39 The Salinity of Ocean Waters
If the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity.
One of these is the subtraction of water from the ocean by means of evaporation - conversion of liquid water to water vapor. In this manner the salinity is increased, since the salts stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind.
The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.
Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the formation and melting of sea ice. When sea water is frozen, the dissolved materials are left behind. In this manner, sea water directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water. In the Weddell Sea Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.
海水盐度
如果我们分析海水的盐度，会发现地区间只有轻微的变化，然而有些小的变化是重要的。
导致海洋的盐度变化的基本过程有三个，其中之一是通过蒸发的方式即把液态水转化为水蒸 气来减少海洋中的水分。 这样由于盐留了下来，所以盐度增大。 当然，如果这种方式走向 极端，将会余下白色的盐晶体。 与蒸发相反的是降水，如降雨，由此水被加入海中，海水 被稀释，从而盐度降低。 这种情形会发生在大量降雨的地区，或江河入海岸处。 因此，盐 度通过蒸发减少水分而上升或通过降水或径流增加淡水成分而下降。 一般来说，在阳光很 强烈的热带地区，海水的盐度略高于世界上其它没有热带那样多的蒸发的地区。 同理，在 江河稀释海水的海岸地带，海水盐度略低于其它海区。 第三个可以变更盐度的过程与海洋 中冰的形成和融化有关。 海水冻结时，溶于其中的物质被留了下来。 这样，在新形成的海 水冰面的正下方的海水比在冰形成之前有更高的盐度。 当然，当冰融化的时候，会降低周 围水中的盐度。 在南极洲边缘的威德尔海中，结冰过程增加低温海水的盐度，从而形成了 浓度最大的海水。 这些大密度的海水下沉，可以在世界海洋的深水域发现。
40 Cohesion-tension Theory
Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plants acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of water in plants have been based on root pressure, a push on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are a [1] [2] 下一页 |
北京 
知识摘要
:点击留言