Backscatter is the reflection of light, radar, radio, or other electromagnetic waves directly back to the direction they came from. This phenomenon is the cause of alpenglow and gegenschein, and is also the principle behind all radar systems.
In weather radar, the strongest backscatter comes from graupel (solid ice). This causes sleet and hail to often show up as much higher rates of precipitation than are actually occurring. Rain has moderate backscatter, being stronger with large drops (such as from a thunderstorm) and much weaker with small droplets (such as mist or drizzle ). Snow has rather weak backscatter, because the spiky crystals tend to scatter in all directions, rather than straight back.