There are two types of mirages: Inferior and Superior
An inferior mirage is caused by a reflection in the air. In deserts and road ways the air right above the surface can be up to 30 degrees Fahrenheit (about 16 degrees Celsius). Since cooler air has much greater density, the light reflection can be bent. The mirage is a reflection of the sky and gives the appearance of water.
A superior mirage is also caused by a reflection in the air. This occurs though when the surface is cooler than the air. These are found in the polar regions and around lakes. With the curvature of the earth, a superior mirage gives the impression of a towering horizon which appears closer than it actually is.