Microwave (MW) imagers observe atmospheric “window” regions where atmospheric absorption mainly by oxygen and water vapour is weak, meaning they can “see through” the atmosphere so that radiation measured comes mainly from the surface and from cloud/rain/ice. Window examples include ~6–7 GHz, 10–11 GHz, 18–19 GHz, 36–37 GHz, 89 GHz, 150 GHz (exact choices differ by instrument).