There are 4 color morphs of northern fulmars: very dark, dark, light, and very light. Color morphs seem to differ in their distribution during the breeding season and in the timing of their molt. The 3 recognized subspecies are distinguished by differences in bill length and thickness and the proportion of the different color morphs, although the subspecies do have individuals of multiple color morphs generally. Individuals of different color morphs seem to mate indiscriminantly, although breeding colonies tend to be made up mainly of a single color morph. Immature individuals cannot be distinguished from adults. Most molting occurs in July. Molting seems to make some populations unable to fly, but not others. Males are slightly larger, on average 835 g whereas females average 700 g (range of masses is 450 to 1000 g). The sexes are similar in overall appearance. Northern fulmars are from 45 to 50 cm long with wingspans of 102 to 112 cm.

Northern fulmars have thick, yellow to gray bills with darker areas over the "tubes." Their legs and feet are flesh-colored to gray. Dark color morphs are more common in the southern portions of their range in the Atlantic and the northern portions of their range in the Pacific. Light color morphs are more common in the northern portion of the range in the Pacific. Atlantic populations tend to have robust bills and are almost exclusively light color morphs, whereas Pacific populations have bills that are more slender and exhibit the full range of color variation. Light morphs are uniformly pale, with head, neck, and ventral surfaces white and with their backs and wings being gray. Dark morphs are uniformly dark gray. Nearly all individuals of any color morph have a light to white patch on the dorsal surface of their wings formed by the exposed lighter portion of their primaries, this is only lacking in the darkest of individuals. Individuals can vary between the very dark ("double dark") and light ("double light") morphs described above. Variation is more of less continuous, but is divided into 4 morph categories for convenience.

Northern fulmars can be confused with pink-footed shearwaters (Puffinus creatopus) or flesh-footed shearwaters (Puffinus carneipes), but can be distinguished by their thick, rounded heads and stubby bills.

Range mass: 450 to 1000 g.

Average mass: 700 to 835 g.

Range length: 45 to 50 cm.

Range wingspan: 102 to 112 cm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry ; polymorphic

Sexual Dimorphism: male larger