Unit 20MarineWeather.doc

Unit 20 MarineWeather 航海气象 Fogs and Other Phenomena Affecting Visibility雾及其它影响能见度的现象1. Advection fog平流雾 This is the most widespread type likely to be encountered at sea and is caused by relatively warm air being cooled by flowing over a cooler sea surface. The latter will be below the dew-point of the air and normally the wind speed will be between 4 and 16 knots ( between force 2 and 4 on the Beaufort Scale). There are only certain localities where the cold Labrador current causes a decrease in sea temperature. The warm ,moist southerly air stream flowing over this is cooled below its dew-point to form advection, or sea fog.这是海上最常见的一种雾，其成因是热气团遭遇较冷海面。

接下来温度将低于该气团的露点温度一般情况下风速为4到6节(蒲福风力2 到4 之间)这种情况仅在某些地方比较盛行其中之一是新大陆的沿海，拉布拉多冷流造成海域温度下降温暖潮湿的南部气团经过该海域是降至露点温度以下，形成水平对流或者是海雾The English Channel is often affected by advection fog when southwesterly winds reach the British Isles from the Azores in spring and early summer.春天和初夏季节，当西南风从Azores到达British Isles时，英吉利海峡经常受到平流的影响In ocean regions, well away from shallows and coastal waters, the sea surface temperature changes very little through solar heating or night radiation. Generally the daily change in sea surface temperature is less than 0.5 .在远离海岸，没有浅水的广阔水域，太阳照射或夜间辐射对海水表层温度的影响很小。

一般海水表层温度的日变化量不高于0.5摄氏度It is possible to estimate the likelihood of the formation of fog from observations of air temperatures, wind direction and other weather signs, plus a knowledge of sea temperatures to be expected on the course ahead. Admiralty Ocean Routeing Charts give information, for each month of the year on.通过对空气温度，风向及其他天气信号的观察再加上预期到达水域海面无额度的了解，就有可能估算到形成海雾的可能性大小英版海洋航行图给出了每月有关以下各项的信息：Mean sea temperatures Mean dew-point temperaturesPercentage frequency of fog (visibility – less than half a mile )Percentage frequency of low visibility (less than 5 miles )Mean air temperaturesMean barometric pressures平均海面温度平均露点温度海雾发生概率（能见度小于1英里）能见度不良发生概率（小于5英里）平均空气温度平均气压2 Frontal or missing fog锋前雾或混合雾This may occur along the boundary when two widely differing air masses meet. Usually associated with either a warm front or a warm occlusion when cold air meet warm moist air ; hence it is normally experienced in temperate or high latitudes. It is caused by the evaporation of relatively warm rain or drizzle which in turn cools the air through which it falls.这种情形可能在两个性质迥异的气团相遇的交界处发生。

当冷空气团遇到暖空气团往往会与冷锋或锢囚锋联系在一起因此，这种情况一般发生在中高纬度相对较暖的雨，毛毛雨的蒸发导致它的发生，反过来降雨也冷却了空气3 Radiation fog 辐射雾 This forms over land, most frequently during autumn and winter over low-lying land, especially if it is damp and marshy and in valleys on quiet nights with clear skies. Under these conditions the land loses heat by radiation and cools the air close to the ground, possibly to below its dew-point. If there is a gentle breeze bowing, ( up to 5 knots) this will cause turbulent mixing but only close to the surface and condensation in the form of fog will take place. A stronger wind will cause the cooling to be diffused through a greater depth of air and dew-point will not be reached.这种雾形成与陆地，多发于秋冬季节的低地。

尤其是晴夜，潮湿泥泞的河谷流域具备这些条件的陆地因辐射而失去热量，使空气冷却，接近地面，可能会低于其露点温度如果再有一阵微风的吹拂，（达到5节），就会在地表形成气流的混合，形成雾的冷凝现象也会发生更强的风则会使降温作用扩散到更大的气团，将不会达到露点温度Since cold air is heaver than warm air ,it will tend to drain down in valleys. Although it never actually forms over the sea, it may drift from the land for several miles but seldom extends for more than 10 miles off-shore. Cloudy skies over-night will reduce the effect of the radiation from the land, or even re-radiate heat back to the surface and radiation fog will not occur under these conditions.由于冷气团重于暖气团，他将倾向于流向河谷。

虽然他并不形成于海上，但它可能从陆地漂移数英里很少伸展到离岸10英里以外夜晚多云的天空可以减缓陆地的辐射作用，或者说热量又被反射回地表这种情况下辐射雾便不会形成Radiation fog will be most dense around sunrise and normally disperses fairly rapidly as the land warms.辐射雾在黎明时候最为严重，当地表变暖时迅速消失4 Arctic sea smoke 北冰洋烟雾 This is a type of fog occurring close to the sea surface when the air is dry and cold-probably at least 9c below the sea surface temperature. Rapid evaporation takes place from the relatively warm sea surface into the colder air and condensation tales place, giving the effect of steam or smoke risking from the sea. 这是一种发生于近海面的雾，当空气干燥且温度低于海面至少9 摄氏度时有可能发生。

快速蒸发发生，从相对较暖的海洋表面蒸发至较冷的空气中，其间冷凝也会发生，造成海绵蒸汽，烟雾的效果It is most common in Arctic and Arctic and Antarctic waters and the Baltic but it can also occur off the eastern coasts of continents in winter e.g. off Newfoundland and over inland seas and lakes. This is one type of fog which may also be associated with strong winds since it requires a continual supply of cold air.这种现象多发于南北极海域和白令海，但在新大陆的东海岸，内陆的湖海也有发生这种无可能会与强风联系在一起，因为该雾形成需要冷空气的持续供应5.mist, dust and haze 薄雾，灰尘，烟雾 As stated above, visibility which is impaired but is more then 1000 metres is described as mist-when caused by water droplets and when the relative humidity is more than 95%. When caused by smoke or dust particles it is described as。