Occult precipitation can be understood as the ability of the vegetation to, through a process of impact or collision, precipitate the small water droplets existing in fog and that in the absence remain suspended in the atmosphere. Fog is understood as any cloud that intercepts the topographic surface. Given its reduced size, only a tiny amount of fog droplets directly precipitates on the ground, so the presence of an obstacle, natural or artificial, promotes the interception of these small droplets that, coalescing, become larger and heavier, precipitating in the soil. The vegetation, due to the continuous movement of its branches and leaves, constitutes the most appropriate obstacle for the collection of water suspended in the fog. The type, size, density, and homogeneity of the forest, as well as the exposure to winds, are also factors that strongly influence the amount of water collected. Trees with a higher exposure to the atmospheric elements -those located in the periphery of a forest patch or in the upper area – collect larger quantities of fog water than their congeners located in the interior of the forest.
Also known in the literature as horizontal precipitation, water potential of fogs, cloud milking, fog drip, or precipitation by direct interception of cloud water, this precipitation only occurs in the simultaneous presence of fog, vegetation, and wind that allows air movement.
Empirically, this phenomenon has long been known in Madeira, thus described by Cecílio Gomes da Silva: “[…] during our intense pedestrian activities through those mountains inside, me and my brothers commented jokingly that up there lived small creatures that always accompanied us: one that provided us with good weather without the terrible “barra” (bar) that hit Altos-Chãos, howled through Portelas and swept Bocas and Eiras; others that from the other side of the mountains made the winds violently blow the Northeast closing everything in a thick humid fog penetrating all the interstices and howling at the edges of bollards and boulders. It did not rain, but within seconds we were completely soaked; […] that forest, I insist, makes it rain within itself, when the Northeast throws against the evergreen slopes of the mountain the colossal masses of air saturated with moisture, causing them to condensate through a rapid rise along the slopes. The resulting thick mists crashing in the dense and diverse forest are collected almost entirely. Drop by drop they fall in the soft carpet of the forest decreasing or practically annulling the speed of the flowing water, which allows a more intense infiltration on the ground” (SILVA, 1997, 104).
Subsequently, several scholars have shown that, in the island of Madeira, the water fog collected by vegetation – the occult precipitation – constitutes an important component of the hydrologic system, reaching, under ideal conditions, more than 60% of the total water that annually falls in the forest ground.
The relief and orientation of the island favors the occurrence of this phenomenon: the values of the fogginess on Madeira are much higher than those over the sea in the region where it is situated, due to the formation of clouds and orographic fogs. The humid air carried by the wind, encountering the island, mountainous barrier with a perpendicular orientation to the prevailing winds from North and Northeast (trade winds), is forced to rise, cooling adiabatically due to the decrease in atmospheric pressure, and condensates in tiny droplets that form clouds and fogs. Locally, this phenomenon is called sea clouds.
In Madeira, fogs are almost exclusively orographic, forming the windward high, with a tendency to disperse leeward, being the annual variation in frequency unclear. The cloud coverage reaches, at Bica da Cana, 235 days / year and in Pico do Areeiro, 229 days / year, normally settling between 600-800 m and 1400-1600 m altitude. Its water content varies between 0,25 g/m3 in the center of the cloud, 0,01 g/m3 at the base, and 0,1 g/m3 at the top. The same process that generates the orographic fogs also originates the so-called rain-shadow, in which the slope leeward (in the case of Madeira, the south) is subject to less rainfall due to the fact that most of this is retained in the north slope and that the air, cold and full of moisture before, decreases and warms when passing to the south, reducing its relative humidity, becoming drier and causing the climatic differences between the different slopes.
Fogs can also occur in the south coast, through this same process, when the wind blows from the south and southwest quadrants. Nonetheless, their frequency is much smaller and is usually associated to the passage of front surfaces from the North Atlantic, or to pressure systems in the vicinity of the Island. It should be also mentioned, a different mechanism of formation of orographic fog that frequently occurs to the south, especially during the hottest months of the year. In this case, the fog is not formed by the rise and cooling of air masses transported by the synoptical winds associated to the global circulation of the atmosphere, but due to the local atmospheric circulation. This phenomenon, which might be regularly observed in Funchal amphitheater, especially during the summer, results from the rise of hot air through the slopes due to the heating of these by the sun. As the day progresses, the sun promotes the warming of the slope and consequently of the air near the surface, also increasing the evapotranspiration and the amount of water in the air. The hot air, being less dense than the cold air, rises through the mountain flanks, adiabatically cooling until it reaches the dew point, from which the water vapor condenses originating clouds and fogs. This phenomenon, different from the mechanism of cloud sea formation originating from the trade winds, is locally called “capacete” (helmet); the fact that the wind is weak in the inside causes lower occult precipitation values than those inside the cloud sea in the north. Another difference between these two phenomena is related to duration. The cloud sea, resulting from the large scale movement of the atmosphere, can last several days. The helmet, since it results from the daily insolation cycle, is formed at the end of the morning, tending to dissipate in the late evening, due to the cooling of the slope and the consequent reversal of the wind direction that starts blowing down the slope, towards the sea.
The factors determining and influencing the occult precipitation are: the existence of favorable conditions for the formation of fog, its frequency and duration, its content in liquid water, dimension of water droplets, wind speed, vegetation presence and characteristics -leaf type, height, size, density and exposure relative to the predominant winds.
The types of natural forest present in the altitudinal range of the fog ring are the Temperate Til Laurisilva (climax vegetation); the Substitution Heath, one of the stages of ecological succession, between 800-1400 m; and the High Heath, climax vegetation, between 1400-1650 m. The occult precipitation can reach 10% of the total water that annually falls down the Til Laurisilva, being this proportion higher during the summer (up to 33% of the total falling water in the forest). Although the largest volume, in absolute amounts, occurs during the winter months, the proportion of the amount of water fallen in the forest ground is more important in summer, when precipitation is scarce and the occult precipitation assumes a more important role regarding the total water entering the forest.
The Substitution Heath collects 13% of the total water that falls on the ground and the High Heath, which trees have compact acicular leaves, more efficient in the intersection of fog droplets, collect on average, 30% of the total water that precipitates in the ground. In specific cases of large isolated heaths, located at Bica da Cana, fog water represents 68% of the total water that precipitates on these, totaling twice the amount of rain that falls in that area. The presence of clouds and orographic fogs leads also to a decreased insolation and temperature that, together with the high relative humidity in the air, reduce the loss of water in the forest by evapotranspiration, increasing thus the volume of water available for infiltration and feed of the underground reserves. This seems to be a widespread phenomenon in all the entire area inserted in the cloud sea, since the underground water in Madeira, especially those coming from altitude springs, have an isotopic signature (content of stable oxygen isotopes, 18O, and hydrogen, 2H), demonstrating that they originate from infiltration of either rain water or fog water.
In addition to the important contribution to the underground reserves, the fog water also seems to be involved in nutrient cycling and ecosystem biogeochemistry. The fog is usually significantly richer than the rain in regard to several nutrients, especially nitrogen, essential for plant growth. The very existence of the Til Laurisilva at the latitude of Madeira might be intimately related with the occurrence of orographic fogs and occult precipitation, responsible for the entrance of extra water in the ecosystem. This vegetation presents certain characteristics, such as complex structure, high richness of epiphytes and abundance of species with little resistance to drought, especially during the hot summer period, such as ferns and bryophytes.
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