Male northern hawk-owls are generally 36–42.5 cm (14.2–16.7 in) long and weigh 300 g (11 oz). Females are slightly bigger with a length of 37.2–44.7 cm (14.6–17.6 in) and a mass of about 340 g (12 oz). Both male and female have similar wingspans of about 45 cm (18 in). The northern hawk-owl plumage is relatively dark brown with an off-white spotting pattern on all dorsal parts of the body with the exception of the back of the neck which boasts a black v-shaped pattern. The underbelly is generally white or off-white which continues to the toes with brown bands on the breast and stomach. It also boasts a long tail with brown banding. The northern hawk-owl has a smokey white face with a black border, a flat head, yellow eyes and a yellow curved beak.
The northern hawk-owl has been said to resemble a hawk in appearance and in behavior. In North America, its appearance in flight is often considered similar to a Cooper's hawk (Accipiter cooperii). It has been suggested that this may be because the hawk-owl may partially fill an important diurnal niche similar to that of day hunters such as hawks.
S. ulula has a variety of calls used by the different sexes in different situations. When attracting a mate the male usually lets out a rolled whistle of ulululululululul and a sound similar to tu-wita-wit, tiwita-tu-wita, wita, when perching at a potential nest site. The female’s call is usually less constant and more shrill.
When alerting to danger, the northern hawk-owl lets out a sound similar to rike, rike, rike, rike. It also releases a high pitched scream followed by a yip when an intruder is near to the nest. To warn of impending dangers to a fledgling, the hawk-owl will let out a noise similar to ki ki kikikikiki. Calls can vary in length from 15 s to 2 min.