In the middle of last March, still winter, practically the entire Iberian Peninsula was subjected to an extraordinary event of haze because of its size, density, duration, disturbance and associated time, consequences, and unusual visions: an invasion of Saharan dust overflowed the Peninsula and reached Great Britain. It was felt that air quality was strongly and negatively affected; It began on the 14th, did not completely disappear from the Spanish Peninsula until the 22nd; It was accompanied by proportionally reduced visibility, turbulent weather and thermal decline with cloudy and orange skies, caused by a large extratropical cyclone named Storm Celia, which covered the snowy surfaces with an earthy layer. Between the 23rd and 28th of the month, with almost no interruption in continuity, new mud rains came, especially noticeable from the south: given the serious risk of slips and falls due to mud on the pavements and streets, staying at home was advised in Málaga. .
The source or home of this dust is the largest planetary desert, the hot subtropical, extreme Sahara; mainly due to subtropical subsidence and continentality, the cold current of the Canary Islands has only been reduced to detail. It covers about 10,000,000 km2 with a length of 5,000 km from the Atlantic to the Red Sea; 2,000 latitude and width between the South Atlas, or the Mediterranean itself, and an imaginary line from San Luis, where the Sahel begins, to Khartoum. Extremely low precipitation (<100 mm) and very irregular; potential evapotranspiration is very high (2,000-6,000 mm). The enormous hyper-arid region, there are years of no rain, 2 to 5 in a row at multiple observatories, and even more in Eastern Sahara. Let’s also remember the big daily temperature fluctuations around 50ºC and sometimes more; It is closely related to the negligible amount of water vapor which provides clear skies for very strong daytime insolation on high albedo surfaces and allows very large heat losses from nocturnal radiation with a marked deficit of the greenhouse effect. hours more heat, in the shade 50ºC can be reached, or even more; on the contrary, at dawn the thermometer may drop below 0ºC.
Without ignoring the presence of other colonial powers (England, Italy; between 1883 and 1976, Spain), it was the French who conquered most of the Sahara at the end of the 19th century (taking Tombouctou, 1894; “Tuaregs”, 1902). It has been its greatest and best scholars; Birot, Capot-Rey, perhaps the best specialist in the Sahara (“Le Sahara française”, 1953), with researchers such as Dresch, Dubief and Ozenda; also of the Institut de Recherches Sahariennes (University of Algeria). ) is also notable for its outstanding contribution, although some mountain ranges are not missing, the endless flat shield of the Sahara, primarily structural plateaus (“hamadas”) swept away by deflation, skeletal soils (“regs”) and dune fields (“regs”) that have been leached out of minor elements. ergs”) are caused by wind deposition of sands, silts, clays and granular fragmentation particles. Sand winds (0.1-1.0 mm) are dust winds that mobilize finer particles (<0.08 mm), preferably these ergs is provided with; and reduce even the dry fog or hazy ones. It is these that feed mud, red or blood rains and, to a lesser extent, ocher snow when necessary. It should be noted that red or bloody rains are a reminder that the Romans were harbingers of numerous disasters, singularly a negative omen.
With these dust sources, the Spanish lands most affected in frequency and intensity by calima are those closest to the Sahara, i.e., in addition to Ceuta and Melilla, Andalusia, the Iberian southeast and, with a greater difference than others, the Canary archipelago. Proximity to the desert and geographic coordinates favor the appearance of haze in the Canary Islands at any time of the year, but with greater incidence and more pronounced consequences in summer and autumn. All invasions of Saharan air, winter or summer, with relatively cold or very hot temperatures, fall under the traditional definition of “southern air”; However, it is a misnomer as the directions of the flows carrying the dust to the islands are east, southeast and rarely northeast. We speak of “southern time” because of the resonant contrast of the above-mentioned circulations with the dominant ones, that is, with the regime of the trade winds. It should be noted that the contrast is not only in the directions -NE or NE- of the trade winds, but also in the origin: continental, that of the Saharan winds; On the contrary, the trade winds are sea winds, as the Greek etymology suggests from their name. The responsible isobaric reliefs are diverse, some depressed and others anticyclonic, and their common denominator is to direct easterly winds to the archipelago. That being said, the results can be particularly disastrous in summer when burning (about 40ºC) and drying (RH < 10%) Saharan winds burn crops and vegetation: 48ºC in Arrecife (Lanzarote) in July 1942. The duration of Kalima ranges from 3 to 15 days in the Canary Islands, with a maximum of 25 consecutive days documented, obsessed at the end, it is noteworthy that the Saharan invasions were felt earlier and more clearly in August 1949. Over the Canary Islands, the central regions of the islands with higher orographic reliefs relative to the cold current, where the Saharan air pushes at a lower temperature and creates a denser air cushion. The dense haze in the Canary Islands for obvious reasons not only adversely affects the quality of the air, but is sometimes perceived in the mouth; it makes the atmosphere very cloudy and sometimes reduces the visibility to less than 1 km, determining the suspension of air traffic. Historically, these streams have been accompanied with some frequency by the locust, an unwanted and frightening traveler in devastating plagues.
There are several atmospheric conditions that cause haze in the Iberian Southeast, a good example being the high pressures, a large Atlantic extratropical cyclone, the talweg crowned with Borrasca Celia, or the last case in mid-March, which is quite different from the thermal depression. , covered this section with mud rain and, much less, mustard-colored snow. However, the most characteristic turbidity-causing disturbances are those known as “Algerian lows”, cyclogenetic developments structured by Saharan advection at the surface, dust in suspension and cold air troughs in the middle and upper tropospheres, gradient exaggeration in the vertical, and low altitude jet streams (“low level jet”). ) marked instability that favors its development. As noted, the disturbances that cause haze in the Iberian Southeast are diverse, inexhaustible in what has been stated. Some storms of frontal nature that enter the Mediterranean via Gibraltar experience a sufficient latitudinal descent so that their depressions and unstable areas reach dune fields (“ergs”), raising the desert dust and giving it a cyclonic spin, with displacement. haunted space. In other cases, the functional mechanism is the pressure difference or gradient that motivates Saharan flows from the second or third quadrants.
Current observations of the last five years, which are less than necessary and complete, seem to show more cases of haze and mud rain in the temperate zone. If this fact is confirmed by sufficiently large series, it could support the hypothesis of subtropical collapse, the latitude expansion and gain of the Hadley Cell, and with it the importance of which should not be overemphasized.