A trailer-based lidar, named Humidity and Aerosol Lidar (HAL), is being built as a remote sensing tool to characterize atmospheric aerosol and water vapor in the line-of-sight. Water vapor and aerosol in the lower atmosphere are critical components affecting the propagation of high-energy laser beams and microwave. The sensor is developed to collect high temporal and vertical resolution data of atmospheric aerosols and water vapor. This ground-based system also serves as a demonstration and an engineering study of a flight-capable sensor for real-time diagnostic of the atmosphere. The lidar, operating on the principles of differential absorption, could measure water vapor to 10 km altitudes. It also measures aerosols and cloud backscatter at altitudes up to 18 km and ranges up to 90 km. Operating with a hemispherical scanner, the sensor could map the 3-dimensional field of aerosols and water vapor and provide vertical as well as horizontal structures. A unidirectional Alexandrite ring laser, operating in single mode near 727.49 nm, is the laser source. The sensor is designed to operate in day and night time. A description of the system, its wavelength calibration unit, the transmitter-receiver system and projected performance will be discussed. Results of the photo-acoustic calibration cell and wavelength selections will be presented. Preliminary results of water vapor and aerosols will be discussed.