The changing tilt of Earth on its axis causes days to become shorter and nights to become longer during winter, and the tilted hemisphere is colder due to the sun hitting the Earth at an oblique angle. North of the equator, winter occurs when the northern hemisphere tilts away from the sun, while the opposite tilt leads to winter in the southern hemisphere.
Lower temperature occur in winter because the sun's rays must travel farther through the atmosphere to reach the tilted hemisphere, which allows more of the heat to dissipate before it reaches the surface. The colder winter temperatures allow the ice crystals that form in clouds to join together and fall to the surface, causing snow in place of rain. A lack of warm air decreases the probability of thunderstorms caused by colliding warm and cold fronts. To cope with falling temperatures, many animal species migrate to warmer areas near the equator. Other species build up reserves of fat and fur to keep warm whether they stay active or hibernate. The winter solstice occurs on the day with the longest amount of darkness and the shortest amount of sunlight. Some of the coldest winters on record occurred between 1683 and 1684, 1739 and 1740, and in 1816.