Saturated Vapour Pressure (kPa) March

The average energy of the molecules in a liquid such as water is governed by temperature. The higher the temperature, the higher the average energy. Some molecules have higher energies than the average, and if they are on the surface of the liquid, can move fast enough to escape the inter-molecular attractive forces holding the liquid together. They then evaporate from the surface of the liquid. This process can continue until the space above the liquid is saturated with water molecules. Equilibrium is then reached with as many molecules leaving the surface of water as are re-entering it. At this point the pressure exerted by that vapour is called the saturated vapour pressure. The symbol used for saturated vapour pressure is ea and the unit of pressure is kPa. Since the molecular kinetic energy is greater at higher temperature, more molecules can escape the surface and the saturated vapour pressure is correspondingly higher. This relationship is illustrated in the shaded box. Daily values of saturated vapour pressure were computed from the 50 year daily time series of temperature generated for South Africa at each one of 429 700 raster points at one arc minute interval, using techniques developed by Schulze and Maharaj (2004).

诸如水这类液体中分子的平均能量由温度决定。温度越高，分子的平均能量便越高。部分分子的能量高于平均水平，若这些分子位于液体表面，则可获得足够的运动速度，挣脱维系液体的分子间吸引力，进而从液体表面蒸发。该过程会持续至液体上方空间被水分子完全饱和，此时达到平衡状态：脱离水面的分子数量与重新回到水面的分子数量相等。此时该蒸气所产生的压强被称为饱和水汽压（saturated vapour pressure），其符号为$e_a$，压强单位为千帕（kPa）。
由于温度越高，分子动能越大，能够挣脱表面束缚的分子数量越多，饱和水汽压也会相应升高。这一关系已在阴影框中予以阐释。
研究人员采用Schulze与Maharaj（2004）提出的技术方法，基于在南非境内429700个以1角分为空间间隔的栅格点上生成的50年逐日温度时间序列，计算得到了逐日饱和水汽压数据集。