In the Northern Hemisphere it is commonly regarded as extending from the  winter solstice (year’s shortest day), December 21 or 22, to the vernal equinox (day and night equal in length), March 20 or 21, and in the Southern Hemisphere from June 21 or 22 to September 22 or 23. The low temperatures associated with winter occur only in middle and high latitudes; in equatorial regions, temperatures are almost uniformly high throughout the year.