Sleet consists of transparent, solid grains of ice that are smaller than 5 millimeters (1/5 inch) in diameter. Sleet pellets are spherical or irregular in shape. They are formed by the freezing of raindrops or the refreezing of partly melted snowflakes. The raindrops or melted snowflakes fall from high altitudes in the atmosphere through a layer of below-freezing air near the earth's surface, changing into sleet. When sleet hits a hard surface, it bounces and makes a rapping sound.