You've probably seen movies or cartoons about characters stranded in the desert. With the sun scorching the sand with incredible heat, they trudge along in search of water. Thirst becomes overwhelming. Sweat soaks their clothes.
They push on, but they never seem to get any closer to the oasis. It remains out there on the horizon, hovering just out of reach. How can this be? Then someone realizes what's going on: it's a mirage!
Although they may be a prominent feature of cartoons, mirages are real. They're caused by light refraction, which is a fancy scientific term that means the bending of rays of light. Mirages are common in hot areas. When you're driving along a blacktop highway in high heat, it's very common to look into the distance and see something that looks like water near the horizon.
Mirages happen when there's a quick change in the density of air in the atmosphere. This occurs when the air in one area is much hotter than the air around it. For example, a hot asphalt highway surface will heat the air just above it to a much higher temperature than the other air in the area.
As light rays pass from the cooler air to the hotter air, they bend and create mirages. The light bouncing toward your eyes at different angles makes it seem like you're seeing a mirror image of what's ahead. Your brain interprets this as a reflection caused by a pool of water.
Of course, all you need for a mirage is a difference in air density caused by a difference in air temperature. Although mirages are often associated with high heat, such as in the desert, they can also occur in icy areas or over cold water.
In these situations, the mirages usually make things appear higher than they really are. For example, a boat might appear taller than it is or a body of land might appear to be floating in the air.
In polar regions, you may from time to time be able to see very special, quickly-changing mirages called Fata Morgana. These complex mirages appear often above mountain ranges and can include a variety of stretched and compressed images that look like they're stacked on top of each other.