Calimas and mud rains

In the middle of last March, still winter, practically the entire Iberian Peninsula suffered an episode of haze that was extraordinary due to its amplitude, intensity, duration, causing disturbance and associated time, consequences and unusual visions of it: the invasion of Saharan dust overflowed the Peninsula and reached Great Britain, was felt strongly and negatively affected the quality of the air; started on the 14th, it did not completely disappear from Peninsular Spain until the 22nd; caused by a large extratropical cyclone with its own name, the Celia Storm, was accompanied by turbulent weather and thermal drop, proportionate, with reduced visibility, cloudy and orange skies, covering snowy surfaces with an earthy layer. With hardly any break in continuity, new mud rains came on the 23rd to 28th, particularly noticeable from the south: in Malaga it was recommended to stay at home, given the serious risk of slipping and falling due to the mud on the sidewalks and streets.

The source or home of this dust is the Sahara, the largest planetary desert, hot subtropical, extreme; essentially due to subtropical subsidence and continentality, reduced to mere detail the cold current of the Canary Islands. It occupies approximately 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 southern Atlas or the Mediterranean itself and an imaginary line from San Luis to Khartoum, where the sahel begins. With extremely low rainfall (<100 mm) and very irregular; the potential evapotranspiration is, on the other hand, very high (2,000-6,000 mm). Immense hyper-arid region, there are years without rain, 2 to 5 in a row in a good number of observatories, even more so in the Eastern Sahara. Let us also remember the huge daily temperature fluctuations, which are around and sometimes exceed 50ºC; closely related to the insignificant amount of water vapour, which provides clear skies for the very strong daytime insolation on surfaces with a high albedo and allows, with a pronounced deficit of the greenhouse effect, enormous heat losses due to nocturnal radiation: thus, during the hours of more heat, 50ºC can be reached in the shade, even more; on the contrary, at dawn, the thermometer can drop below 0ºC.

Without ignoring the presence of other colonial powers (England, Italy; between 1883 and 1976, Spain), it was the French who, at the end of the 19th century, conquered most of the Sahara (taking of Tombouctou, 1894; submission of the “Tuaregs ”, 1902). French have also been the biggest and best scholars of this geographic space; with researchers of the stature of Birot, Capot-Rey, perhaps the best connoisseur of the Sahara (“Le Sahara française”, 1953), Dresch, Dubief and Ozenda, among others; Also noteworthy is the extraordinary contribution of the Institut de Recherches Sahariennes (University of Algiers). Although some mountainous areas are not lacking, the endless flat shield of the Sahara is divided primarily between structural plateaus swept away by deflation (“hamadas”), skeletal soils whose minute elements have been washed away (“regs”) and dune fields (“ ergs”) resulting from the aeolian accumulation of sands, silts, clays and granular disintegration particles. Sand winds (0.1-1.0 mm), dust winds, which move finer particles (<0.08 mm), are preferably supplied with these ergs; and lower even those of dry mists or haze. It is these that nourish, where appropriate, rains of mud, red or blood, as well as, to a lesser extent, ocher snow. It is to be remembered that the red or bloody rains constituted for the Roman augurs the announcement of countless calamities, a singularly adverse omen.

With these dust springs, the Spanish lands most affected by the calima, in frequency and intensity, are those closest to the Sahara, that is, in addition to Ceuta and Melilla, the Andalusian, the Iberian southeast and, more than any other, with great difference, the Canary archipelago. Proximity to the desert and geographical coordinates favor the appearance of haze in the Canary Islands any season of the year, although with greater incidence and more pronounced consequences in summer and autumn. All invasions of Saharan air, whether winter or summer, with relatively cool or very hot temperatures, are included under the traditional designation of “southern weather”; It is, however, an improper denomination, since the directions of the flows that carry the aforementioned dust to the islands are east, southeast and, rarely, northeast. We speak of “southern time” due to the resounding opposition of the aforementioned circulations to the predominant ones, that is, to the regime of the trade winds. It should be noted that the contrast is not only in directions -NE or NNE in the trade winds-, but also in origin: continental, that of the Saharan winds; On the contrary, the trade winds, as their own name, of Greek etymology, indicates, are maritime winds. The responsible isobaric reliefs are diverse, some depressive and others anticyclonic, with the common denominator of directing easterly winds towards the archipelago. As has been said, the consequences can be especially disastrous in summer, when the burning (around 40ºC) and drying (relative humidity <10%) Saharan winds burn crops and vegetation: in July 1942, 48ºC in Arrecife (Lanzarote) . The duration of the calima varies, in the Canary Islands, between 3 and 15 days, with a documented maximum of 25 consecutive days, obsessive at the end, in August 1949. It is noteworthy that the Saharan invasions are felt earlier and clearly in the midlands of the islands with higher orographic reliefs, in relation to the cold current of the Canary Islands that forms on it a cushion of air at a lower temperature and more dense, ridden by the Saharan air. Intense for obvious reasons, the haze in the Canary Islands not only negatively affects the quality of the air, sometimes it is perceived in the mouth; it makes the atmosphere very cloudy and sometimes reduces visibility to less than 1 km, determining the suspension of air traffic. Historically, these flows were, with some frequency, accompanied by an undesirable and fearsome passenger, the locust, in devastating plagues.

There are several atmospheric situations that cause haze in the Iberian Southeast, a good example of this is the recent one in mid-March, quite different from the talweg or thermal depression crowned by high pressures, a large diameter Atlantic extratropical cyclone, the Borrasca Celia, that closed the episode with mud rain and, much less, ocher snow. However, the most characteristic haze-causing disturbances are those known as “Algiers lows”, cyclogenetic developments configured by Saharan advection on the surface, with dust in suspension, and cold air troughs in the middle and upper tropospheres, with gradient exaggeration in the vertical and marked instability, which favor the development of low-altitude jet streams (“low level jet”). As has been indicated, the disturbances that cause haze in the Iberian Southeast are diverse, they are not exhausted in those mentioned. Certain storms with a frontal structure, which enter the Mediterranean via Gibraltar, experience a sufficient latitudinal descent so that their depressed and unstable area reaches the dune fields (“ergs”), raises the desert dust and gives it a cyclonic turn, with displacement into the haunted space. On other occasions, the functional mechanism is the pressure difference or gradient, which motivates Saharan flows from the second or third quadrants.

The available observations of the last five years, less abundant and complete than necessary, seem to show a greater incidence of haze and mud rains in the temperate zone. This fact, if confirmed with sufficiently large series, could support the hypothesis of dilation and gain in latitude of the Hadley Cell and, with it, of subtropical subsidence, an issue whose importance should not be emphasized.