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The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator.
Due to the Earth's axial tilt, winter lasts from the winter solstice (typically December 22) to the vernal equinox (typically March 20), while summer lasts from the summer solstice (typically June 21) through to the autumnal equinox (typically September 21).
The Arctic is the region north of the Arctic Circle. Its climate is characterized by cold winters and cool summers. Precipitation mostly comes in the form of snow. The Arctic experiences some days in summer when the Sun never sets, and some days during the winter when it never rises. The duration of these phases varies from one day for locations right on the Arctic Circle to several months near the North Pole.
Between the Arctic Circle and the Tropic of Cancer lies the Northern Temperate Zone. The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. However, a temperate climate can have very unpredictable weather.
Tropical regions (between the Tropic of Cancer and the equator) are generally hot all year round and tend to experience a rainy season during the summer months, and a dry season during the winter months.
In the Northern Hemisphere, objects moving across or above the surface of the Earth tend to turn to the right because of the coriolis effect. As a result, large-scale horizontal flows of air or water tend to form clockwise-turning gyres north of the equator. These are best seen in ocean circulation patterns in the North Atlantic and North Pacific oceans. South of the equator, the directions are reversed.
For the same reason, flows of air down toward the northern surface of the Earth tend to spread across the surface in a clockwise pattern. Thus, clockwise air circulation is characteristic of high pressure weather cells in the northern hemisphere. Conversely, air rising from the northern surface of the Earth (creating a region of low pressure) tends to draw air toward it in a counterclockwise pattern. Hurricanes and tropical storms (massive low-pressure systems) spin counter-clockwise in the Northern Hemisphere (by contrast, they spin clockwise in the Southern Hemisphere).
The shadow of a sundial moves clockwise in the Northern Hemisphere (opposite of the Southern Hemisphere). During the day, the Sun tends to raise to its maximum at a southerly position, whereas in the Southern Hemisphere it raises to a maximum that is northerly in position (as 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 very different. The North Pole faces away from the galactic centre of the Milky Way; this results in there 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.
List of continents and countries 
Continents of the Northern Hemisphere 
- all of Europe
- all of North America, Central America and the Caribbean
- the vast majority of Asia, except East Timor and Indonesia (mainly in Southern Hemisphere)
- about 2/3 of Africa, just above the "horn"
- 1/10 of South America, north of the mouth of the Amazon River.
See also 
- Merriam Webster's Online Dictionary (based on Collegiate vol., 11th ed.) 2006. Springfield, MA: Merriam-Webster, Inc.