CURRENT EFFECTS 51 



various sizes, smaller or larger, will be running across the older swells 

 in one direction or another if the wind is strong enough at the time to 

 produce a wave pattern (fig. 13). In any case, when two separate 

 trains of waves are present, travelling in different directions, the 

 result of the interference will be a series of peaks rather than ridges. 

 (See Sverdrup, Johnson, and Fleming, 1942, p. 531, fig. 133.) 



The most obvious cause for these irregularities is that whenever 

 the wind changes in direction, or when a new wind springs up from a 

 new direction after a calm, a cross sea develops on top of whatever 

 :>lder waves may already be in existence. But irregularities also de- 

 velop from the immediate effects of a freshening wind, because the 

 latter is never steady, but conies in gusts that vary so widely in strength 

 (also in direction) that every fresh gust sets up a new set of short- 

 crested wavelets on the backs of the older waves. This is true, even 

 in the Trade Wind Belts where the wind is more nearly constant in 

 strength and in direction than it is anywhere else. And it is largely 

 because the new wave systems set up by a dying wind are progressively 

 smaller and smaller that old swells are so much more even in contour 

 than storm waves are (p. 45). 



During storms, the contour is still further complicated by the fact 

 that peaks occasionally shoot up to great heights when two waves come 

 together from different directions, as illustrated on a small scale in 

 figure 17. Reports have it, in fact, that the most tumultuous and 

 dangerous seas of all are those that develop in the area of calm air at 

 the "eye" of a tropical hurricane as a result of the interference between 

 the wave trains that meet there. Nautical periodicals contain repeated 

 accounts of the damage done even to well-found ships, steam as well 

 as sail, by the masses of water that may fall on board when such seas 

 break ; of decks swept clean of boats and houses, of bulwarks carried 

 away, and of hatches stove in by the mere weight of water. Many a 

 ship has been lost with all hands under such circumstances. 



If a strong head wind blows up suddenly, or if the wind suddenly 

 shifts while still blowing strongly, the waves — even very small ones — 

 often break backwards, so that their crests are driven over into the 

 troughs behind. Similarly, a short steep sea, and often a very tumul- 

 tuous and irregularly breaking one, tends to develop if waves are run- 

 ning against a strong tide or current, since the effect of the latter in 

 such cases is to increase the heights of the oncoming waves, but at the 

 same time to decrease their lengths and thus to render them steeper, 

 as is described more fully on page 53. 



THE EFFECTS OF CURRENTS ON THE DIMENSIONS OF WAVES 



The preceding pages have taken no account of the possibility that 

 the surface stratum of the sea may be moving in one direction or 



226794 O - 53 - 5 



