Water vapor...OR if you want to be anal and minus me try this. (mÄ­räzh') , atmospheric optical illusion in which an observer sees in the distance a nonexistent body of water or an image, sometimes distorted, of some object or of a complete scene. Examples of mirages are pools of water seen over hot desert sand or over hot pavement; at sea, an inverted image of a ship seen in the heavens or, also at sea, some object that is actually over the horizon but seems to loom up a relatively short distance away. These phenomena can be explained by the facts (1) that light rays undergo refraction, i.e., are bent, in passing from a medium of one density into another of different density and (2) that the boundary between two such media acts as a mirror for rays of light coming in at certain angles (see reflection). Ordinarily the density of the atmosphere gradually decreases with altitude. Variations in temperature disturb the normal state (the density of warm air is less than that of cold air), producing unusual variations in the density of the atmosphere. The “lake” mirage in the desert is essentially a reflection of the sky. Light rays coming at a grazing angle from the sky just above the horizon are thrown upward by the surface of the area of extremely hot air just above the sand, and the effect to an observer is a shimmering reflecting expanse resembling the surface of a body of water. The inverted image of a ship seen in the heavens at sea is caused by a layer of dense, cool air over the water; this layer bends the rays of light from the ship (below the horizon) in a curved path that arches over the horizon and back to earth. The image formed appears to be that produced by an object somewhere distant in a straight line from the observer and, therefore, at a position in the sky. It is sometimes inverted because in the bending process the light rays coming from the object are changed in relative position. The type of mirage described as looming, in which distant objects appear much nearer than they actually are, is explained in the same way as the image of the ship, except that the image is not inverted; the density variations may also act as a magnifying glass. Mirages can be photographed. The strange phenomenon known as the fata morgana [Ital.,=Morgan le Fay, of the Arthurian legend, the supposed author of the mirage] is a complex mirage especially in evidence at the Strait of Messina; in this mirage images of objects such as ships, houses, or men, often two of the same object with one inverted, are seen suspended in the air over the object itself or on the water. furthermore...Cold air is denser than warm air, and has therefore a greater refractive index. As light passes from colder air to warmer air it bends away from the direction of the temperature gradient (the "normal" in the figure at right); when it passes from hotter to colder, it bends towards the direction of the gradient. The diagram on the right shows a light ray coming from the sky toward the hot ground. If the air near the ground is warmer than that higher up, the light ray bends in a concave upwards trajectory. Once the ray reaches the viewer’s eye, the eye traces it as the line of sight, which is the line tangent to the path the ray takes at the point it reaches the eye. The result is that an inferior image for the above sky appears on the ground. The viewer may incorrectly interpret this sight as water reflecting the sky. In the case where the air near the ground is cooler than that higher up, the light rays will of course curve downwards, producing a superior image.

Frequently, while driving along a highway on a clear, hot day, we see what appears to be pools of water some distance ahead. However, as we approach it, we find that the road surface that seemed wet before is actually dry. On looking ahead , the wet patch appears to have moved further away on the road. When the sun goes high in the sky, the road gets heated first and then the layers of air above it. Thus the layer of air closest to road is hottest and optically least dense whereas, the layers higher up are colder and optically more dense. The rays from the sky travel from an optically denser to a rarer medium and hence bend away from the normal. This bending continues and a stage is reached where the angle of incidence becomes greater than the critical angle and total internal reflection takes place. The totally reflected rays that reach the eyes appear to come from a point on the ground where the image of the sky is formed. Thus one sees a reflection of sky giving an illusion of water, though there is no water around.

Commander's answer was X L ent. The phenomenon is called a mirage, the ones in the desert have the same cause, a layer of hot air just above hot sand. Or the Fata Morgana at sea, hot air over real water. Sometimes they can actually reflect objects over the horizon miles away, so you see an actual oasis with palm trees that is not really where you think it is. Also you can see camels that seem to have legs twenty feet tall, the legs are reflected on the layer of air. I've never been in a camel desert, but in the US deserts and the palins where I grew up, I've seen twenty foot tall horses and people, I've seen plenty of those wet highways and reflected trucks and cars with tall skinny oval tires. I always get tensed up at the wheel, expecting an out of control skid when I hit that water, luckily the trucks are so high off the ground I slide right under them.

Up north when there was black ice on the road they seemed wet.