energy of water vapor the earth would undergo extreme temperature fluctuations like those of the moon o Latent heat stored energy in a form that is not sensible detectable by ordinary o Where air rises in convention currents air pressure at the surface is low Where air is sinking or subsiding air pressure is high o Rising air tends to cool with altitude releasing latent heat which causes further senses rising 15 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 surface o 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 the Northern Hemisphere and counterclockwise into a low pressure zone o Ocean currents similarly curve clockwise in the Northern Hemisphere and counterclockwise in the south o 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 land o Surface ocean currents result from wind pushing on the ocean surface as well as o As surface water moves deep water wells up to replace it creating deeper ocean o Differences in water density depending on the temperature and saltiness of the o Huge cycling currents called gyres carry water north and south redistributing heat from the Coriolis effect currents water also drive ocean circulation from low latitudes to high latitudes Gulf Stream o 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 life o Sometimes these seasonal rains are extreme causing disastrous flooding o Sometimes the rains fail causing crop failures and famine o 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 continent o 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 year o As the earth orbits the sun the tilt of its axis creates seasons with varying amounts of wind rain heat or cold o 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 air o Cold fronts generate strong convective currents and often are accompanied by violent surface winds and destructive storms o The weather after the cold front passes is usually clear dry and invigorating Warm front a long wedge shaped boundary caused when a warmer advancing air mass slides over neighboring cooler air parcels o A warm front will have many layers