![]() ![]() The evaporation rate will therefore depend on the humidity gradient between this saturated microlayer and the planetary boundary layer as a whole. In the first few millimetres above the ocean surface the air is in equilibrium with the underlying water surface: it is saturated with respect to water vapour. The determination of E is more difficult. It depends slightly on temperature, but for our purposes can be taken as Lv = 2.5 x 106 Jkg-1. The latter is the energy required to vaporize one kilogramme of water. Where E is the evaporation (in kgm 2s 1) and L v is the latent heat of vaporization. The latent heat, Q E (Wm-2), added to the atmosphere by the ocean is In §2.11.1 we will see that hurricanes are the most dramatic consequences of this tropical latent heat excess, but Chapter 5 will show that this tropical energy release can significantly influence the climate of the extratropics as well. Tropical weather systems can thus be much more energetic, because of the greater potential source of latent heat. Air in the tropics, therefore, can contain much more vapour than air in polar latitudes. At temperatures near 0☌ only a few grammes of water vapour, per kilogramme of air, need to be present in the atmosphere before saturation is reached, while at 30☌, 30 grammes are required. 2.5, in terms of the saturated specific humidity. The variation with temperature of the saturation level is shown in Fig. We have already implicitly assumed knowledge of this property in discussing condensation air containing more water vapour than its saturated level tends to form clouds, with the water vapour condensing to form droplets (the climatically important process of cloud formation, including the influence of the ocean, will be discussed in more detail in §2.8.3, §4.4, and Chapters 5 and 7). This means that the same number of water molecules enter the body of water from the air, per second, as are evaporated from the surface. A parcel of air said to be saturated with respect to water vapour is one where its constituent water vapour is in equilibrium with an underlying flat surface of water. Ītmosphere - possible saturation with respect to water vapour - makes this form of heat exchange highly temperature-dependent. Variation of saturated specific humidity with temperature at a pressure of 1000 mb. Latent heat transfer is therefore an important means of re-cycling energy through the ocean-atmosphere system. Such additions of heat can eventually assist in the creation of new pressure gradients within the atmosphere, thus driving large-scale atmospheric motion. This internal source of heat adds to a cloud's buoyancy, allowing it to penetrate higher into the troposphere. When the water molecules condense to form water droplets, usually in clouds, this energy is released to heat the surrounding air. When water is evaporated from the ocean surface energy is supplied to the molecules to free them from the strong inter-molecular bonds within liquid water. The most important of these is latent heat transfer. Other physical mechanisms do, however, also contribute to the net heat flux. Radiation dominates the exchange of heat between the atmosphere and ocean. ![]()
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