There are many atmospheric phenomena in the world. Some are overwhelming, like a lightning storm; others are beautiful, like the appearance of a rainbow; or delicate like drops of dew on the leaves of the vegetation. If I had to define the calima with an adjective, it would be lugubrious.
The calima is a heat haze, a mass of air that contains enough quantity of particles in suspension to make visibility difficult on a clear day. It could resemble a fog bank, except that it is made up of solid particles like sand, silt or clay, rather than moisture.
You could also think of a sandstorm in the middle of the desert, but it is not exactly that: the calima can reach much more height than a sandstorm, exceeding 2,000 meters, and does not have the violence of surface wind gusts which is unleashed in a sandstorm, as it is generated by the vertical convection of hot air.
- Trade winds and ITCZ. Source: ultramarin.nl
Thus, the particles in suspension can travel thousands of kilometers carried by the prevailing winds of the upper layers of the atmosphere, reason why the countries and regions affected are very diverse: from Western Europe, the African countries of the Sahel and the Canary Islands, to overseas territories such as the Caribbean, Central America and South America.
Where does so much dust come from?
Evidently, the desert areas of the planet are the perfect source for the origin of the calima. However, they must be located along a strip called the Intertropical Convergence Zone (ITCZ), which is the low pressure belt in which the trade winds from the northern hemisphere converge with those of the southern hemisphere. In this strip is conformed what is known as Dust Belt.
- Dust Belt. Source: Aeolian Dust Blog
We would be talking about the ‘Thermal Ecuador’, the area of the terrestrial globe in which the highest temperatures are recorded due to the incidence of 90° of the solar rays in these latitudes, and that generate the greater vertical convection of hot air in the planet. In this way, the large deserts located in the area of influence of the ITCZ are the source of the calima.
The impact of calima
When defining the calima as lugubrious, you must imagine a translucent and enveloping mass that diminishes the brightness of the sun, and unleashes a curious phenomenon called ‘rain of blood’. This macabre term has its explanation in the color of the drops of water, mixed with the particles of the desert. It is generally known as ‘mud rain’, but differentiation is made when the pigment of the residue is reddish after precipitation.
Being suspended, the particles of the calima act as condensation nucleus for moisture, which facilitates the formation of water drops. Once the drops are so large that the air is not able to keep them suspended, they fall to the earth by the effect of gravity and, when impact against its surface, the desert dust impregnates it with its ocher, reddish or yellowish hue, reaching up to 2-4 gr/m² after these events.
- ‘Mud rain’ over a car in Madrid (20/07/16). Source: Photo taken by myself.
However, there are other effects which are directly imperceptible to the human eye. In Spain, for example, the calima is associated with an increase in temperature, since the air mass that carries the charge of particles has its origin in the Sahara Desert.
In this sense, the Canary Islands are the most affected by this mass of air when the wind comes from the East (a common thing following the direction of the prevailing winds), but the most remarkable thing is that the calima carries a cooling of the atmosphere, and that the multitude of transported sediments exert a considerable refraction on the rays of light, limiting the percentage of light that reaches the ground and acting as a cloudy day.
- A day with calima in the city of Madrid. Source: Tormentas en acción.
The consequences of this situation affect the whole ecosystem, starting with the human being. The dust load associated with the calima is a serious problem for the at-risk population: the elderly, children and people with a respiratory problem.
In fact, scientists, through the European MACC (Monitoring Atmospheric Composition and Climate) project, have found enough evidence to claim that the calima is directly related to the epidemic of meningitis in some countries of the Sahel, such as Burkina Faso, Senegal , Niger and Mali.
Fortunately, thanks to its monitoring, it is possible to predict the appearance of the calima 48 hours in advance, which facilitates the management of local warnings for prevention (staying at home, not performing extreme physical activities, and staying hydrated).
In addition to these problems, the sedimentation of the particles that compose the calima has harmful effects on vegetation and agriculture (preventing photosynthesis), on the soil (creating a thin layer that modifies its chemical composition), and contaminating the water from rivers, lakes and aquifers. The occurrence of diseases in some coral reefs has even been observed, after the sedimentation of the dust in the ocean.
Relation with Climatic Change
Studies on Climate Change focus on the presence of greenhouse gases in the atmosphere, but little is known about the impact of aerosols on the climate of planet Earth.
In recent years, observer experts at the State Meteorological Agency (AEMET) in Spain are checking how they can affect the characteristics of suspended particles to the climate system by studying their amount, size and mode of light scattering.
Until now, it can be verified that there is an increase of dust in the atmosphere and its associated phenomena, like the rain of mud, since the 90’s. Its explanation resides in a greater insolation, which favors the ascending movements of the air, and the expansion of the desert areas in North Africa and the Sahel.
According to Nature Climate Change, 75% of extreme temperature events are attributable to climate change; however, it cannot be stated categorically that it is behind these processes.
- Sahara mass air loaded with dust. Source: zonu.com
What is undeniable is the feedback the climate has with the presence of atmospheric dust, since the concentration of particles in the atmosphere can modify the climate, causing a decrease in temperature on the surface of the Earth and giving rise to colder periods. But in turn, the climate determines the volume of dust particles available in the atmosphere, depending on the existence and extent of deserts and drought on the planet.
So… Is there a relationship between suspended particles and greenhouse gases? That is the million-dollar question among the experts who study the different theories on Climate Change. It is believed that the concentration of dust in the atmosphere conditions the presence of greenhouse gases in the atmosphere.
For example, one of the main theories proposed by oceanographer John Martin (1988) argues that iron deficiency as a micronutrient in the oceans decreases the presence of phytoplankton in surface waters, which are essential for the removal of atmospheric CO2, so the average temperature of the planet will be higher. This is the so-called ‘iron hypothesis’.
We should remind that, thanks to the calima, these iron particles can reach the ocean and thus increase the volume of phytoplankton, which translates into a decrease in atmospheric CO² and a thermal reduction on a planetary scale. So we can deduce that the greater the amount of dust in the atmosphere, the lower the presence of CO².
Now then, should the human being, according to this premise, contribute to the fertilization of the oceans with iron minerals to stop or mitigate Climate Change? Is it not too late? Or even worse, could they reduce CO² levels to the point where a small ice age could be occasioned?
As we try to answer, we will still feeling the warm breath of our African neighbor, as a warning that reminds us how close we are to an inhospitable and indomitable but alive and changing world.