FORECASTING FOGS

Dominion, Volume 29, Issue 226, 20 June 1936, Page 11

FORECASTING FOGS

Meteorological Service To

Airliners

NETWORK OF STATIONS

“Generally it is possible to tell when conditions are favourable for fog, but it is so local a phenomenon, especially on the coast's of this country, that the only way to deal with it satisfactorily is by means of reports at frequent intervals from a close network of reporting stations,” Dr. E. Eidson, Government Meteorologist, said in an interview with "The Dominion,’’ in which he discussed fog development in New Zealand, a weather vagary which has been responsible for some delays on the airline between Palmerston North and Dunedin, and for which the installation of radio beacons has been suggested by Mr. Harold Gatty.

Dr. Kidson defined fog as a surface phenomenon, which had to be distinguished from low cloud. It was produced through the air being cooled by the land or ocean surface beneath it to below its dew point. There was always a certain amount of water vapour present in the atmosphere, but the amount it could hold increased rapidly as the temperature rose. The dew point was the temperature at which the amount of water vapour present was enough to saturate it or, in other words, was as much as it could hold. Cooling bexow the dew point caused some of the water vapour to condense out and become visible as cloud or fog.

Types of Fog.

There were various ways in which the cooling could be brought about, and consequemiy several different types of fog. "The commonest is what is called radiation fog,” he said. “At night time, when it is clear, the ground radiates heat freely out into space, and the surface becomes cold. The cold ground in turn cools the air in contact with it and so may cause fog. Radiation fogs occur mainly in winter, when nights are long and the ground is cold. They are generally local in character, tending to form on plains and in hollows, and are seldom of great depth. They usually disperse by about 10 a.m. in New Zealand.

“The next most frequent way in which fogs are caused is by warm, or relatively warm, and moist air moving on to a cold surface. Very dense fogs may be produced in this way and they may increase gradually to considerable depths. The logs of the Newfoundland Banks arc of this type. Over the Gulf Stream is air which is unusually warm and moist for its latitude, and when this is moved over the cold Labrador current fog is rapidly formed. “An effect of this kind is common In winter on the east coast of the South Island, where the humid and mild portli-easterly breeze blows over the cold land. Combined with radiation it is responsible for most of the fogs in that area.

“The reverse action .of, cold air moving over warm water may cause fog if the current is slow and the air is saturated, because if ebld and warm air are mixed, both being saturated, condensation takes place. The fogs so produced are usually thin and shallow only. They occur principally over lakes, estuaries and reservoirs. They are common on some of the estuaries of North Auckland. “The most widespread fogs in New Zealand occur in connection with cyclones. The principal cause is the cooling by the sea surface of warm air which has had a far northern origin, but it is assisted by the cooling due to the 'falling air pressure as it approaches the cyclone centre and to the slow ascending currents which are known to occur in that area. Almost invariably these fogs occur in air which has a component of motion from the east, so that in winter in eastern districts a number of influences may combine to produce dense and extensive fog. Patches of fog are sometimes produced in dull and showery weather by the cold rain falling through the lower layers of the atmosphere.

System of Forecasts.

“In the case of fog due to cyclones it is usually possible to forecast the probability of its occurrence. Generally, also, it is possible to tell when conditions are favourable for fog. But it is so local a phenomenon, especially on the coasts, that the only way to deal with it satisfactorily is by means, of a close network of stations, reporting at frequent intervals.” Dr. Kidson said this system was now used on the main air routes, covering Palmerston North-Dunedin,. Wellington-Nelson-Blenheim and Napier-Gisborne. Reports are received from about 50 stations all over New Zealand. The majority on or near airlines report at 6 am, 9 a.m., noon and 3 p.m., and the others less frequently, but generally at 9 .a.m. and 3 p.m. at least, the reports giving a variety of information, including the force and direction of the wind, the amount and height of cloud, the visibility and the temperature. These are received at Wellington, the 6 a.m. ones being sent by telephone or wireless, and a report of the state of the weather along the route, together with a forecast of probable changes is then conveyed to the aviation companies before the planes leave on their ilights. These are sent by telephone to aerodromes before the planes take off. In addition, the pilot makes a telephone call to the next aerodrome to find the state of weather there before beginning a flight, and ground radio stations are provided at the main aerodromes so that advice may be sent to aeroplanes in the air of changes in conditions at landing points along the routes. The meteorological service will be extended as the airlines develop. One improvement which is hoped for is that radio stations will be provided at all aerodromes, so that commercial machines equipped with wireless, when unable to land on one ground through a change in conditions, will be able to call up another by radio and see if conditions are favourable there. From Christchurch and Wellington special reports are now given as to the speed and direction of upper air currents, ascertained by pilot balloon ascents at observatories and barometric pressure at aerodromes is included in the reports so that pilots may adjust their altimeters for landing. The weather reports and forecasts are available to all pilots at. main aerodromes along the main ■ airlines, not Only to those of the commercial services, but it is hoped that this service will be extended in time to provide this information at all aerodromes. At present club pilots on most cross-coun-try flights ascertain conditions ahead by telephoning to the next aerodrome.