Visibility

• Although heavy fog has been observed every month, the frequency of occurrence does not constitute a serious hazard to local operations.

• Conditions most favorable for formation of heavy fog occur during the winter rainy season. On clear nights following a moderate or heavy rain, when weak onshore winds prevailed during the day, rapid cooling begins soon after sunset. Within a few hours, a northeast quadrant drainage wind develops, advecting cool air to the local area. The thickest fog forms when the northeast drainage winds advect fog from the marshes to the local area. Duration of the resulting heavy fog depends on the depth and intensity of the surface inversion. Heaviest fog may form by 0200L and persist until 1500L. Heavy fog occurring on two or more consecutive days is unusual.

• On occasion, fog may develop following a Levante, especially if a strong Levante has persisted for 2 days or more. If the Levante ends with the onset of an afternoon or early evening sea breeze, the warmer moist air to the west will be advected over the colder, upwelled water near the Spanish and Portuguese coasts (the Huelva Front). This air will then condense forming low stratus, and occasionally fog, depending on the height of the inversion and the strength of the subsidence capping the marine layer. Post-Levante fog is most common from late spring to early autumn. This fog normally dissipates by 0900L.

• Fog and low ceilings can also be advected northward from the coast of Morocco and Western Strait of Gibraltar region with this same post-Levante situation. This usually occurs with the development of a Gulf of Cadiz Eddy. This type of fog can advect into the area between sunset and sunrise with southerly flow.