 Air near the surface flows down and away in a high pressure system (left) and air flows up and together at a low pressure system (right). Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky. But what you don’t see is air – lots of it. We live at the bottom of the atmosphere, and the weight of all the air above us is called air pressure. Above every square inch on the surface of the Earth is 14.7 pounds of air. That means air exerts 14.7 pounds per square inch (psi) of pressure at Earth’s surface. High in the atmosphere, air pressure decreases. With fewer air molecules above, there is less pressure from the weight of the air above. Pressure varies from day to day at the Earth’s surface - the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where the air is warmed often have lower pressure because the warm air rises. These areas are called low pressure systems. Places where the air pressure is high, are called high pressure systems. A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses, forming clouds and often precipitation. Because of Earth’s spin and the Coriolis effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator. This is called cyclonic flow. On weather maps, a low pressure system is labeled with red L. A high pressure system has higher pressure at its center than the areas around it. Winds blow away from high pressure. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward. On a weather map, you may notice a blue H, denoting the location of a high pressure system. How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. Historically, measurements of air pressure were described as “inches of mercury.” Today, meteorologists use millibars (mb) to describe air pressure. Air pressure depends on temperature and density.When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon. This means the density of air is high inside the balloon. When the density of air is high, the air pressure is high. The pressure of the air pushes on the balloon from the inside, causing it to inflate. If you heat the balloon, the air pressure gets even higher. Air pressure depends on the temperature of the air and the density of the air molecules. Atmospheric scientists use math equations to describe how pressure, temperature, density, and volume are related to each other. They call these equations the Ideal Gas Law. In these equations, temperature is measured in Kelvin.
This equation helps us explain how weather works, such as what happens in the atmosphere to create warm and cold fronts and storms, such as thunderstorms. For example, if air pressure increases, the temperature must increase. If air pressure decreases, the temperature decreases. It also explains why air gets colder at higher altitudes, where pressure is lower.
GEO 302C EXAM 2 Fall 2003 The mean score of this test is 78. Your grade was posted on eGradebook on 10/10/03. You may not refer to any other materials during the exam. For each question (except otherwise explicitly stated), select the best answer for that question. Read all choices before selecting an answer and make sure your choice answers the question asked. 1. Ignoring the contributions of wind, a baseball would be expected to travel a _______ distance in cold air than in hot air. a. longer b. shorter 2. This satellite image shows a tropical cyclonic system developed in the __________. a. Northern Hemisphere. b. Southern Hemisphere. 3. Tightly packed isobars produce _______ pressure gradient forces and ______ winds. a. strong, high. b. strong, light. c. weak, high. d. weak, light. 4. If surface air pressure decreases, the height of the column in a mercury barometer would: a. remain constant b. increase c. decrease d. change momentarily, but return to its earlier reading 5. If water than mercury were used in a barometer the height of the column in the barometer would: a. increase significantly b. decrease significantly c. remain the same d. not provide an accurate measure of atmospheric pressure 6. The unit of pressure most commonly found on a surface weather map: a. inches of mercury b. millibars (mb) c. pounds per square inch (psi) d. atmospheres 7. Sea level pressure is typically around a. 2000 mb b. 1000 mb c. 500 mb d. 100 mb e. 10 mb 8. The surface pressures at the bases of warm and cold columns of air are equal. Air pressure in the warm column of air will with increasing height than in the cold column. a. decrease, more rapidly b. decrease, more slowly c. increase, more rapidly d. increase, more slowly 9. Which of the following instruments measures pressure? a. barometer b. thermometer c. psychrometer d. hygrometer 10. The surface weather map is a sea level chart. Thus, a surface weather map is also called: a. a constant pressure chart b. a constant height chart c. an isobaric chart d. a constant latitude chart 11. Lines connecting points of equal pressure are called: a. isobars b. millibars c. contours d. isotherms 12. Pressure changes: a. more rapidly in the horizontal direction than in the vertical b. more rapidly in the vertical direction than in the horizontal c. at the same rate in the horizontal and vertical directions d. more rapidly in the vertical over land than over the ocean 13. The Coriolis force is the force that causes the wind to blow. a. true b. false 14. Which of the following forces does not have a direct effect on horizontal wind motions? a. pressure gradient force b. frictional force c. gravitational force d. Coriolis force 15. Which of the following forces can not act to change the speed of the wind? a. pressure gradient force b. frictional force c. Coriolis force d. none of the above 16. The force that would cause a stationary parcel of air to begin to move horizontally is called the: a. Coriolis force b. pressure gradient force c. frictional force 17. Which of the following produces the strongest Coriolis force? a. fast winds, low latitude b. fast winds, high latitude c. slow winds, low latitude d. slow winds, high latitude 18. If the earth stopped rotating which of the following would not be true? a. surface winds would blow from high toward low pressure b. there would still be a Coriolis force c. there would still be a pressure gradient force d. there would still be a gravitational force 19. The vertical pressure gradient force is directed a. downward b. upward c. horizontally 20. The winds aloft in the middle latitudes of the Southern Hemisphere generally blow: a. from west to east b. from east to west c. from north to south d. from south to north 21. Surface winds blow across the isobars at an angle due to: a. the Coriolis force b. the pressure gradient force c. the frictional force 22. The wind around a surface high pressure center in the Northern Hemisphere blows: a. counterclockwise and outward from the center b. counterclockwise and inward toward the center c. clockwise and outward from the center d. clockwise and inward toward the center 23. We can generally expect the air to be above areas of surface low pressure and above areas of surface high pressure. a. rising, rising b. rising, sinking c. sinking, sinking d. sinking, rising 24. Which of the following associations is most accurate? a. microscale - chinook wind b. synoptic scale - sea breeze c. mesoscale - land breeze 25. An instrument used to measure wind speed is called a(an): a. anemometer b. thermometer c. psychrometer d. barometer 26. An extremely strong downslope wind that occurs in parts of Antarctica would be considered a a. katabatic wind b. Chinook wind c. mountain breeze d. monsoon circulation e. diurnal breeze 27. Which of the following conditions favor the development of dust devils? a. hot, moist days b. hot, dry days c. cold, moist days d. cold, dry days 28. The summer monsoon in eastern and southern Asia is characterized by: a. wet weather and winds blowing from land to sea b. dry weather and winds blowing from land to sea c. wet weather and winds blowing from sea to land d. dry weather and winds blowing from sea to land 29. Which below is not an assumption of the single-cell model of the general circulation of the atmosphere? a. the earth's surface is covered with water b. the earth rotates once in 24 hours c. the sun is always overhead at the equator 30. Chicago, Illinois (latitude 42o N) is located in the . a. northeast trades b. southeast trades c. westerlies d. doldrums 31. The intertropical convergence zone (ITCZ) is a region where: a. the polar front meets the subtropical high b. northeast trades meet the southeast trades c. northeast trades converge with the subtropical high d. the Ferrel cell converges with the Hadley cell e. polar easterlies converge with the air at the doldrums 32. In terms of the three-cell model of the general circulation, areas of surface low pressure should be found at: a. the equator and the poles b. the equator and 60o latitude d. 30o latitude and 60o latitude 33. The majority of the United States lies within this wind belt: a. westerlies b. easterlies c. northerlies d. trades e. southerlies 34. The world's deserts are found at 30o latitude because: a. the intertropical convergence zone is located there b. of the sinking air of the polar front c. of the convergence of the prevailing westerlies and the northeast trades d. of the sinking air of the subtropical highs e. of the doldrums 35. Which of the following does not describe the subtropical jet stream? a. generally blows from east to west b. is found at the tropopause c. is normally equatorward of the polar front jet stream 36. The average winds aloft are strongest in: a. summer b. winter 37. Which of the following is true about the oceans? a. Approximately 3% of the total water on Earth is located in the oceans b. The oceans cover about 30% of Earth’s surface c. The average depth of the oceans is infinite d. The sea surface is level for all the oceans e. The oceans have a huge heat capacity, so their temperature is less variable than the air temperature. 38. The largest reservoir of water on Earth is a. oceans b. ice caps/glaciers c. lakes/rivers d. atmosphere 39. What is the primary salt in the ocean? a. CO2 b. FeO2 c. NaCl 40. Major surface ocean currents that flow parallel to the coast of North America are: a. Labrador, Canary, California b. California, Gulf Stream, Labrador c. Kuroshio, California, Labrador d. California, Texas, Florida 41. In the Northern Hemisphere, the surface ocean currents in the Atlantic and the Pacific move in a generally circular pattern. The direction of this motion is in the Atlantic and in the Pacific. a. clockwise, counterclockwise b. counterclockwise, counterclockwise c. clockwise, clockwise d. counterclockwise, clockwise 42. The cold water observed along the northern California coast in summer is due mainly to: a. the California current b. oceanic fronts c. upwelling d. cold air moving over the water e. evaporation 43. The Ekman Spiral describes: a. the turning of water with depth b. the air flow into a center of low pressure c. the circulation of surface water around a gyre d. the air flow out of a region of high pressure e. the wind-flow pattern in a jet stream 44. Upwelling is: a. the lifting of air along the polar front b. the rising of cold water from below c. increasing heights in an upper-level ridge d. the rising air motion found in a low pressure center 45. Upwelling occurs along the northern California coast because a. winds and the Ekman transport cause surface waters to move away from the coast b. of seismic activity on the ocean bottom c. of gravitational attraction between the earth and the moon d. water flows from the Atlantic ocean into the Pacific because they are at different levels 46. The name given to the current of warm water that replaces cold surface water along the coast of Peru and Ecuador around Christmas is: a. Brazil current b. Humbolt current c. Benguela current d. El Niño e. California current 47. During a major El Niño event: a. Peruvian fishermen harvest a record amount of fish near Christmas time b. extensive ocean warming occurs over the central and eastern tropical Pacific c. the Northeast trade winds increase in strength d. Texas experiences severe drought conditions e. climatic abnormalities are only limited to the tropical Pacific areas 48. A condition where the central and eastern tropical Pacific Ocean turns cooler than normal is called: a. El Niño b. La Niña c. the Southern Oscillation d. the Ekman Spiral 49. The reversal of the positions of surface high and low pressure at opposite ends of the tropical Pacific Ocean is called: a. El Niño b. the Southern Oscillation c. upwelling d. La Niña 50. Which of the following associations is most accurate for El Nino, La Nina, or both? a. Both – Influence weather and climate around the world b. La Nina – Easterly trade winds weaken c. La Nina – The thermocline in the eastern equatorial Pacific Ocean is deeper than average d. El Nino – Is part of ENSO process, but La Nina is not e. Both – A phenomenon in the Atlantic Ocean |
The wind around a surface low pressure center in the Southern Hemisphere blows
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