The brine in sea ice works to keep salinity in equilibrium with temperature. Therefore, when the temperature of sea ice changes, its salinity moves to the equilibrium level, by separating or thawing ice crystals in the sea ice. When rising summer temperatures bring sea ice close to the melting point, even if the air temperature is below the freezing point, the brine volume of the sea ice increases and nearly a half of the weight of the sea ice comes to consist of brine. This marked increase in brine volume rapidly weakens the sea ice.
The brine above the water surface collects in a depression on the sea ice surface, forming pools called puddles. The puddles absorb solar radiation and thaw the ice that surrounds them, growing larger and deeper. When the air temperature temporarily drops, however, the puddle becomes covered with a thin lid of ice; given a sufficiently strong wind, even when the temperature stays around the freezing point the lid is preserved due to latent heat of evaporation. The water inside the puddle covered by the lid absorbs solar radiation, which often keeps it higher than the freezing point. Because the salinity of the water in the puddles is lower than the water in the sea below it, even if the thawing process thins the puddle bottom and produces a hole in it, the puddle water's lower density will prevent it from leaking down into the sea. On multi-year ice, puddle water serves to wash out brine, forming a layer of surface ice with almost zero salinity. In fact, in summertime the water from puddles on multi-year ice is often used as drinking water.
Puddle formation is deeply affected by the snow cover, beginning in the fast ice at lower latitudes and gradually proceeding northward to the center of the Arctic Ocean. On the NSR coast, puddles begin to form in late May; puddle formation in the center of the Arctic Ocean starts one month later, in late June. The appearance of puddles heralds the arrival of the brief Arctic summer. The sea ice gains a huge thermal inertia due to the latent heat in the thawing process of ice, stabilizing the temperature in ice-covered waters.