再论相对湿度对区别都市霾与雾(轻雾)的意义

在都市,霾的出现有重要的空气质量指示意义。而雾或轻雾的记录,有明确的天气指示意义,与特定的天气系统相联系。由于经济规模的迅速扩大和城市化进程的加快,都市霾现象或者是灰霾天气日趋严重,霾与雾的区分成为一个非常现实,又迫切需要解决的问题。在全国气象系统的台站观测中,区分霾与雾(轻雾)的判据比较混乱,缺乏可比性,东南沿海各省不成文规定的用相对湿度区分的标准普遍偏低,将大量霾记成了轻雾或雾。实际上近地层大气中每时每刻总是有霾存在的,而雾滴的存在是少见或罕见的;霾滴要想通过吸湿增长成为雾滴,必须有足够的过饱和度,能够越过过饱和驼峰才行,这在自然界并不容易。在非饱和条件下,不但非水溶性的霾不能转化成雾滴,既便是水溶性的霾粒子一般也不可能吸湿转化为雾滴。实测资料表明出现雾时,极端最小相对湿度是91%,在相对湿度低于90%的情况下,没有观测到雾。降温是达到饱和形成雾滴的最主要、最重要的物理过程,在自然界中的霾滴通过吸湿过程增长成雾滴几乎不可能。历史上我国各级气象部门从来不存在以相对湿度70%界定轻雾与霾的补充规定,由于理解的问题,将大量霾记成了轻雾。区分霾和雾,应该根据影响天气系统的变化,结合宏观特征的各种判据来确定。建议将相对湿度的阈值定为90%,作为区分轻雾(雾)与霾的辅助判据是合理的。

More Discussions on the Differences between Haze and Fog in City

The appearance of haze has important indication meaning to air quality,while the fog or light fog has specific weather indication relating to particular weather systems.With the rapid development of economy and urbanization,haze or haze weather become increasingly severe,so that,to distinguish haze and fog becomes a very realistic problem that need to be solved imminently.At the meteorological stations of China,the criterion to distinguish the haze and fog is relatively confusing and uncomparable.The widely adopted criterion of relative humidity to distinguish the haze and fog in many southeast coastal provinces is generally on the low side,leading to many haze weather events being recorded as light fog or fog.In the near surface atmosphere there always exists haze,while fog droplet hardly exists.Only under the enough supersaturation can the haze nuclei grow into fog droplet through moisture absorption,and such process is not easy to happen.Under non-saturation condition,it is impossible for even the soluble haze nuclei to grow into fog droplet not to speak of the insoluble ones.The observational data show that the minimum relative humidity for fog to occur is 91%,and there is no fog when the relative humidity is lower than 90%.The most important physical process to form fog droplets is cooling, down to the saturation point.It is almost impossible for haze nuclei to grow into fog droplet only through the natural moisture absorption.There is no such a supplement regulation ever saying that the relative humidity 70% shall be used to distinguish light fog and haze at any level of meteorological departments.Many haze weather events being recorded as light fog is purely due to misunderstanding.So it is necessary to take into consideration the change of weather system and other the macro-characteristics when distinguishing fog and haze,and it is reasonable to use the threshold value of relative humidity 90% as an assistant criterion to distinguish fog and haze.