English: On January 15, 2020, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired a true-color image of ice on Alaska’s Cook Inlet. Cook Inlet receives fresh water from a number of rivers, creating areas of nearly-fresh water. The variations in salinity create a complicated pattern of ice formation. Freshwater is unlike most substances because it becomes less dense as it nears the freezing point, so very cold, low-density fresh water stays at the surface. In contrast, the salt in ocean water causes the density of water to increase as it approaches the freezing point, so very cold ocean water tends to sink. As a result, sea ice forms more slowly that ice in fresh water, because salt water sinks away from the cold surface before it freezes. In addition, while the freezing temperature of fresh water is 32°F (0°C), the temperature in ocean water must reach 28.8°F (-1.8°C) in order to freeze. In general, the top 300-450 feet of ocean water must be cooled to the freezing temperature before sea ice can form.