Sea ice concentration is a useful variable for climate scientists and nautical navigators. It is defined as the area of sea ice relative to the total at a given point in the ocean. This article will deal primarily with its determination from remote sensing measurements. Significance Sea ice concentration helps determine a number of other important climate variables. Since the albedo of ice is much higher than that of water, ice concentration will regulate insolation in the polar oceans. When combined with ice thickness, it determines several other important fluxes between the air and sea, such as salt and fresh-water fluxes between the polar oceans (see for instance bottom water) as well as heat transfer between the atmosphere. Maps of sea ice concentration can be used to determine ice area and ice extent, both of which are important markers of climate change. Ice concentration charts are also used by navigators to determine potentially passable regions—see icebreaker. Methods In situ Measurements from ships and aircraft are based on simply calculating the relative area of ice versus water visible within the scene. This can be done using photographs or by eye. In situ measurements are used to validate remote sensing measurements. SAR and visible Both synthetic aperture radar and visible sensors (such as Landsat) are normally high enough resolution that each pixel is simply classified as a distinct surface type, i.e. water versus ice. The concentration can then be determined by counting the number of ice pixels in a given area which is useful for validating concentration estimates from lower resolution instruments such as microwave radiometers. Since SAR images are normally monochrome and the backscatter of ice can vary quite considerably, classification is normally done based on texture using groups of pixels—see pattern recognition. Visible sensors have the disadvantage of being quite weather sensitive—images are obscured by clouds—while SAR sensors, especially in the higher resolution modes, have a limited coverage and must be pointed. This is why the tool of choice for determining ice concentration is often a passive microwave sensor. Microwave radiometry All warm bodies emit electro-magnetic radiation: see thermal radiation. Since different objects will emit differently at different frequencies, we can often determine what type of object we are looking at based on its emitted radiation—see spectroscopy. This principle underlies all passive microwave sensors and most passive infrared sensors. Passive is used in the sense that the sensor only measures