Turbulent flow over a boundary is a complex phenomenon for which there is no complete theory. Nevertheless, much experimental data has been collected on flows over solid surfaces, both in the laboratory and in nature, so that from an engineering perspective, the situation is well understood. The force exerted on a surface varies with the roughness of that surface and approximately with the square of the wind speed at a fixed height above it. A wind of 10 meters per second (about 20 knots, or 22 miles per hour) measured at a height of 10 meters will produce a force of some 30 tons per square kilometer on a field of mown grass, or of about 70 tons per square kilometer on a ripe wheat field. On a really smooth surface, such as glass, the force is only about 10 tons per square kilometer.

When wind blows over water, it is more complicated. The roughness of the water is not a given characteristic of the surface but depends on the wind itself. Not only that, the elements that constitute the roughness, the waves, themselves are mostly in the direction of the wind. Recent evidence indicates that a large portion of the momentum transferred from the air into the water goes into waves rather than directly into making water currents; only as the waves break or otherwise lose energy does their momentum become available to generate currents. Waves carry a substantial amount of both energy and momentum (about as much as is carried by the wind in a layer about one wavelength thick), and so the wave-generation process is far from negligible.

A violently wavy surface belies its appearance by acting, as far as the wind is concerned, as though it were very smooth. At a wind of 10 meters per second, the force on a wavy surface is much less than the force would be over mown grass and scarcely more than over glass; in light winds (2 or 3 meters per second) the force on a wavy surface is even less than it would be on glass. The waves’ motion seems to modify the airflow so that air slips over the surface more freely than if it were smooth. This is not the case at higher wind speeds (above about 5 meters per second), but the force remains quite low relative to other surfaces.

Unfortunately, there are no direct observations under conditions when high winds, greater than about 12 meters per second, have had time and fetch (the distance over water) enough to raise substantial waves. A few indirect studies, however, suggest that the water’s apparent roughness may increase under high wind conditions, so that the force on the surface increases more rapidly than the square of the wind speed.

If the force increases at least as the square of the wind speed, high-wind conditions will produce effects far more important than their frequency of occurrence would suggest, as five hours of 60-knot storm winds will put more momentum into the water than a week of 10-knot breezes. If it should be shown that, for high winds, the force on the surface increases even more than the square of the wind speed, then the transfer of momentum to the ocean will turn out to be dominated by the occasional storm rather than by the long-term average winds.

1. According to the passage, several hours of storm winds (60 miles per hour) over water would:

(A) be similar to the force exerted by light winds for several hours over glass.
(B) create a surface roughness which reduces the force exerted by the high wind.
(C) be more significant in increasing the momentum of the water than constant light winds over a period of a few days.
(D) create a force not greater than six times the force of a 10-mile-per-hour wind.
(E) directly affect water current.

Type: Main Idea
(C) In the last paragraph, we are told that “several hours of 60-knot storm winds will put more momentum into the water than a week of 10-knot breezes.” In this question we are asked about winds of 60 miles per hour. How do these winds compare? If we look back to the first paragraph, we see that 20 knots is equal to 22 miles per hour. Therefore, these are roughly equivalent measures of comparison, and we can relate the information in the last paragraph to our question, choosing (C), that the high winds contribute more momentum to the ocean than do the lighter winds over a longer period.
Choice (A) is not correct, since we are told in the fourth paragraph that at 10 meters per second (equivalent to 22 miles per hour), the surface force is slightly more that the force over glass. Since this is a much greater wind, we would expect the force to be even greater, so choice (A) is not correct. Choice (B) suggests that the roughness created by the waves would actually decrease the force of the wind. According to paragraph three, this is true, though only at lower wind speeds, not the high speeds described in this question. Choice (D) compares the relative forces of two winds. Though you might guess that a wind that is six times faster might exert a force six times greater, we are told in the passage that the force actually appears to increase proportionally with the square of the wind speed. Thus, the force is closer to thirty-six times greater for the higher wind.

2. The main purpose of the passage is to discuss:

(A) oceanic momentum and current.
(B) turbulent flow of wind over water.
(C) wind blowing over water as related to causing tidal flow.
(D) the significance of high wind conditions on ocean momentum.
(E) experiments in wind force.

Type: Definition of a word of phrase
(B) The main idea of this passage is the turbulent flow of wind over water. The passage introduces the topic in the first paragraph, by describing the basic physics of turbulent flow over surfaces. The next four paragraphs then expand this concept to water, and illustrate the complications in understanding the force of wind on the water surface. Choices (A) and (C) are not correct, in that these concepts are only briefly mentioned in passing. High winds, like in choice (D), are mentioned frequently, though not specifically for their effect on the momentum of the ocean. Indeed, the whole passage is applicable to water in general, not only the ocean. Choice (E) is not correct. Although experiments in wind force are described here, it is only to bolster the author’s argument, not as the main subject of the passage.

3. The author’s suggestion that the transfer of momentum to the ocean is dominated by the occasional storm would be most weakened if which of the following were true:

(A) Air momentum is converted directly into increased ocean current.
(B) High speed winds slip over waves as easily as low speed winds.
(C) Waves do not move in the direction of wind.
(D) The force exerted on a wheat field was the same as on mown grass.
(E) The force of wind under normal conditions increased as the square of wind speed.

Type: Inference
(B) The suggestion that the occasional storm will make a large contribution to momentum relies on the main argument that high winds can contribute more to the momentum of water than light winds (see paragraphs two and four, specifically). High winds can increase the intensity of waves, or increase the apparent roughness of the surface, which allows the exertion of more force. With the increased roughness, the force increases even more than the square of the wind speed (which is the usual estimation of the force exerted by wind on a surface). Choices (A) and (D) are not relevant to the statement, and thus would not weaken it if they were true. Choice (C), if true, would affect the transfer of momentum from both high and low winds to water, so this is not a good choice. Choice (E) actually is true, and does not weaken the argument. Thus (B) is correct.