Environmental Problems, Their Causes, and Sustainability 1

ecules of gases (mostly nitrogen and oxygen) in the atmospherezipping around at incredible speeds andhitting and bouncing off anything they encounter.Atmospheric pressure is greater near the earth’ssurface because the molecules in the atmosphereare squeezed together under the weight of the airabove. An air mass with high pressure, called a high,contains cool, dense air that descends toward theearth’s surface and becomes warmer. Fair weather followsas long as the high-pressure air mass remainsover an area.In contrast, a low-pressure air mass, called a low,produces cloudy and sometimes stormy weather. Becauseof its low pressure and low density, the center ofa low rises, and its warm air expands and cools. Whenthe temperature drops below a certain level where condensationtakes place, called the dew point, moisture inthe air condenses and forms clouds. If the droplets inthe clouds coalesce into large and heavy drops, precipitationoccurs. Recall that the condensation of watervapor into water drops usually requires that the aircontain suspended tiny particles of material such asdust, smoke, sea salts, or volcanic ash. These so-calledcondensation nuclei provide surfaces on which thedroplets of water can form and coalesce. Now youknow how rain forms.What Are Tornadoes and Tropical Cyclones?Weather GodzillasTornadoes and tropical storms are weather extremesthat can cause lots of damage but can sometimeshave beneficial ecological effects.Sometimes we experience weather extremes. Two examplesare violent storms called tornadoes (which formover land) and tropical cyclones (which form over warmocean waters and sometimes pass over coastal land).Tornadoes or twisters are swirling funnel shapedclouds that form over land. They can destroy housesand cause other serious damage in areas when theytouch down on the earth’s surface. The United States isthe world’s most tornado-prone country, followed byAustralia.Tornadoes in the plains of the Midwest usually occurwhen a large, dry cold air front moving southwardfrom Canada runs into a large mass of humid air movingnorthward from the Gulf of Mexico. Most tornadoesoccur in the spring when fronts of cold air fromthe north penetrate deeply into the midwestern plans.As the large warm-air mass moves rapidly overthe more dense mass of cold air it rises rapidly andforms strong vertical convection currents that suck airupward, as shown in Figure 6-3. Trace the flows in thisDescendingcool airSeverethunderstormRisingwarm airSevere thunderstormscan trigger a numberof smaller tornadoesTornado forms whencool downdraft andwarm updraft of airmeet and interactWarm moist air drawn inRisingupdraftof airFigure 6-3 Formationof a tornado ortwister. Althoughtwisters can formany time of the year,the most active tornadoseason in theUnited States isusually Marchthrough August.Meteorologists cannottell us with greataccuracy when andwhere most tornadoeswill form.http://biology.brookscole.com/miller14103