A radar optimized to detect clouds. This is usually a short wavelength radar often called millimeter radar, since its wavelength is usually near 8 mm (Ka band) or 3 mm (W band). Such shorter wavelengths give radars an advantage in cloud detection because of the radar scattering cross section for small particles varying as the inverse radar wavelength to the fourth power. Because attenuation caused by atmospheric liquid and from water vapor is serious at these short wavelengths, such radars are usually operated at ranges closer than about 20 km. They can be either of the scanning type or fixed in the vertical. Their good spatial resolution allows them to depict finescale cloud features and, when used in combination with other active (e.g., lidar) and passive (microwave and IR radiometers) sensors, can, under some conditions, be used to quantitatively map cloud ice and liquid water contents, particle sizes, and concentrations.