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HOMEWORK 3 ANSWER KEY ATMOSPHERIC MOISTURE GEO 131 Spring 2007 True/False: 1. The special properties of water (universal solvent, high boiling point) are related to its low molecular weight. False - it is related to its asymmetrical shape which creates a charge imbalance (i.e., a polar charge distribution). The hydrogen end is positively charged and the oxygen end is negatively charged. This charge imbalance enables water molecules to pull apart other molecules, thereby making water a universal solvent. The charge imbalance also causes the water molecules to be strongly attracted to one another, thereby raising the boiling point of water. 2. Water can dissolve almost anything because of the shape of the water molecules and the charge distribution on them. True - The positive charge on one end and the negative charge on the other end of the Vshaped water molecule creates an electrical attraction for the molecules near to them, and can thereby pull molecules apart. 3. If ice sufficiently compressed, it will melt- even at temperatures below freezing. True - Because ice must expand in order to develop the ordered crystalline structure characteristic of the frozen state, compression destroys this structure and results in melting. 4. On a cold winter day, an unfrozen lake will lose heat energy faster than a lake with a frozen surface. True - Ice acts like a layer of insulation because it slows the rate at which heat energy can escape from the underlying water. 5. The charge distribution on water molecules is relevant to dissolving things, but not to the phase changes that water undergoes. False - the charge distribution results in an attraction between water molecules - the positive end of one water molecule attracts the negative end of another. As a result, this increases the amount of energy required for phase changes. If it weren't for this fact, water would not serve as the "fuel for storms." 6. Many substances on earth undergo phase changes within the temperature range that occurs naturally on earth. False - only water does this - which is one of the reasons why it is so special. It is also why the hydrologic cycle occurs. If water did not possess this property, life as we know it would be impossible on earth. 7. The reason temperature changes result in phase changes is because the change in temperature affects the speed of the molecules. True - by affecting the speed of the molecules, a change in temperature affects the ability of the chemical bonds to hold molecules together. For example, as the temperature increases, the speed of the molecules increases, thereby reducing the effectiveness of the bonds. This leads to a change from either a solid to a liquid, or a liquid to a gas, depending upon the original state of the matter and the amount of temperature increase. 8. The molecules in liquid water move faster than those in water vapor. False - the molecules in a gas move much faster. They move so fast that they overcome the forces of attraction between individual water molecules and move freely in all directions. The molecules in a liquid move more slowly, and those in a solid are locked in place. 9. The energy required for evaporation is known as the heat of fusion. False - the process of evaporation involves the heat of vaporization (the key word is vapor). The heat of fusion refers to the energy released when water freezes, i.e., when water molecules fuse together, so to speak, into a rigid matrix to form solid ice. Note that the "heat of fusion" has nothing to do with "nuclear fusion" in stars, which was discussed earlier in the course. 10. During a phase change, the temperature of substance does not change. True - the energy is being used to weaken molecular bonds rather than to increase the temperature of the substance. Thus, during a phase change, no change in temperature occurs. 11. It would take 30 calories to heat ten grams of water from 90 C to 110 C False - it would take 6,100 calories. One hundred calories would be required to raise the temperature from 90 C to 100 C, 5900 calories would be required for the phase change from liquid to gas, and 100 more calories to raise the temperature from 100 C to 110 C. The steps used to make this calculation are as follows: The amount of energy required to raise the temperature of one gram of ice, water, or steam by 1 C is 1 calorie, i.e., the specific heat of water is 1 cal/gram/ C. Therefore: 1 cal/g/ C x 10 g = 10 calories/ C Since the temperature increase from 90 C to 100 C is 10 C, multiply this answer by 10 C. 10 cal/ C x 10 C = 100 calories The latent heat of vaporization is 590 cal/g. Because no temperature increase occurs during a phase change, simply multiply the number of grams of water by the latent heat of vaporization. Thus, 590 calories/g x 10 g = 5900 calories The last step is identical to the first step because it involves the same amount of temperature increase. Therefore, the increase in temperature from 100 C to 110 C also requires 100 calories. Finally, add the values of each step together: 100 + 5900 + 100 = 6100 calories * Remember that much of this energy is derived from the air and therefore, the phase change from liquid to gas cools the air. Note also that the same amount of energy would be given off by the water were it to cool from 110 C to 90 C and that this energy would heat the air. It is this energy, which is liberated by condensation that fuels storms! 12. If the water vapor in the previous question were cooled from 110 C back down to 90 C, it would release exactly the same amount of energy as was required to raise its temperature from 90 C to 110 C. True - the energy of phase changes is conserved. This means that, whatever the amount of energy required to raise the temperature of a substance that is the amount that will be released when it is cooled to it starting temperature. THE ENERGY THAT IS RELEASED DURING THIS COOLING PROCESS IS THE ENERGY THAT SERVES AS THE FUEL FOR STORMS. IN SHORT, CONDENSATION FUELS STORMS BY RELEASING HEAT ENERGY TO THE ATMOSPHERE AND WARMING IT UP! THIS CREATES A LOW PRESSURE REGION, A PRESSURE GRADIENT AND WINDS. 13. If the latent heat of vaporization were 80 calories/gram (rather than 590 calories/gram), water would evaporate more readily than it does now. True - this would be true because less energy (80 calories/gram vs. 590 calories/gram) would be required for the phase change. This would also mean that life might become impossible on earth because water would boil more easily! 14. Condensation of water vapor in a storm decreases the air pressure True - Because the heat energy released by condensation acts to heat the air, the air pressure decreases. Remember, warm air is associated with low pressure because warm air has a lower density (fewer molecules per volume) than cold air. Warm air also tends to have more moisture in it, which also makes it lighter. THIS IS WHY ALL STORMS ARE LOW PRESSURE SYSTEMS AND WHY A WEATHER FORECAST THAT PREDICTS A DROP IN PRESSURE USUALLY SIGNALS THAT STORMY WEATHER IS ON THE WAY. 15. Most of the worlds freshwater is stored in rivers and lakes False - Most freshwater is stored in glaciers. The second largest reservoir is below ground in the form of groundwater. 16. Compared to the oceans, the land contributes a relatively small fraction of water to the atmosphere through the process of evaporation. True - it contributes only about 15%. The low amount is due to the simple fact that there is less water on the land than in the ocean. 17. As temperature increases, relative humidity increases- assuming no change in specific humidity. False - as temperature increases, the amount of water that can potentially enter the air increases because of the greater amount of energy available for evaporation. In other words, more water can evaporate and remain in the gaseous state because chemical bonds are so much less effective at binding water molecules together when the molecules are moving at high speeds. However, if water does evaporate into the air, this would mean that the relative humidity was increasing. (* Note vapor pressure is another way to indicate the amount of water in the air. The greater the amount of water vapor, the greater the amount of vapor pressure.) RH = Actual vapor pressure x 100 Potential vapor pressure Example: Assuming that the potential vapor pressure (PVP) is 2 mb, then: Original condition: RH = (1 mb/2 mb) x 100 = 50% If we increase the temperature and raise the potential vapor pressure to 4 mb, then: RH = (1 mb/4 mb) x 100 = 25% If we increased the AVP to 1.2 mb while keeping the temperature and potential vapor pressure constant at 2 mb, then: RH = (1.2 mb/2 mb) x 100 = 60% Thus, when the temperature increases - and no additional water enters the air - the amount of water in the air relative to the amount it could hold decreases. This is a very important concept that is crucial to understanding weather, so learn it! 18. The lower the relative humidity, the greater the wet bulb depression on a sling psychrometer. True - the drier the air, the greater the amount of evaporation, and therefore the greater the amount of cooling experienced by the wet bulb. 19. The more humid the air, the cooler you will be on a hot day. False - on a hot, humid day, water cannot evaporate from your skin as readily as it would on a dry day because the air is nearly saturated. As a result, the water, which has absorbed heat from your body and the air, remains in contact with your skin, thereby, preventing your body from cooling. This is why hot, humid days are described as "muggy and uncomfortable". 20. If the air temperature in late afternoon is 78 F and wet bulb depression is 1 , one could expect cloud formation, or fog to occur when the temperature drops to 77 F True - if the temperature drops to 77 F, the relative humidity will have reached 100% and condensation will begin. In this example, 77 F represents the dew point temperature. The air temperature will not drop below 77 F because the process of condensation will release heat energy to the air, thereby preventing further cooling. Note that this means that, at dew point, the wet bulb temperature and the actual air temperature are the same. 21. The only condition required for condensation to occur is relative humidity of 100% False - condensation nuclei also must be present. Without them, the relative humidity would have to reach about 400% before condensation would occur. It is also worth mentioning that certain types of condensation nuclei can cause condensation to occur when the relative humidity is less than 100%. 22. A hair hygrometer is used to measure absolute humidity False - it is used to measure relative humidity. 23. Stable air is likely to produce precipitation False - stable air is defined as air that resists vertical motion - either upward or downward. Therefore, it is unstable air that is associated with storms because unstable air can rise and cool, thereby causing condensation and precipitation to occur. 24. The expansion of a gas causes it to cool True - heat energy is used for the "work" involved in expansion, and this causes the temperature to drop. This constitutes an adiabatic temperature change, i.e., a change in temperature that results when heat energy is used to move molecules apart; it is not caused the addition or subtraction of total energy from the air. In other words, when an adiabatic temperature change occurs, no energy is added to, or subtracted from, the air. Rather, the energy that is already present in the air is used to perform work, and this causes the temperature to change. Specifically, expansion causes cooling, and compression causes heating. 25. The dry adiabatic lapse rate is less than the moist adiabatic lapse rate, i.e., the DAR causes the air to cool at a slower rate than the MAR. False - it is greater than the wet adiabatic lapse rate because it does not involve condensation. When condensation occurs, heat energy is released to the atmosphere, thereby heating the atmosphere. This process slows down the rate at which air temperature decreases as it rises to higher altitudes. 26. As air rises, its relative humidity increases True - as air rises, it is cooled and this causes its relative humidity to increase. This results in cloud formation. Remember, the potential vapor pressure of cool air is less than that of warm air. 27. Frost forms when dew freezes False - it forms by sublimation, i.e., the gaseous water in the air goes straight to a solid form at the surface without passing through a liquid phase first. 28. Advection fog is caused by a vertical rise of air into cooler portions of the atmosphere. False - the word advection refers to the horizontal motion of air. Advection fogs form when moist, mild-temperature, ocean air flows over a cooler land surface causing condensation to occur. 29. Snow is formed when water droplets freeze False - it is formed by sublimation, which does not involve a liquid state. 30. In the Huntsville area, orographic uplift is the main cause of precipitation during the summer. False! - Orographic uplift is defined as uplift caused by the movement of air over a mountain range. Because there are no mountains in the area, this can't be the answer. In the Huntsville, area, the main cause of summertime rainfall is convectional uplift that occurs when the air is heated by the hot surface and moisture evaporates into the air. Both processes lead to convectional uplift. 31. Within a given air mass, humidity and temperature can vary substantially from one region to another at the same elevation False By definition, an air mass has similar physical properties (i.e., temperature, moisture content) throughout an area or region. 32. Source regions typically occur along coasts False Source regions are typically include two environments those over land (continental) and those over the oceans (maritime). 33. A cP air mass is cold and dry True 34. mP Pacific air masses originate over Asia as cP air masses and become unstable as they move over the ocean. True 35. Air circulation around the Icelandic Low can send moist air (and stormy conditions) into the Northeastern U.S. even though this area lies in the Westerlies. True 36. The greater the speed of motion of an air mass, the slower its rate of modification. False The faster the air mass moves, the faster it will enter an environment having different physical characteristics (i.e., temperature and moisture content), and thus, the quicker it will change from its original characteristics to inherit the new characteristics. 37. An air mass is a frontal system False! An air mass does not a frontal system make! It is the boundary between two air masses that have unlike characteristics that defines a frontal boundary. 38. Fronts are named on the basis of the air mass being overtaken False they are named on the basis of the air mass that overtakes. 39. Cold fronts have steeper frontal slope than warm fronts True 40. Weather is usually more intense along a warm front than along a cold front. False 41. After the passage of a warm front, the temperatures increase True 42. And occluded front involves only two air masses False An occluded front if formed by the interaction of three air masses (warm, cold and cool; or, cold, warm and cold). 43. Which pair of air masses would produce the most rapid uplift of air along a frontal boundary based on differences in temperature and moisture? cP and mT these two air masses have the most contrast in temperature and moisture properties. 44. Just prior to, and during, the passage of a cold front, the pressure drops True 45. As a front approaches, the clouds become lower and usually thicker True 46. The presence of mountains can lead to the formation of a stationary front True 47. Isobars are lines that connect points of equal temperature False! Isobars are lines that connect points of equal PRESSURE! 48. In an anticyclone, air flows downward and outward True 49. Why would a cloud most likely have a flat bottom? e) Because it is higher than the condensation level. 50. Cyclones rotate clockwise in the southern hemisphere True ... View Full Document

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