Fog and Cloud When air is cooled below its dew point, then the water vapour within air starts to condense out into water droplets. These water droplets are called either fog or cloud, depending on the heights and process of formation of water droplets. Fog forms through cooling of the air by conductive process, where as cloud normally forms by adiabatic cooling of the rising air. Again fog forms near earth surface, where as cloud forms at higher altitudes. Fog Fog forms when the difference between temperature and dew point is generally less than 2.5 °C, with relative humidity of about 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. So fog may form, (1) by cooling air to its dew point, or (2) by adding moisture to air, near the surface. So for fog formation, it is better to have low dew point and high humidity. At hygrometer there are two thermometers, wet bulb thermometer and dry bulb thermometer. The difference between these two thermometer’s readings is called “depression of wet bulb below the dry bulb”. From page….., it is understood, a small depression of the wet bulb below the dry indicates the higher humidity, whereas a large depression of the wet bulb below the dry indicates the lower dew point, both are favorable for fog formation. For that reason it is called, small and large depressions of the wet bulb, at hygrometer below the dry bulb indicate the formation of fog. Initially fog forms over land with shallow depth and air turbulence brings this fog to upwards & upper air to downwards. Then this downwards air becomes fog, when it comes in touch with cold surface. This way an up & down cycle is going on and making a deeper and stronger fog layer. This air turbulence has to be smaller; otherwise it will throw the fog to more upwards, then it will be no more fog, but a cloud layer at higher altitude. This way turbulence helps to continue fog life. So fog usually lies in relatively shallow layer and turbulence is an essential condition for maintaining the fog. Fog is a visibility hazard. While navigating at fog, we should be more active. Range of visibility is to be considered with supreme importance. When coasting, range of visibility is determined by the distances of shore objects, at the time of first sighted or last sighted, with the help of admiralty charts and radar. In the open sea, visibility can be calculated by noting the radar range of a ship, when that ship is first sighted visually from disappear position, at foggy weather. Visibility-meter is also a good instrument to measure the distance of visibility, as well as the presence of fog at sea. 1 Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury Visibility at Bay of Bengal is normally good. Here fog is normally rare. The average frequency of fog in ship observations is less than 2% over the whole area. Some parts are free of fog in all months but a few patches occur near the cost of Orissa in June. Here fog is mainly confined to the dawn period and extends over inshore waters for an hour or so. Sometimes, during early winter-morning, dense fogs are observed for 3 / 4 hours, at the coast of Bay of Bengal and Karnafuly River. There are mainly 2 types of fog, as below – 1. Radiation fog 2. Advection fog. There are some less common types of fog also. Radiation fog is formed by the cooling of land surface, during night time, by thermal radiation in calm conditions with clear sky. The cold ground produces condensation in the nearby air by conduction, which produces the water droplets for radiation fog. In perfect calm, this fog layer can be less than a meter deep but turbulence can promote a thicker layer of 150 meters. It does not last long after sunrise. Its maximum time is about half hour after sunrise, as air temperature goes to its lowest position at this time. It is a relatively shallow fog. It may be dense enough to hide the entire sky or may conceal only part of the sky. Favorable conditions for radiation fog are clear sky, with little or no wind. Radiation Fog is disappeared due to below reasons, 1. The spreading of a cloud sheet over fog reduces the cooling, which disappears fog. 2. Heat from the ground lifting the fog into low cloud (Stratus). 3. Increasing of wind causes fog dissipation. 2 Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury 4. In mature fog, the settling of droplets to the surface contributes significantly to the fog dissipation. 5. Except in mid-winter at higher latitudes, solar radiation can penetrate to the ground in places of thinner fog, heats the ground and evaporates the Fog. Radiation fog also called “Ground Fog”, as it normally forms over land. There is a large diurnal range of air temperature over land, but over sea, it is very less. So giving off heat over land is much quicker than sea, accordingly the air over land becomes cold, which is much faster than air over sea. On the other hand, the sea surface can not be so cool to form this fog. For that reason, radiation fog forms over land but not over sea and can drag towards costal sea areas. Radiation fog, which forms over land, can drift towards rivers or other coastal areas. The fog at Karnafuly River, at winter time is the good example of this type of fog. Due to same reason, radiation fog may be encountered in UK coastal waters also. Advection fog forms, when moist air moves over colder land or water surfaces. When this moist air comes in contact with colder surface and becomes cool below its dew point, then the excess water vapour condense into small droplets of water, on minute particles of dust or salt, which is moving through air. This way advection fog is developed. It is most common along coastal areas but often develops deep in continental areas. But it is mainly formed at sea, so it is called “Sea Fog” also. Advection fog deepens as wind speed increases up to about 15 knots. Wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds. Advection fog is very common on the Grand Banks of New foundland. Here the warm and moist westerlies first blow over the warm Gulf Stream and gets hotter and more moisturized, then cross over the cold Labrador Current and forms advection fog. These fog banks are occurring along the coasts of Newfoundland & Labrador, with quite deep height. It often continues for days at a time and hampering shipping activities. 3 Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury This advection fog is also available at east cost of Japan (here moist westerlies first blow over warm Kuro Shio, then over cold Oya Shio), south coast of UK, west coast of the United States, central and eastern United States (with extension as far north as the Great Lakes). Forecasting of advection fog can be done by plotting on a graph paper. Here sea temperatures and dew point temperatures of fixed intervals of time are needed to plot on this graph paper, where temperature is the vertical line and local time is the base line. Then we will get two graph lines of dew point and sea temperature, if we extent these two lines (in figure which is given as dotted line), then we will get the forecasting time of advection fog, (which is 1200 hrs local time, at below graph). So the knowledge of sea surface temperature is very important in forecasting of advection fog (sea fog). 4 Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury 5 Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. Haz For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury e is an atmospheric condition, where dust, smoke and other dry particles obscure the visibility. It often occurs, when dust and smoke particles accumulate in relatively dry air. When weather conditions block the dispersal of smoke and other pollutants, then they concentrate and form a low-hanging blanket of foggy condition, which impairs visibility. Industrial pollution also can produces dense haze. Mist is formed, when small droplets of waters are suspended at air. It can occur as part of natural weather or volcanic activity. The only difference between mist and fog is “range of visibility”. This phenomenon is called fog, if the visibility is one kilometer or less, otherwise it is known as mist. So mist has better “range of visibility” then fog.