However, it was only with the development of remote sensing instruments operating from satellites that the cause for this global disruption of weather patterns was discovered. Remote sensing technology uses satellite instruments to view the Earth in wavelengths extending from the visible part of the electromagnetic spectrum through the thermal infrared to the microwave region. Satellite instruments are mostly passive, which means they record reflected or emitted radiation from the surface. Passive sensors more frequently operate only during the day and cannot penetrate clouds.
Active sensors provide their own sources of illumination.
One common type of active sensor that operates in the microwave region is called radar. Radar instruments can operate in any type of weather and day or night. They record data either on film or convert light to electrical signals that are recorded digitally. Making observations in the light bandwiths beyond the visible part of the spectrum requires the use of detectors rather than film. These detectors are sensitive to reflected or emitted electromagnetic radiation in other parts of the spectrum and usually record the received signals digitally for transmission back to receivers on the Earth.