Wallraff's gradient theory overcomes the problem of distance limitation via different means. It proposes the existence of long-range, stable atmospheric odour gradients. The foundation for this navigational map is a spatial representation in which two or more environmental odours have a particular intensity. Odour gradient differs along dissimilar directional axes and, therefore, the pigeon can compare the intensity of the scent at a particular location to its concentration at the home loft. This mechanism in principle could operate over vast distances, but would require the detection and interpretation of minute differences in odour concentration. However, a more poignant question is the existence of predictable odour gradients. Meteorologists deny that odour gradients, as required by this hypothesis, exist in nature. The olfactory navigation hypothesis states that pigeons learn an odour map by associating smells perceived at the home loft with the directions from which they are carried by winds. Therefore, attempts to manipulate the development of that have involved changing the direction of wind, shielding birds from winds of a certain direction and exposing the pigeons to artificial odorants. The predication is that the experimental pigeons should learn an altered map and thus when released, they should fly according to their distorted perception. Such an experiment was conducted, where two groups of pigeons were reared in separate, although identical aviaries composed of bamboo. Group one had air blown from the south containing olive oil and air from the north containing synthetic turpentine. This was reversed for group two. The pigeons were then released east of the loft; half had a drop of synthetic turpentine added to the bill, while the others were given a drop of olive oil. Pigeons from group one exposed to olive oil flew north, contrary to birds sentient to synthetic turpentine, which flew south. Consistent, but reversed results were found in group two. However, it is important to note that there has been a failure to replicate these results in other countries, such as Germany, Italy and the United States, even when considerable effort has been made to employ identical procedures. Nevertheless, further experiments applied two different methods – namely the placement of fans near the home coop in order to reverse wind direction and usage of deflector lofts to shift the apparent direction of the wind by 90°. Deflector lofts comprised wooden or glass baffles, which deflected wind course and therefore any signature odours. Findings were that pigeons raised in such lofts oriented themselves with a magnitude of a 90° error, known as the ‘deflector loft effect’.