We present a powerful tool to analyze two-dimensional field-aligned images of equatorial plasma depletions taken from mid-latitudes. By shifting each individual image by the sum of the longitudinal offsets obtained by performing a cross-correlation between successive images, a single composite image can be formed for the entire night. It is shown that these field-aligned composite images give information on the observation times of the depletions associated with spread-F as they pass by Hawaii. In addition, they can be used to estimate the formation time and location of the depletions to the west of the observing site. This technique also yields estimates of the eastward velocity and velocity shear of the depletions. We suggest that the information gained by using these images can be used to predict satellite scintillations at stations to the east of the observing station.