Study of visibility and fog at Malta

Abstract

A statistical survey extending over five years shows that good visibility at Malta is associated with northerly winds, low relative humidity and high barometric ­ pressure. Other favorable factors are the presence of cumulus cloud or alternatively an almost total absence of cloud. The effect of the force of the wind varies according to the wind direction, but in general the visibility deteriorates if the wind becomes strong, especially in easterly winds; yet if the wind is NW and strong the visibility may often be exceptionally good. On summer evenings the visibility at the Observatory in Valletta becomes very good if the wind is from SW and this is regarded as a fohn effect. Diurnal and seasonal effects are also present and the visibility normally becomes good or very good in the afternoon, especially in autumn. The diurnal effect is due to convection, but it tends to be suppressed if the wind is strong or the sky continuously overcast or if the wind blows from SE. Observations of the sea horizon at selected points at various altitudes above sea level indicate that the visibility over t he open sea undergoes a variation which is seasonal but not diurnal ; while the diurnal changes in visibility seawards from Malta are due to the existence of mist or haze in the immediate vicinity of the island. Visibility over the open sea is normally very good in winter with NW. winds and is of the order 30 miles or more. Thick fog is very rare. It is observed over the island in the early morning and less frequently in the evening and also over the open sea in the form of banks or patches. Its frequency at the Observatory is approximately one observation in 18 days in January and February and about half that figure in July and August. It occurs in shallow depressions near a feeble cold front or in regions of light and indefinite barometric gradients in summer; but in the latter case it is also probably a boundary phenomenon between masses of air of different temperatures and humidities.