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Chapter 19

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  1. Chapter 19 Air Pressure Astronomy/Meteorology

  2. Understanding Air Pressure 19.1 • Air Pressure Defined • Pressure exerted by the weight of the air above • Exerted in all direction • http://www.youtube.com/watch?v=vGulvutZJpg - 6 minutes • Measuring Air Pressure • Barometer – device used to measure air pressure • Bar= pressure • Metron = measuring instrument • Pressure increases  pushes mercury up • unit = millibar • Air pressure at sea level is 1013.2 mbars

  3. Factors Affecting Wind 19.1 • Air moves from high pressure to low pressure • Wind result of differences in air pressure • Differences in air pressure b/c of unequal heating (temp) • Solar radiation source of wind • Air doesn’t move in straight lines b/c • Pressure differences • Coriolis Effect • Friction

  4. Pressure Difference 19.1 • Great difference = greater wind • Isobars – lines on a map that connect places of equal air pressure • Pressure gradient – spacing between lines • Close: steep pressure gradient = high winds • Widely spaced: weak pressure gradient = light wind • http://www.youtube.com/watch?v=hKGQJEp-Pys • 2 minutes • Driving force of wind • High to low

  5. Coriolis Effect 19.1 • Describes how Earth’s rotation affects moving objects • Northern Hemisphere = deflected to the right • Southern Hemisphere = deflected to the left • Only Affects wind direction • Strong winds equals strong deflection • Strongest at poles • Basically nonexistent at equator http://www.youtube.com/watch?v=mcPs_OdQOYU 2 minutes

  6. Friction 19.1 • Affects wind speed and direction • Only affects a few km from Earth’s surface • Jet streams = fast moving rivers of air that travel between 120 – 240 km/hr from east to west

  7. Pressure and Centers and Winds 19.2 • Highs and Lows • Cyclones – centers of Low Pressure • Pressure decreases from the outer isobars towards center • Northern Hemi: counterclockwise • Low = Left • Southern Hemi: clockwise • Anticyclones – centers of High Pressure • Pressure increases from outer to inner • Northern Hemi: clockwise • Southern Hemi: counterclockwise

  8. Pressure and Centers and Winds 19.2 • Weather and Air Pressure • Rising air = clouds and precipitation • Surface Convergence = Cyclonic (low) • Sinking air = clear skies • Surface Divergence = Anticyclone (high)

  9. Pressure and Centers and Winds 19.2 • Weather Forecasting • Low-pressure centers – can produce bad weather • Lows move W to E • Predicting is difficult

  10. Global Winds = Non-Rotating Earth Model 19.2

  11. Global Winds = Rotating Earth Model 19.2

  12. Regional Wind Systems 19.3 • Local Winds – small-scale winds • Caused by either topographic effects or variation in surface (land and water) • Land and Sea Breezes • Land heats and cools faster than water

  13. Valley and Mountain Breezes • Air on slopes of mountains heated more during the day than the valley floor • Slope air = less dense  glides up slope to generate a valley breeze • Air cools quickly at night = more dense = sinks into valley

  14. How Wind is Measured 19.3 • Wind Direction • Prevailing Wind – wind consistently blows more often from one direction • US = westerlies move from west to east • Wind Speed • Anemometer – measure wind speed

  15. El Nino and La Nina • El Nino = irregular intervals of 3 to 7 years of warm countercurrents that replace normal cold offshore waters • Affects Ecuador and Peru • La Nina= temperature in eastern Pacific are colder than average

  16. Normal Conditions El Nino Conditions

  17. http://www.youtube.com/watch?v=SR5VPAqVQBw • 3 minutes • http://www.youtube.com/watch?v=fN_NmCpry38 • 4 min 30 sec • http://www.youtube.com/watch?v=mHpZhKUldBg • 3 min 15 sec