Physics 137 - Introduction to the Atmosphere and Weather                                            Homework Key - Assignment #11

Revised  11/02/09

11-QT2. How would the general circulation of air appear in summer and winter if the earth were tilted on its axis at an angle of 45° instead of 23½°?

The ITCZ would shift further north in the northern summer and further south in the northern winter. Similarly the subtropical highs and polar fronts would shift further from the equator during the summer and closer to the equator in the winter in both hemispheres than is now the case.

11-QT4. Explain why icebergs tend to move at right angles to the direction of the wind.

Since the mass of an iceberg is about 90% beneath and only 10% above the ocean surface, ocean currents are far more important than winds in determining the course of an iceberg. Hence, since the Coriolis force causes the Ekman spiral which in turn causes the average current direction to be at right angles to the direction of the wind (in the clockwise sense as seen from above), the direction taken by an iceberg is also at right angles to the wind.

11-QT8. Explain why the surface water temperature along the northern California coast is warmer in winter than it is in summer.

The prevailing wind along the northern California coast tends to be southward, parallel to the shore, during summer, but nearly onshore, with only a weak north component during winter. Since upwelling is strongest when the wind is parallel to the coast, the strongest upwelling and therefore the coolest surface water temperatures occur during the summer.

11-QT10. The Coriolis force deflects moving water to the right in the Northern Hemisphere and to the left in the southern hemisphere. Why, then, does upwelling tend to occur along the western margin of continents in both hemispheres?

Strong subtropical highs cause prevailing winds to be northerly along the western boundaries of northern continents and southerly along the western boundaries of southern continents. In both cases the Coriolis force, by means of the Ekman spiral, causes a net westward migration of coastal waters and associated upwelling.

11-PE1. Locate the following cities on a world map. Then, based on the general circulation of surface winds, predict the prevailing wind for each one during July and January. (a) Nashville, Tennessee, (b) Oklahoma City, Oklahoma, (c) Melbourne, Australia, (d) London, England, (e) Paris France, (f) Reykjavik, Iceland, (g) Fairbanks, Alaska, (h) Seattle, Washington.

The prevailing winds for July and January respectively, based on the maps of Figs.11.3, are approximately (a) S, NNW; (b) S, NW; (c) NW, SE; (d) NW, SW; (e) NW, SW; (f) SW, S; (g) W, SE; (h) N, W.

11-PE2. Suppose a balloon is released over southern Baja California on the morning of March 10,1998 (Refer to Fig. 11.11, p. 296.) If the balloon rises to the 300-mb level and is "caught" by the jet stream, calculate about how long it would take the balloon to exit the United States off the coast of Georgia.

The distance from southern Baja California to the coast of Georgia is about 2000 miles. The average speed of the subtropical jet along a path from southern Baja to Georgia on March 10, 1998, was about 100+ knots, or 115+ miles per hour. Hence the required time for a balloon to travel the Baja-Georgia route would be about t = distance/speed @ (2000 mi)/(115+ mi/hr) @ 17 hr.