TO PHYSICAL METEOROLOGISTS FOG and haze are not merely nuisances or hazards. They are also fascinating aspects of weather that provide opportunities to observe how water vapor in the air precipitates as droplets of water.

A water droplet cannot grow in the atmosphere unless it contains a minimum of about 5,000 molecules of water. The probability of so many molecules assembling in one place by mere chance is very low. Physicists agree that condensation must be triggered by a nucleus-a speck of some material in the air that attracts water molecules. If this is so, then every one of the myriad droplets comprising fog and haze and every drop of rain that falls must start with a nucleus. Most nuclei are crystals of sea salt. All rain, even that which falls in the deep interior of the continents, contains traces of salt [see "Salt and Rain," by A. H. Woodcock; SCIENTIFIC AMERICAN, October, 1957]. But because the estimated volume of spray thrown into the air by whitecaps and by surf breaking on beaches does not appear to be great enough to supply a salt crystal for every droplet that forms in the air, the question remains open.

The collection and analysis of the water droplets is complicated by the fact that no two fogs are exactly alike. Moreover, they usually appear at irregular times and for unpredictable intervals. In a few regions, however, a persistent fog occurs as a regular feature of the annual weather pattern. One example is the deep haze that blankets Venezuela every year, usually from early December to the middle of April. The mystery of how this haze forms is deepened by the fact that it appears only during the dry season, when the relative humidity approaches that of a desert. This year Guillermo Zuloaga, a geologist of Caracas who is chairman of the research committee of the Creole Petroleum Corporation, decided to turn amateur meteorologist long enough to make a study of the fog.

"Traditionally," Zuloaga writes, "people have called our haze the calina, an old Spanish word defined as 'a thick hot air, like a fog, which rises in very hot weather and sets the air on fire.' Because the presence of the calina coincides with the time of burning of the dry pasture lands and with brush fires, the general belief is that it is caused by smoke.

"This year the calina was particularly intense From February to the end of May visibility was seriously impaired, and there were several grave consequences. Air traffic in the interior was hampered, with some airports closed for days at a time. A squadron of jet fighters flying from eastern Venezuela to its base in Maracay became dispersed and several aircraft were lost. Ships ran aground, and several automobile accidents were caused by the poor visibility. Many residents of Caracas, which lies in a narrow valley surrounded by mountains, lost the beautiful sight of Mount Avila, only four or five kilometers to the north [see illustrations].

"For several years I have been observing this phenomenon and wondering what causes it. I noted first of all-as apparently no one had before-that it comes from the Caribbean Sea and is carried by the wind toward the interior of the country. In many flights I have found the calina visible as far as 100 kilometers out to sea north of the Venezuelan coast. The trade winds during the dry season blow steadily from the northeast toward the coast. If the calina were smoke, one would scarcely expect to find it upwind.

"As seen from the ground the calina is bluish, but as one looks down on it from a plane it appears brownish. The calina layer ends abruptly at about 9,000 feet and the upper level is remarkably flat, so that it makes a horizon. It appears to be absolutely independent of any clouds that move through it and it contrasts sharply with them in color and shape.

"Unlike ordinary fog, the calina is remarkable for its uniformity and constancy. It covers thousands of square miles for months at a time: the valleys and the mountains, the llanos (the great plains in the interior of Venezuela) and the marshes of the Maracaibo Basin. It even extends at times as far as Bogota in Colombia, over 600 miles southwest of our Caribbean coast. For months on end we have spectacular red sunsets, and the moon looks yellowish when it is near the horizon. The calina seems more intense in the morning and evening and seems to disappear during the night; this is because it is more visible in lateral lighting. The air feels heavy; people complain of high humidity, whereas actually the air is abnormally dry.

"Puzzled by the calina and not finding any satisfactory description in the literature, I decided to carry out systematic observations to try to find its composition and to explain its origin. I cleaned up my old polarizing microscope, adapted a camera to it and set out to discover what sort of particles nucleate the calina droplets.

"My observation that the calina came from the sea naturally led me to believe that it should contain microscopic crystals of salt, and so my first efforts were aimed at capturing such crystals in the air. Previous attempts to do this, by workers at the Woods Hole Oceanographic Institution seeking to confirm the presence of salt in cumulus clouds, had been only partially successful. Being hygroscopic, the salt absorbed water during the plane's descent in the humid atmosphere and tended to redissolve. It occurred to me that the crystals could be collected on microscope slides that had been smeared with Canada balsam. This mounting medium, commonly used for mineral preparations, is anhydrous, and salt is therefore insoluble in it. I prepared the microscope slides by first placing a small drop of balsam in the center of the square centimeter. Crystals were collected simply by exposing the prepared slide to the air for a few seconds. The cover glass was then pressed over the specimen, so that the particles caught in the balsam were permanently preserved.

"When collecting from an airplane, I exposed four slides at a time by holding them with binder clips to a piece of 1/4-inch glass about 12 inches long and the width of the microscope slides. I was able to hold them out a few inches into the slip stream even at speeds of 200 miles per hour. When working from an automobile, I clipped the slides to a piece of cardboard, which I then attached to a ventilator window opened to face the wind.

"Early in the study I found a few salt crystals and immediately concluded that I had hit the full explanation of the calina. My theory was simple: The strong, steady trade winds that blow over the Caribbean, whipping up whitecaps and the large waves that break along the Venezuelan shore, raise a cloud of droplets that make up the sea spray; this spray is carried inland by the wind in great volume; as the water evaporates from the droplets in the atmosphere, tiny salt crystals are formed that, being very small, float away with the wind. These crystals, I assumed, were the nuclei of the calina droplets.

"But, as usual in these cases, what at first appeared simple became more complex as I probed deeper into the problem. Careful examination of the slides soon disclosed that I was catching not only salt crystals and other things to be expected in the air, such as grains of pollen, dust, vegetable fibers and fragments of insects, but also a great number of tiny spherical particles evenly distributed on the slides and almost invisible in the Canada balsam. The particles were a few microns in size (a micron is one thousandth of a millimeter), too small for identification by ordinary optical methods. They were present on all the slides. My theory did not explain these particles.

"In an effort to gather more evidence I sought the help of friends who were flying or driving to the interior; I gave them slides, cover glasses and balsam and detailed instructions on how to collect the particles. I made two special flights over Caracas to gather more material, take pictures and record the temperature and relative humidity of the air in the thick of the calina. The temperature was between 28 and 80 degrees centigrade (82.4 to 86 degrees Fahrenheit), and the relative humidity was 40 to 45 per cent at 5,000 feet-much hotter and drier than normal. During the 'noncalina' portion of the year the average relative humidity in Caracas is 89 per cent at 7:00 a.m., 64 per cent at 1:00 p.m., 89 per cent at 7:00 p.m. and 93 to 98 per cent during the night.

"It turned out that even the hot, dry air over the llanos, near the ground, contained salt crystals in suspension. Slides prepared at a ranch in the inland state of Cojedes contained small but perfect crystals. (Incidentally, if one square centimeter of balsam captured two or three crystals, then there must be millions of them in the ambient air, and these crystals must fall to the ground when the wind dies down or when it rains. What will this eventually do to the fertility of the soil?) The extremely low relative humidity appeared to be contrary to what one would expect with salt in the air, which I then thought should tend to increase the humidity.

"This was another indication that I had been jumping to conclusions. I had to be more systematic and start again. I began to keep records of the relative humidity and noted that it had dropped to levels never reported before: 50 per cent at 7:00 a.m., 30 to 40 per cent in the course of the afternoon. During the night we did not even approach the dew point, which in normal times is reached every night. Intrigued by all this, I decided to study the process of the formation of the calina from its very beginning: at the seashore.

"At 7:00 a.m. on a Saturday I went down to the shore at Catia de la Mar (Caracas, although 1,000 meters above sea level, is only some 15 miles from the sea by a superhighway) and exposed some slides for five minutes to the sea spray: four without balsam and four with balsam. The temperature of the air was 30 degrees C. and the relative humidity three meters from the surf and two meters above it was only 77 per cent at 8:00 a.m. The trade wind from the northeast was blowing moderately, and the waves were normal.

"Then I took the road that goes east along the shore toward Playa Grand here, over a stretch of about one kilometer and some 100 meters away from the water, I exposed another four slides while driving against the wind. Subsequently I went up the hill to the Playa Grande terrace, where I was 60 meters above sea level and some 200 meters from the water (500 in the direction the wind), and exposed another four slides.

"From this high point there is a panoramic view of the shore, and I could the calina coming from the sea and being blown up toward the mountains, blurring both the horizon and the profile of the coastal range. The temperature was 34 degrees C. and the relative humidity was 54 per cent.

"Then I exposed a number of slides along the autopista, the superhighway, between Caracas and the sea. Relative humidity was between 40 and 45 per cent, the temperature was around 34 degrees C. The series gave me an excellent sampling of particles from the shore up to 1,000 meters.

"With great curiosity I went to the microscope to examine the slides. The first surprise came when I found some perfect salt crystals preserved in the balsam of slides exposed near the surf. How could they have crystallized out so soon? I found them again and again, associated with sea-water drops, in slides exposed on the road along the shore and in those exposed at the Playa Grande terrace. There was no doubt: the sea air, even close to the shore, contained salt crystals. It could be, of course, air turbulence had brought these crystals from a great height or a long distance But their frequency made this unlikely. Apparently crystallization takes place over a very short distance-a sort of spray-drying caused by the atomization of the sea-water drops into a relatively dry atmosphere.

"Then, in the Playa Grande slides, I found a most interesting thing: all the stages of the process of formation of the salt crystals (which are perfect cubes) could be seen, starting with drops of sea water and moving through the intermediate steps. Furthermore-and this was particularly important-there were great numbers of small, spherical droplets around the large drops that contained the salt crystals. These small droplets resembled the mysterious spherical particles that universally accompanied the salt crystals on all slides of the calina. The resemblance was not only in shape but also in size. I was struck by one particularly fascinating detail that was difficult to photograph because of the extremely flat depth of field: the microdrops showed a tendency to cluster around both the crystals and the drops that were in process of crystallization [see illustration in Figure 3].

"The more I observed this phenomenon, the more puzzled I was..At first I thought that the clusters were a 'splash' effect resulting from the impact of the crystal, or the drop containing the crystal, against the Canada balsam. But when I saw the phenomenon repeated time and again, I became convinced that even in the air the microdrops were clustered around the salt crystals like tiny satellites.

"Pondering on this curious phenomenon, it occurred to me that the particles might be charged with static electricity and so be attracted to a crystal nucleus of opposite charge. An obvious experiment was to charge the microscope slides by rubbing them with an electrifying agent. This I did on the roof of our office building, holding the slides with plastic tongs and rubbing some with a silk handkerchief and others with a camel's-hair brush. After 20 seconds of exposure to the air the electrified slides were protected with cover slips held in place by Scotch tape stuck along the edges. It was a hot, dry afternoon (30 degrees C and 28 per cent relative humidity in the shade). The results were dramatic: not only had I captured the particles in great numbers, but also they had attached themselves to the slides in the very spots where the glass had been rubbed! On the slides electrified with the camel's-hair brush trails of microdrops actually traced the path of each hair [see illustration in Figure 4]. I had captured almost exclusively the microdrops, which, probably due to difference of charge intensity, had divorced themselves from the salt crystals.

"The dryness of the air again pointed up the question of the composition of the microdrops. How could they subsist, without evaporating, in such a dry, hot atmosphere?

"The season of the calina was now approaching its end and it was urgent to gather enough drops for chemical analysis. Some of my technical friends hurriedly built a small electrostatic separator, obtaining the necessary 15,000 volts from a television set. With this apparatus we were able to collect a few milligrams of the microdrop material before the calina disappeared. Spectrographic analysis of the metallic components, which is all that could be done on such a small sample, indicated the presence of sodium, magnesium and calcium along with copper from the electrical apparatus and some aluminum, probably from clay in the dust collected with the calina particles.

"In the few remaining days of the season I prepared most of the slides without balsam by electrifying the glass with either the camel's-hair brush or with Saran Wrap, a plastic film [see illustration in Figure 5]. This gave me dry mounts of the particles. These particles were more visible than those embedded in balsam, and the salt crystals, simultaneously collected on the slides, lasted many days before absorbing humidity. In contrast with the behavior of the salt crystals, the microdrops grew considerably in size, indicating they were still able to absorb water from the air.

"What may be reasoned from the study of the slides? I have the impression that the chemical composition of the microdrops is that of a brine that remains after the crystallization from the sea water of the sodium chloride-a concentrated solution of sodium and magnesium chlorides, magnesium sulfate and calcium chloride, all quite soluble and hygroscopic salts. This might explain their resistance to evaporation. The metallic elements shown by spectrographic analysis to be present in the microdrops are those one would expect to find in the brine.

"It is, of course, possible that the salts in some of the droplets do crystallize out if they reach an exceedingly dry atmosphere. The probability, in this ease, is that minute crystals of magnesium chloride hydrate (MgC₁₂-6H₂O) would be formed-and this is a very hygroscopic salt that would again rapidly dissolve into droplets. We do not know, of course, what chemical reaction the droplets of brine actually undergo because of the action of the sun's rays during their travels with the wind.

"There are other circumstances related to the calina that are also puzzling. Ordinary fog is supposed to start by condensation of water in the atmosphere around small particles of hygroscopic salts (or dust, or sulfur from fires) even when the air is unsaturated. But for condensation to continue, the relative humidity has to increase above the value at which condensation started. Since the effect of condensation is normally to dry the air by removal of water vapor, thereby decreasing the relative humidity, condensation cannot ordinarily proceed unless somehow the relative humidity increases by evaporation of more water into the air or by cooling of the air.

"But in the case of the calina, persistent fog and dryness of the air appear to go together. Could it be that the calina, made up as it is of hygroscopic crystals of sodium chloride and those droplets containing particularly thirsty salts, is one factor contributing to the dryness? At least this year the calina, an intense and prolonged dry season (the worst in 40 years), and excessive heat have coexisted. There may be a relationship of cause and effect. Stronger trade wind would produce more spray and hence more calina, which would tend to dry the air and retard the rains. It will take further studies to confirm or prove wrong all this theorizing.

"Venezuela is south of the hurricane zone of the Caribbean Sea, and as a general rule we do not have the big changes in atmospheric pressure observed at higher latitudes. We have instead a remarkably uniform diurnal pressure tide of small magnitude. This stability of atmospheric pressure may be one of the reasons why the calina stays in the air for such long periods.

"One last point of some industrial interest: the calina particles seem to be a cause of short circuits on high-voltage lines in Venezuela, not only near the shore but in the interior. The high voltage attracts the particles, and the insulators get covered with them. There has been a serious increase of this kind of accident this year.

"Now, at last, it is raining on the parched soil of Venezuela. Between showers the air is clear again and I can enjoy the beautiful view of Mount Avila from my office window."

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