Page:Handbook of Meteorology.djvu/151

 Pressure is an important factor in visibility. When the air is misty and the seeing generally is poor a very slight increase in pressure clears it up at once. As a cyclonic depression advances the seeing becomes poorer, because of rain or snow, until the trough passes. Then the seeing at once begins to improve, with increase of pressure.

The foregoing, turbulence, convection, and inversion—that is, the formation of a lid—are the principal movements of the air which impair visibility. The factors themselves are moisture, dust, smoke and refraction of light. The dust and smoke differ merely in origin; the moisture may appear in the form of fog, mist, rain, or snow.

Fog, Cloud, Mist.—In marine transportation, fog is the worst factor in the impairment of visibility. Practically it is the only one. If the temperature is brought below the dew-point, fog results from condensation of the water vapor. The brisker the wind, the thicker the fog blanket. A convectional updraught does not destroy the turbulence; convection merely carries it higher.

The thickening of a sea fog is an illustration. If the sea water is colder than the air, which is the case when polar waters intrude within lower latitudes, warm air blowing over it will give up its vapor in the form of fog. So long as the eddying movements of the air are constantly bringing warm air next the cold water, fog condensation increases. The fog blanket thickens; its upper part marks the height at which the temperature of the air is above that of saturation.

The fogs of the Newfoundland Banks have been the terror of the sailing route between American and British ports. They will prove a much greater hazard to air transportation unless a circuitous southerly route is followed. An Arctic current and a southwest wind laden with water vapor constitute the working machinery of this fog factory. Radio-telegraphy now forewarns the sailing master when and where he will encounter the fog blanket that hovers over an ocean graveyard. Times and positions of these fogs are approximately known, but definite forecasts cannot now be made.

During summer, fog along the steamship lanes in the vicinity of the Banks averages between ten and twenty per cent of the time; in winter it may be expected about one-third of the time.