Effect of Air-Mass Stability
Fog and stratus are typical phenomena of awarm-type (stable) air mass, since they require saturation of the layer at or near the surface of Earth. A warm-type air mass is one with a temperature greater than that of the underlying surface.
Instability causes vertical mixing, which tendsto create an adiabatic lapse rate and a uniform moisture distribution throughout the turbulent layer. If initially the moisture decreases with height and the temperature lapse rate is less than dry adiabatic (the usual state of the atmosphere), mixing results in a net transfer of moisture up-ward and heat downward, decreasing the relative humidity at the surface and increasing it aloft. If the instability extends to only a short distance above the ground, a stratus or stratocumulus deck may form. If the instability extends to great heights, any clouds will be of the convective type.
Cooling from below exerts a stabilizinginfluence. Bear in mind, however, that there may be diurnal changes in the characteristics of an air mass, because of the heating of the land in the daytime and the cooling at night. Overland, the stability of an air mass is increased during the night, reaching a maximum near daybreak; during the day, the stability decreases, reaching a minimum in the late afternoon. Over the sea, the diurnal effect is small and usually negligible. An overcast condition reduces the heating and cooling effect and, consequently, the diurnal variation on stability.
Radiation cooling of the surface occurs mainlyduring clear, calm nights, such as those occurring in high-pressure areas. Meteorological soundings show the stability of an air mass, the presence of inversions, and the degree of saturation of the air. A warm air mass is stable in the lower layers, frequently showing a surface- or low-level inversion; the cold, unstable air mass has a near-adiabatic lapse rate. Fog is usually associated with a surface inversion; strato-type low clouds, with a low-level inversion above the surface.
Air-mass fogs consist of radiation, advection,advection-radiation, and upslope fogs. These fogs occur within an air mass. Air-mass fogs are not primarily the result of frontal activity. Radiation causes or contributes heavily toward the forma-tion of nearly all fogs over the continents.
RADIATION FOGS. —Radiation fogs consistof ground fogs and high-inversion fogs. Both types of radiation fogs occur only overland. Ground fogs are the result of radiational cooling of the surface of the ground in a single night. High-inversion fogs, on the other hand, are due to the net loss of heat from the surface of the ground by radiation over a period of weeks or months. High-inversion fogs may last several days. Ground fog usually dissipates during the daytime a few hours after its formation.
Ground fogs are quite prevalent on calm, clearnights after a day during which there has been cloud cover. The cloud cover helps keep the temperature down during the day, so the temperature is already relatively low when radiation starts. These fogs are quite general over Earth’s land surfaces, appearing frequently in mountain valleys and on the coastal plains of the United States.
High-inversion fogs take place only over thecontinents outside the tropics in winter. The winter fogs of the low valleys in the far western United States are good examples of high-inversion fogs. They occur most frequently in areas where mP air has become trapped in a valley and the moisture condenses when sufficient radiation has taken place. If an anticyclone with its accompanying subsidence causes the air mass to stagnate, the fog may last for several days. High-inversion fog owes its name to the strong inversion that is always found above the fog.
ADVECTION FOGS. —Advection fogs arecaused by the transport of warm air over a cold surface or the transport of cold air over a warm surface. Included in advection fogs are land- and sea-breeze fogs, sea fogs, tropical-air fogs, and st earn fogs.
Land- and sea-breeze fogs, which form alongthe eastern coast of continents, occur in summer. They also occur over large inland bodies of water, being quite frequent over the Great Lakes. Most of the fogs of the Great Lakes region are of this type. Radiation processes play a large role in the formation of these fogs.
Sea fogs occur any place where sea air iscooled over a cold ocean current. Therefore, sea fogs are not confined to coastal areas. Two outstanding examples of sea-fog formation are the prevalence of fogs in the vicinity of the Grand Banks of Newfoundland, and the Kamchatka Peninsula, in Asia. These fogs may persist even during times of strong winds and great turbulence, especially when the difference between the air temperature and the water temperature is large. Sea fogs are also quite persistent along the California coast, where sea air passes over the cold California Current.
Tropical-air fogs result from the gradualcooling of air as it flows over the oceans from low latitudes to high latitudes. They can also occur overland in winter as tropical air passes over cold land surfaces. This is probably the most common type of fog over open ocean areas. Some of the most widespread fogs in the United States are also of this type. Tropical-air fogs develop best over western Europe and the eastern North Atlantic. Sea fogs and tropical-air fogs are sustained up to much higher wind speeds than is the case for other fogs. Wind speeds of 30 knots in the midst of a sea fog or tropical-air fog are not uncommon.
Steam fogs occur when cold air with a low vapor pressure passes over warm water. If the water is quite warm, the air does not have to be very cold for the fog to form. The big factor is the difference between the vapor pressure of the air and the water. Wind speeds and temperature lapse rates do not have an appreciable effect on the formatiorl of steam fogs. The Great Lakes region has steam fogs quite frequently in mid-winter, when cold continental air passes over the Great Lakes at a time when the temperature of the lake water is just above freezing.
ADVECTION RADIATION FOGS. — Advection radiation fogs occur when air that has come inland from the sea during the day undergoes nighttime radiat ional cooling. The special circumstances under which they form are what set them apart from other kinds of radiational fogs. The moisture-laden air comes in from the ocean, where its temperature was kept relatively low; it does not, therefore, take much radiational cooling for the air to reach its dew-point temperature and form fog. Advection radiation fogs take place mainly in late summer and autumn, when the water is warm enough to cause the air to have a high dewpoint and the nights are long enough to afford sufficient radiational cooling. This type of fog occurs primarily along the Gulf Coast, the Atlantic coastal plain and piedmont, in the vicinity of the Great Lakes, and in the coastal valleys of California.
UPSLOPE FOGS. —Upslope fogs form in regions where the land slopes gradually upward, as a result of the cooling of the air by adiabatic expansion as the air moves to higher elevations. One such region is the Great Plains of the United States and Canada. This type of fog can occur in relatively high wind speeds. The faster the air moves up the slope, the more rapid will be the cooling. And to some extent, there will be a counteraction of the downward transport of heat by turbulence. Often these fogs are formed as the combined result of the ascent of the air and radiation and, occasionally, by increases of moisture due to rain.