Atmospheric circulation

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Answers to STUDY BREAK Questions

Essentials 5th

Chapter 7


1. How is weather different from climate?
Weather is the state of the atmosphere at a specific time and place, while climate is the long-term average of weather in an area.

2. Which is denser at the same temperature and pressure: humid air or dry air?
Curiously, humid air is less dense than dry air at the same temperature—because molecules of water vapor weigh less than the nitrogen and oxygen molecules than the water vapor it displaces.

3. How does air’s temperature change as it expands? As it is compressed?
Air becomes cooler when it expands, and warms as it is compressed. Air descending from high altitude warms as it is compressed by the higher atmospheric pressure near Earth’s surface.

4. Can more water vapor be held in warm air or cool air?
Warm air can hold more water vapor than cold air (at the same atmospheric pressure).

5. What happens when air containing water vapor rises?
Water vapor in rising, expanding, cooling air will often condense into clouds because the cooler air can no longer hold as much water vapor. If rising and cooling continue, the droplets may coalesce into raindrops or snowflakes.

6. What is meant by thermal equilibrium? Is Earth’s heat budget in balance?
Over long periods of time the total incoming heat (plus that from earthly sources) equals the total heat radiating into the cold of space; so Earth is in thermal equilibrium. Some variation is observable over shorter time spans -- the current episode of global warning is an example.

7. How does solar heating vary with latitude? With the seasons?
Near the poles light approaches the surface at a low angle, favoring reflection. At tropical latitudes, sunlight strikes at a more nearly vertical angle, which distributes the same amount of sunlight over a much smaller area. Tropical latitudes thus receive significantly more solar energy than the polar regions. As Earth revolves around the sun, the constant tilt of its rotational axis causes the Northern Hemisphere to lean toward the sun in June but away from it in December. The sun therefore appears higher in the sky in the summer but lower in winter.

8. What is a convection current? Can you think of any examples of convection currents around your house?
A convection current is a single closed-flow circuit of rising warm material and falling cool material.

9. Describe the Coriolis effect to the next person you meet. Go ahead—give it a try!
Understanding Coriolis effect depends on communicating the idea that objects “inherit” their eastward momentum from their originating latitude. They bring that eastward movement with them as they move north of south. In a sense, the apparent motion of Coriolis effect is the difference in expected east-west position.

10. If all of Earth rotates eastward at 15º an hour, why does the eastward speed of locations on Earth vary with their latitude?
The angular velocity of Earth is 15° an hour (think of the pie slice of Figure 7.10). But the linear velocity depends on latitude. See how much farther Quito must travel to make it around Earth in one day in that Figure?

11. How many atmospheric circulation cells exist in each hemisphere?
Three air circulation cells exist in each hemisphere.

12. How does the Coriolis effect influence atmospheric circulation?
Instead of continuing all the way from equator to pole in a continuous loop in each hemisphere, air rising from the equatorial region moves poleward and is gradually deflected eastward; that is, it turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This eastward deflection is caused by the Coriolis effect.

13. What happens to air flow between circulation cells? (Hint: What causes Earth’s desert climates?)
The great deserts of both hemispheres, dry bands centered around 30° latitude, mark the intersection of the Hadley and Ferrel cells. Air falls toward Earth’s surface in these areas, causing compressional heating.

14. Draw the general pattern for the atmospheric circulation of the Northern Hemisphere (without looking at Figure 7.12). Now locate these features: the doldrums (or ITCZ), the horse latitudes, the prevailing westerlies, and the trade winds.
Do your drawing without looking at the Figure, and then check your accuracy.

15. What’s a monsoon? Do we experience monsoons in the continental United States?
Heating of the great landmass of Asia draws vast quantities of warm, moist air from the Indian Ocean. Winds from the south drive this moisture toward Asia, where it rises and condenses to produce a months-long deluge (a monsoon). Monsoons occur in North America as warming and rising air over the South and West draws humid air and thunderstorms from the Gulf of Mexico.

16. How do sea breezes and land breezes form?
Warm land transfer heat to the air, which expands and rises, creating a zone of low atmospheric pressure. Cooler air from over the sea then moves toward land to form a sea breeze. The situation reverses after sunset, with land losing heat to space and falling rapidly in temperature.

17. What are the two kinds of large storms? How do they differ? How are they similar?
Large storms are either tropical cyclones or extratropical cyclones. Tropical cyclones (sometimes called hurricanes or typhoons) form in a single tropical air mass. Extratropical cyclones (named because they form outside the tropics) are the frontal storms familiar to winter residents of mid-latitude continents. Extratropical cyclones form at a front between two air masses.

18. What is an air mass? How do air masses form?
An air mass is a large body of air with nearly uniform temperature, humidity, and therefore density throughout. Air pausing over water or land will tend to take on the characteristics of the surface below.

19. What causes an extratropical cyclone? How are air masses involved?
Energy is required to mix air masses. Since that energy is not always available, a dense air mass may slide beneath a lighter air mass, lifting the lighter one and causing its air to expand and cool. Water vapor in the rising air may condense. All of these effects contribute to turbulence at the boundaries of the air masses and can lead to the formation of an extratropical cyclone.

20. What’s a weather front? Is it typical of tropical or extra-tropical cyclones?
The boundary between air masses of different density is called a front. Fronts are typical of extratropical cyclones.

21. Why do extratropical cyclones rotate counterclockwise in the Northern Hemisphere?
This apparent anomaly is caused by the Coriolis deflection of winds approaching the center of a low-pressure area from great distances. In the Northern Hemisphere there is rightward deflection of the approaching air. The edge spin given by this approaching air causes the storm to spin counterclockwise in the Northern Hemisphere.

22. What causes the greatest loss of life and property when a tropical cyclone reaches land?
Three aspects of a tropical cyclone can cause property damage and loss of life: wind, rain, and storm surge. Of these, storm surge is the most devastating.

23. What things were unique about the 2005 Atlantic hurricane season?
A record 27 tropical cyclones formed, and 15 of these became hurricanes. Three hurricanes reached Category 5 strength. One of these was hurricane Katrina, cause of the costliest natural disaster to befall the United States.

24. Of a tropical cyclone’s three most dangerous properties (wind, rain, storm surge), which of Katrina’s characteristics caused the greatest loss of life? Of property?
Storm surge caused the greatest loss of life and property when Katrina struck the Gulf coast. The storm surge in Bay St. Louis was 10.4 meters, or 34 feet, high.
25. How do large tropical cyclones affect the human-built coastal zone? The natural coastal zone?
The effects of a large tropical cyclone on a built-up area were clearly seen in the city of New Orleans. Much of the city was built on land below sea level, and when the levees failed because of storm surge stress, much of the city flooded. The natural coast was also affected. Salt water driven inland by storm surge killed marsh grasses along the Louisiana and Mississippi coasts. In Louisiana alone, more than 260 square kilometers (100 square miles) of wetlands were torn apart, their ability to support a rich variety of species essentially destroyed.

26. Is there a proven link between global warming and the apparent growing intensity of Atlantic hurricanes?
Althought there is a strong suggestion of a relationship between the growing greenhouse and intensification of tropical cyclones, researchers are unable to link the two with certainty

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