Two less-prominent sources of low-level wind shear deserve brief discussion: gusty or strong surface winds, and land/sea breezes. Fluctuations of 10 knots or more from the mean sustained wind speed, or strong winds blowing past buildings and structures near a runway can produce localized areas of shear. This type of shear can be particularly hazardous to light aircraft. Observing the local terrain and requesting pilot reports of conditions near the runway are the best means for anticipating wind shear from this source.

Land and sea breezes commonly occur near large lakes, bays, or oceans. The flow to or from the water is caused by the differential heating and cooling of land and sea surfaces. The sea breeze is a small-scale frontal boundary and can reach speeds of 15 to 20 knots. It can move inland 10 to 20 miles, reaching its maximum penetration in mid- or late afternoon. The depth of the breeze is approximately 2,000 feet. Land breezes occur at night, because the land becomes cooler than the water. The land breeze has less intensity than the sea breeze, and unless aircraft penetrate it during a long, low over-water approach, there is little threat to flying safety.

Learning Objective: Recognize the pro-cesses within the atmosphere that bring types of fog.

Stratus clouds and fogs occur at or near the surface of Earth and can seriously restrict visibility at low levels. Therefore, they are a very important consideration in aircraft operations, particularly in connection with landings and takeoffs. Fogs are especially significant to the pilot who limits his flying to visual flight rules, because ceilings under stratus clouds often are very low, and visibilities in fog conditions often are not sufficient to permit navigation by visual reference. Too, fogs are of great concern to ships. In harbors and over open ocean areas, fogs constitute a definite hazard to safe navigation. For these reasons it is important that you have a basic understanding of the factors leading to the formation and dissipation of stratus and fogs, and the basic types of fog.

Stratus and fogs result from the condensation of excess water vapor out of a saturated air mass. Both are composed of small water droplets or small ice particles suspended in air. The main difference between them is that fog is a layer of suspended droplets adjacent to Earth's surface, while stratus is fog that has been lifted or has formed some distance above the ground. Moisture will not condense out of pure (nuclei-free) air until a high degree of supersaturation (from 400 to 800 percent humidity) is reached. The air within the troposphere is not nuclei-free. Moisture can condense on nuclei when relative humidity reaches approximately 100 percent. (Some nuclei become effective at relative humidities as low as 70 percent, which may produce haze and the beginning stages of fog.) The atmospheric nuclei believed important in the condensation process are sea-salt particles and certain hydroscopic products of combustion. The combustion nuclei are more common over industrial areas, but some condensation nuclei appear to be universally present.

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