The Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half ball'. It is also that half of the celestial sphere north of the celestial equator. Earth's northern hemisphere contains much of the land and most (about 90 %) of the human population.

In the temperate regions of the hemisphere, winter lasts from December 22 to March 20 (though winter weather can begin as early as September and last into May or even June in arctic regions) and Summer from June 21 to September 22.

Earth's perihelion (when the earth is closest to the sun) occurs in Northern hemisphere winter. This is because the earths axial tilt has the larger effect on weather than the distance from the sun. Because winter occurs at periphelion (during which its orbital motion is fastest), winter has a technically shorter duration than the southern hemisphere. However the moderating effect of the larger southern hemisphere oceans more than compensates for this effect, resulting in the northern hemisphere having colder and more severe winters. Tropical regions tend to experience a rainy season during the 'summer' months, and a dry season during the 'winter' months.

Regions north of the Arctic Circle will experience some days in summer on which the sun never sets, and some days in winter on which the sun never rises. The duration of these phases varies from one day for places right on the Arctic Circle to several months near the North Pole itself.

Hurricanes and tropical storms spin counter-clockwise in the northern hemisphere due to the Coriolis effect. In contrast they spin clockwise on the southern hemisphere. The shadow of sun dials moves clock-wise on the northern hemisphere (opposite of the southern hemisphere). During the day the sun tends to raise to maximum at a southerly position, where as in the southern hemisphere it raises to a maximum that is northerly in position (as the it tends towards the direction of the equator). In both hemispheres the sun rises in the east and sets in the west.

Also the Moon appears "upside down" compared to a view from the southern hemisphere and the view of the stars is different. The north pole faces away from the galactic centre of the Milky Way, this results in their being far fewer and less bright visible stars in the Northern Hemisphere compared to the Southern Hemisphere, making the Northern Hemisphere more suitable for deep-space observation as it is not 'blinded' by the milky way.