A multi-spectral imager useful for weather mapping, comprising an array of filters on at least one focal plane array (FPA) including pixels. Each of the filters are associated with a different set of the pixels, and each of the filters transmit a portion of electromagnetic radiation, comprising a different band of wavelengths, to the set of the pixels associated with the filter. A circuit connected to the pixels reads out a signal outputted from each of a plurality of different pixels in the set and outputs the signals to an adder. The adder sums the signals from each of the plurality of different pixels in the set to form a sum used for generating a weather map.