Urban visibility measurements during tropical weather events using image processing

Visibility measurements are useful in areas such as air traffic control, air pollutant monitoring, urban traffic monitoring, vehicle safety, and also rain monitoring. Instead of using transmissometers and nephelometers, a simple off the shelf camera was deployed on the 11th floor of a multi-tenant building to sample a scene with a wide depth of field. The camera was automated to take pictures once per minute for seven days during and after a typhoon. Selected daytime events were processed. Three different patches of varying distances from the camera were processed for 6 different channels (RGB and HSV) for the entire duration using 6 different criteria - mean, column contrast, row contrast, gradient mean, spectral norm, and information entropy. In a separate deployment over the same area, an acoustic sensor was deployed on the field that corresponds to one patch in the image. It was demonstrated that the contrasts move together with the gradient, the spectral norms move together with the mean, while the saturation entropy has an inverse relationship with the contrast as well as the mean. It was also demonstrated that the envelope of the RGB mean follows the envelope for the acoustic sensor.