75% of the images we receive in July are daytime imagery, because we are in summertime and the days are long. For these daytime images, cloud removal is easy. AVHRR not only sees temperature, but also visible light and Near IR, which is almost visible. To remove clouds from an image, one simply needs to choose a threshold in either channel 1 or 2, and set any pixel in an image that is above this threshold, or reflectance, as bad. Clouds reflect a lot of light, while the ocean does not.

But in night data, these visble and Near-IR channels are not available because it is dark. So, to remove data from these images one typically compares every pixel to climatology (an average, or known temperature for that area). So, let's compare to climatology (figure 5) .