I have never worked on a farm, nor have any source to tell me why, but I'm just going to take a shot in the dark. I assume that the silo is not only used for storage, but also a dispensary, where trucks or trailers or other containers come to be filled. This being the case it would be similar to water towers used to fill trucks and trains, or even supply water to a home or group of homes. The main reason is the fact that it is a gravity feed system. The higher the water tower, the higher the pressure and the more homes it can supply while still having a decent pressure, with the silo, the higher and narrower the structure, the easire it is to completely empty the product without having to build conveyer systems or manually empty it. Why, exactly? At the bottom there must be a cone, concave up, which the grain slide down into a chute, which diresct the flow into the desired location. The cone must have a minimum slope to ensure that the grain can overcome friction forces and actually slide or tumble, and not get stuck. The larger the diameter of the building, the higher the cone must be, and results in more unusable storage space under the cone. So the narrower the structure, the higher the ratio of useable space to wasted space, lowering storage costs as it reduces storage facility construction costs by utilizing less material, less labour and takes up less space. Having a high tower also allows the nozzle used for filling trucks or trailers to be higher, since the pressure of the grain inside the tower pushing down on the grain below it might actually force it uphill through the nozzle, up to the height of the grain inside, minus a bit for friction forces, the smaller tower means that if the level does not go down to 0, whatever is left over is a small radius, therefore low volume of residue left in the "empty" tower. This also enables higher sides on the trailers so they can be loaded more heavily, minimizing transportation costs by maximizing payloads. Sorry if the answer meanders a bit, the inability to insert diagrams limits the clarity of this particular answer.

Many things including storage need is what determines the size and shape of grain silos. The size and shape depends on the volume of grain to be stored or if it's inland storage and shipping. I live in an area of inland shipping and grain is one of our main exports. ------------------------- Firstly: grain exerts a horizontal pressure on the structure which contains it. A round store will resist this pressure through the development of hoop tension forces which are very efficiently resisted (eg by steel reinforcement). A rectangular structure must resist grain loads through the development of bending stresses which are less efficiently resisted than tensile loads since both tensile and compressive forces have to be resisted. In addition, in the case of a rectangular 'horizontal' store, the walls act as retaining walls and their foundations have to resist overturning moments caused by the grain loads. Foundations for cylindrical structures have mostly to resist the vertical loads imposed on them from the walls. Secondly: the roof structure of a rectangular structure has to carry its loads in bending, compared to the roof of a cylindrical structure which can be designed as a shell (for instance a cone), which carries its loads in direct compression and tension. Another important advantage of cylindrical structures is that they have less joints. In the case of silos where the bins are independent (i.e. not connected to each another), there is a joint between the wall and the floor, and a joint between the wall and the roof. It is thus usually a relatively simple matter to seal these joints to make the structure air-tight and suitable for fumigation. Ideally the roof should be rigidly attached to the wall, since this not only makes sealing easier, but also greatly increases the stiffness of the wall in resisting bending stresses. In such cases silicone type sealants are useful for sealing the roof-wall joint, however where a sliding joint is required bituminous based mastic sealants sandwiched between the joining surfaces have been successfully used. Rectangular structures, by comparison, have more joints (for instance at the comers) and by the nature of their construction, sealing for fumigation is generally more difficult to achieve. Horizontal stores have another inherent disadvantage, in that they require more complicated and longer conveying systems to place grain in them. Usually an internal conveyor is required with a tripper (or similar device) to spread the grain over the floor surface. A cylindrical store, on the other hand, requires only a central point for filling. Where insects are present, temperatures are high, and most especially where moisture content is above safe levels, then storage of grain becomes both risky and difficult, and losses will be difficult to avoid. It is in these circumstances that the type of store and its design become critical to the safety of the stored grain. Where harvested grain is to be received over a short period, dried over a longer period and dispatched over 12 months, it is a simple matter to calculate the buffer storage requirements for wet and dry grain. More complex models are required where the rates of receival and out-turn are less clearly defined; for instance at a shipping import terminal where the rate of receival will depend on ship arrival rates, berth availability etc. and where out-turn rates may depend on the inland transport system. In such situations computer models are very helpful in optimising various design parameters including the storage volume, number of berths, ship unloader capacity and out-turn capacity. An optimum-cost solution can thus be determined through sensitivity analysis, by varying the values of the input parameters. Tall or Short In the case of flat bottom stores, structural efficiency is also increased by minimising the height of the structure. For a given volume of storage, the lower the height of the walls, the more grain pressure is applied directly to the floor surface and the less load there is on the walls. Furthermore, the minimum structural surface area (and hence cost) for a given volume of grain, is achieved if the wall height is relatively low. For instance in the case of a cylindrical 'tank' with a conical roof, the minimum surface area of walls and roof is achieved when the wall height is around half the radius of the bin. It is thus no coincidence that the lowest cost stores are generally in the shape of squat cylindrical 'tanks' where the walls are relatively low compared to the diameter. http://www.fao.org/documents/show_cdr.asp?url_file=/docrep/T1838E/T1838E19.htm

In addition to the point made in the other answers about the use of gravity to dispense the grain from the tall silos... If you had the choice between 2 silos, both would hold the same amount of grain, but one of which would take up a HUGE amount of your land, and the other would take up much less, which would you choose? Especially if you thought that later you might need more than one? If you could build 2 or 3 tall silos on the area of land where you could have built just one very wide silo, wouldn't it make sense to build the tall ones? With land prices so high in some areas, it makes sense to build the structures which take up less space. That way you can use the extra land for something else.

There are many reasons why grain is stored in vertical storage facilities, but some of them stem back more than a century. In the end, it is cheaper to store the product in an upright storage facility. Several tall silos take up less land than one short and wide silo, requiring smaller foundations and lowering land costs. The facilities can also take advantage of gravity when vehicles are unloaded and loaded. Grain is stored in different ways, depending on which stage of the handling process you observe. When wheat and other grain crops are harvested, the grain is separated from the chaff by the combine-harvester and dumped directly into dump trucks or trailers in the field. It is then transported to grain elevators or to storage facilities at the farm if it is not sold at that time or the farmer sells directly to a buyer. At the grain elevator, the grain is received, weighed, graded, and dumped into a holding pit. The grain is then lifted to the top of the storage facility by an elevator with bins on a continuous belt that scoop up the grain, lift it to the top of the elevator, and dump it into the storage area. Older elevators were build of wood and had a single storage area with a square cross-section; newer ones are usually round and made of steel. Somewhere in my father's records, there is a photo of the grain elevators my grandfather owned, before he sold out to the co-ops in the 1930s. Grain elevators are usually marked with the name of the co-op, company, or individual who owns them. The grain is then loaded onto rail grain cars or, less commonly, large transport trucks for shipment to larger facilities. The grain is loaded using gravity - one of the advantages of storing the grain above the level of the highest vehicle it loads. The grain is transported to large centralized storage facilities, usually in cities with access to the water. It is then transported by ship or other means to commercial or international buyers.

First let me apologize, I misread xander's "concave", duhhh, next time draw a picture, xander. Let's make sure we know what you are talking about here. By "grain silo" I think you mean "grain elevator" and not the silo beside the red barn, familiar from romanticized pictures of farms. ( That silo is used for storing fodder (animal feed) which is kept fresh by allowing a certain amount of more or less controlled fermentation, such preserved fodder is called silage. The word silo has come to be applied to all kinds of tall cylindrical structures, or even holes, 'missle silos.' But a silo does not have to be tall or cylindrical, in fact the word comes from Spanish. silo, from L. sirum ,from Gk. siros "a pit to keep corn in." or possibly from an unrelated but similar sounding and similarly defined, non Roman, Basque word, zulo "dugout, cave or shelter for keeping grain." So a missile silo is more of a silo than a grain elevator is. The tall 'silage' silo is primarily of American origin. The main advantage is that it allows gravity to compress the fodder as an aid to fermentation, other methods require some sort of mechanical compression. The chopped fodder is blown thru tubing to the top of the silo and allowed to settle. Another reason concerns xander's funnel at the bottom, which allows collection of the liquid byproduct of the fermentation process where it can be drawn off. Or "tapping a silo" as it has become known to countless underage farmboys and unfortunates farming in 'dry' counties. Actually the liquid is mostly nitric acid which makes a lousy Manhattan even in Kansas. So xander wasn't far off in assuming that gravity plays a part, the "less fresh" silage is removed from the bottom thru various chutes and mechanical lifts and fresh fodder can be added at the top in a more or less continuos process. Luckily I see that Alatea has given a nice explanation of the engineering concerning cylinders so I don't gotta. Many 'siloish' structures are constructed of precurved metal panels, the curve gives the individual panels a certain stiffens that makes them easier to handle during construction. There are grain storage facilities that are not tall, like the squat tanks Alatea described, they are becoming way more common than elevators, cheaper to build, etc., but an advantage of being tall is that it takes less ground space. That can be important in crowded ports and RR yards. Of course the 'grain elevator', strictly speaking, is the device for lifting the grain into the storage tower or 'silo.' I grew up in the Texas Panhandle and the entire complex of silos and other buildings is called an elevator, just as a cotton facility is called the gin. The elevators are so common, back there that I don't think I really noticed them until I moved away and then had a vague feeling that something was missing up there in the sky. While traveling, a grain elevator appearing over the horizon was more a sign that we were nearing a town than a water tower. There was medium sized elevator, only 16 silos, on the edge of town just a few blocks from our house, where we would go with our BB guns and air rifles to shoot rats. We were usually welcome to do so as long as we were careful "with them guns, donchew boys go shootin at the shed. And keep away from the pit!" There were a lot of rats, attracted as they are to tall cylindrical structures. Not all elevators are round, in fact lots of them that announced the nearness of a small town or railroad siding were rectangular. Today the only sign that there used to be a town is the lone remaining elevator. Most of those elevators are smaller and for a more temporary storage, grain is delivered by wagon, truck, or even directly from a combine and then transferred to larger trucks or railroad grain cars to the larger ones. Lots of the smaller ones are no longer used, as transportation costs lowered to where smaller loads can be carried directly to the larger facilities, which are usually cylindrical. And usually made of concrete, that is another reason they are cylindrical. For the same reasons Alatea explained a round form is stronger than a square one and can be lighter as the concrete is poured and cured, the forms are raised and another layer poured. xander's theory that the height allows the grain to be emptied into the tops of trucks and RR cars is also correct, but grain elevators are loaded and EMPTIED from the bottom to the top. A full truck is weighed, the grain dumped into the pit, then the empty truck weighed. The grain is carried to the top by buckets on a chain like a conveyor belt, the actual elevator, at the top it is dumped into a hopper, a spout from the bottom of the hopper can be positioned over which ever silo is wanted, different kinds or grades of grain, or just whichever one is emptiest, and the grain is delivered by gravity for storage. To empty grain, a gate is opened at the bottom of xander's funnel and the grain drops into a hopper for weighing, then dumped into another pit under the silos, then the grain is carried back UP to the top by the buckets and back into the hopper up there where the nozzle is connected to the delivery pipes where gravity carries it down to the delivery spout. xander, I again apologize for misreading your answer, One more thing, I, too, wonder why you have never worked on a farm, I wonder what the source for such an answer would be? I HAVE worked on a farm and I highly recommend it as something to be avoided if at all possible.