Our previous investigations using real-time total integrated scattering (TIS) have shown a very distinct onset of hillock formation in optical quality Al films on Si wafers at temperatures well below 100 degrees C. This paper extends those investigations by studying in real time the initial development of the lateral hillock distribution. After the first hillocks appeared the number of hillocks accelerated rapidly with increased temperature, in particular for higher heating rates. The lateral hillock distributions were further statistically analyzed by the Quadrate counts method to get more objective results. This analysis indicates that hillocks tend to be clustered over lar er areas. The nearest neighbor distance between hillocks was also calculated. The hillocks had a tendency of lining up in distorted ring shaped patterns. This is interpreted as an effect of inhomogeneous stress distributions at the local grain level. The observations were made possible by using an imaging partial integrated scattering (PIS) system comprising of a dark field microscope objective and a charge coupled device (CCD) camera with an image processor and a data acquisition system.