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Franklin Explains The Waterspout
In Physical and Meteorological Observations: Conjectures and Suppositions likely compiled by Franklin in 1751and forwarded to his friend Peter Collinson and later published in theRoyal Society's Philosophical Transactions, LV (1765), Franklin addressed the weather conditions which form large eddies or whirlings. “Thus these eddies may be whirlwinds at land, waterspouts at sea.” He conjectured that these were formed by air ascending or descending that had attained a circular motion. The air within receded “from the middle of the circle by a centrifugal force, and leaving there a vacancy; if descending, greatest above, and lessening downwards; if ascending, greatest below, and lessening upwards; like a speaking trumpet standing its big end on the ground. When the air descends with violence in some places, it may rise with equal violence in others, and form both kinds of whirlwinds.”
He continued: “The air in its whirling motion receding every way from the center or axis of the trumpet, leaves there a vacuum; which cannot be filled through the sides, the whirling air, as an arch, preventing; it must then press in at the open ends. …. The air entering, rises within, and carries up dust, leaves, and even heavier bodies that happen in its way, as the eddy, or whirl, passes over land. If it passes over water, the weight of the surrounding atmosphere forces up the water into the vacuity… A body of water so raised may be suddenly let fall, when the motion, &c. has not strength to support it, or the whirling arch is broken so as to let in the air; falling in the sea, it is harmless, unless ships happen under it. But if in the progressive motion of the whirl, it has moved from the sea, over the land, and there breaks, sudden, violent, and mischievous torrents are the consequences.”
John Perkins Questions The Theory
In October 1752, Massachusetts physician John Perkins, who had read Franklin’s conjectures on the nature of waterspouts, questioned some aspects of it. He had doubts as to whether “water in Bulk or even broken into Drops ever ascends into the Region of the Clouds… I cannot conceive a Force producible by the Rarifaction [sic] and condensations of our Atmosphear [sic] in the Circumstances of our Globe capable of carrying Water in large portions into the Region of the Clouds. Supposing it to be raised it would be too heavy to continue the ascent beyond a considerable Height, unless parted into Small Drops: And even then by its centrifugal Force from the Manner of Conveyance it would be flung out of the Circle and fall scatter’d like Rain.”
In other words, Perkins could not conceive of an ascent of water from the surface into the cloud, only the descent of water from it to the surface.
A week later, Perkins sent on a second letter furthering his comments after viewing some drawings of waterspouts in the Mediterranean provided by a Mr Stuart in the Philosophical Transactions of the Royal Society. Perkins felt from observations of what ed called the “bush” — the hazy region where the spout touches the water surface and the spattering of the falling water occurs. “On the place of this Spattering arises the Appearance of a Bush into the Center of which the Spout comes down. This Bush I take to be form’d by a Spray made by the force of these Drops which being uncommonly Large and descending with unusual Force by a Stream of Wind descending from the Cloud with Them increases the height of the Spray which Wind being repulsed by the Surface of the Waters rebounds and Spreads…”
Further. “Over every one of Stuart’s Figures I see a Cloud: I suppose his Clouds were first, and then the Spout; I dont know whether it be so with all Spouts, but Suppose it is. Now if Whirlwinds carry’d up the Water I Should expect them in fair weather but not under a Cloud; as is observable of whirlwinds. They come in fair Weather, not under the Shade of a Cloud, nor in the Night since Shade cools the Air.”
He concludes: “This is easy to conceive in Case of descending Parcels of Drops through various winds, at least till the Cloud condenses so fast as to come down as it were Uno rivo: But it is harder to me to conceive it in the ascent of Water that it should be convey’d along, Secure of not leaking or often droping through the under Side in the prone part. Another Thing is the appearance of the Spout coming from the Cloud. This I cant account for on the notion of a Direct Spout. But in the real descending one it is easy. I take it that the Cloud begins first of all to pour out drops at that particular Spot or Foramen.”
“If Spouts ascend it is to carry up the warm rarified Air below, to let down all and any that is colder above: And if so they must carry it through the Cloud they go into (for that is cold and dense I imagin [sic]) perhaps far into the high Region making a wonderfull [sic]appearance at a convenient Distance to Observe it, by the Swift rise of a body of vapour above the region of the Clouds. But as this has never been Observ’d in any Age, if it be supposable thats all.”
Perkins then asked: “Whether a Violent Tornado of a Small Extent and other Sudden and Strong Gusts be not Winds from Above descending nearly perpendicular. And whether many that are call’d WhirlWinds at Sea are any other than these, and so might be call’d Air Spouts, if they were Objects of Sight.”
In February 1753, Franklin answered the Perkins letter, presenting a detailed, nearly 4000-word, description of his theories on the topic. First, he agreed that waters could not be drawn up into the air by a vacuum to a height greater than 30 feet. Franklin’s viewing of the Stuart diagrams led him to believe they favored his hypothesis.
Franklin answered Perkins’ last question first. Franklin supposed a whirlwind and spout as one and the same, save their location of occurrence. He is quite correct if he considered a whirlwind as a dust devil rather than a tornado, which is of the same genus Vortex, but a different species. I make this interpretation based on this sentence in his letter: “Whirlwinds generally arise after Calms and great Heats: The same is observ’d of Water Spouts, which are therefore most frequent in the warm Latitudes. … Whirlwinds and Spouts are not always tho’ most commonly in the Day-time.” If he had meant tornado rather than dust devil, Franklin would have emphasized the presence of a thunderstorm.
Later in the letter, Franklin does speak of a terrible Whirlwind hitting Rome on the night of 11 June 1749, which likely, from the damage described was actually a tornado. “In that Account the Whirlwind is said to have appear’d as a very black long and lofty Cloud, (discoverable notwithstanding the Darkness of the Night by its continually lightning or emitting Flashes on all Sides) pushing along with a surprizing Swiftness, and within 3 or 4 feet of the Ground. Its general Effects on Houses, were stripping off the Roofs, blowing away Chimneys, breaking Doors and Windows, forcing up the Floors, and unpaving the Rooms: (Some of these Effects seem to agree well with a supposed Vacuum in the Center of the Whirlwind); and the very Rafters of the Houses were broke and dispersed, and even hurled against Houses at a considerable Distance, &c.”
Franklin correctly determined that “A Fluid moving from all Points horizontally towards a Center, must at that Center either ascend or descend. Water being in a Tub, if a Hole be open’d in the Middle of the Bottom, will flow from all Sides to the Center, and there descend in a Whirl. But Air flowing on and near the Surface of Land or Water from all Sides toward a Center, must at that Center ascend; the Land or Water hindering its Descent.”
Franklin then remarked: “There may be Whirlwinds of both kinds [ascending (rising) and descending], but from the common observ’d Effects, I suspect the Rising one to be the most common; and that when the upper Air descends, tis perhaps in a greater Body, extending wider and without much whirling as in our Thunder Gusts. When Air descends in a Spout or Whirlwind, I should rather expect it would press the Roof of a House inwards, or force in the Tiles, Shingles or Thatch; force a Boat down into the Water, or a Piece of Timber into the Earth than that it would lift them up and carry them away.” [Perhaps this is the first description of a downburst from a thunderstorm?]
“The Augmentation of the Cloud, which, as I am inform’d is generally if not always the case during a Spout, seems to show an Ascent rather than a Descent of the Matter of which such Cloud is composed. For a descending Spout one would expect should diminish a Cloud.”
In the discourse, Franklin understood that “the lower Region of Air is often more heated and so more rarified, than the upper; consequently specifically lighter.” In the situation of the waterspout, “heated Air may be very moist, and yet the Moisture so equally diffus’d and rarified, as not to be visible, till colder Air mixes with it, when it condenses and becomes visible. Thus our Breath, invisible in Summer, becomes visible in Winter.”
He then set out a situation where clean and calm daytime weather in the summer will greatly heat the surface of the land/sea “together with the lower Region of Air Contact with it, so that the said lower Air becomes specifically lighter than the superincumbent higher Region of the Atmosphere, in which the Clouds commonly float. … The Consequence of this should be, as I imagine that the heated lighter Air being press’d on all Sides must ascend, and the heavier descend; and as this Rising cannot be in all Parts or the whole Area of the Tract at once, for that would leave too extensive a Vacuum, the Rising will begin precisely in that Column that happens to be the lightest or most rarified; and the warm Air will flow horizontally from all Points to this Column, where the several Currents meeting and joining to rise, a Whirl is naturally formed…”
Franklin then argued that the region around the base of the spout should have the greatest forces acting on it and these forces should diminish with altitude, and it is these differing forces, which form the long, sharp cone of the spout. The figure here below comes from that letter.
“In Fig I. which is a Plan or Ground Plot of a Whirlwind, the Circle V represents the central Vacuum. Between aaaa and bbbb I suppose a Body of Air condens’d strongly by the Pressure of the Currents moving towards it from all sides without, and by its Centrifugal Force from Within; moving round with prodigious Swiftness, (having as it were the Momenta of all the Currents united in itself) and with a Power equal to its Swiftness and Density.”
“It is this whirling Body of Air between aaaa and bbbb that rises spirally. By its Force it tears Buildings to Pieces, twist up great Trees by the Roots, &c. and by its spiral Motion raises the Fragments so high till the Pressure of the surrounding and approaching Currents diminishing can no longer confine them to the Circle, or their own centrifugal Force encreasing grows too strong for such Pressure, when they fly off in Tangent Lines as Stones out of a Sling, and fall on all Sides and at great Distances.”
“If it happens at Sea, the Water between aaaa and bbbb will be violently agitated and driven about, and parts of it raised with the spiral Current, and thrown about so as to form a Bushlike Appearance.”
“Fig II. is to present the Elevation of a Water Spout; wherein I suppose PPP to be the Cone, at first a Vacuum till WW the rising Column of Water has fill’d so much of it. SSSS the Spiral Whirl of Air surrounding the Vacuum and continu’d higher in a close Column after the Vacuum ends in the Point P. till it reach the cool Region of the Air. B.B. the Bush describ’d by Stuart, surrounding the Foot of the Column of Water.”
“Now I suppose this Whirl of Air will at first be as invisible as the Air itself tho’ reaching in reality from the Water to the Region of cool Air in which our low Summer Thunder Clouds commonly float; but presently it will become visible at its Extremities. At its lower End by the Agitation of the Water, under the Whirling Part of the Circle, between P and S. forming Stuart’s Bush, and by the Swelling and Rising of the Water in the beginning Vacuum, which is at first a small low broad Cone whose Top gradually rises and sharpens as the Force of the Whirl increases. At its upper End, it becomes visible by the Warm Air brought up to the cooler Region, where its Moisture begins to be condens’d into thick Vapour by the Cold, and is seen first at A. the highest Parts, which being now cool’d, condenses what rises next at B. which condenses that at C; and that condenses what is rising at D. The Cold operating by the Contact of the Vapours faster in a right Line downwards, than the Vapours themselves can climb in a spiral Line upwards; they climb however, and as by continual Addition they grow denser and consequently their centrifugal Force greater, and being risen above the concentrating Currents that compose the Whirl, they flie off, spread and form a Cloud.”
“It seems easy to conceive, how by this successive Condensation from above the Spout appears to drop or descend from the Cloud, tho’ the Materials of which it is composed are all the while ascending.”
“The Condensation of the Moisture contain’d in so great a Quantity of warm Air as may be suppos’d to rise in a short Time in this prodigiously rapid Whirl, is perhaps sufficient to form a great Extent of Cloud, tho’ the Spout should be over Land as those at Hatfield; and if the Land happens not to be very dusty, perhaps the lower Part of the Spout will scarce become visible at all; Tho’ the upper or what is commonly call’d the descending Part be very distinctly seen.”
“The same may happen at Sea, in case the Whirl is not violent enough to make a high Vacuum and raise the Column, &c. In such Case the upper Part ABCD only will be visible, and the Bush perhaps below.”
“But if the Whirl be strong, and there be much Dust on the Land, or the Column WW be rais’d from the Water; then the lower Part becomes visible, and sometimes even united to the upper Part. For the Dust may be carried up in the Spiral Whirl till it reach the Region where the Vapour is condens’d, and rise with that even to the Clouds.”
In this description, Franklin hit close to our understanding of vortices, whirls and spouts, but assumed that the water/dust is raised by “vacuum” forces rather than carried by the winds themselves, drawn upward by rising thermals. But considering the lack of personal observation and measurements, he laid a good groundwork for later scientists to improve on.
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