The winter solstice of the Northern Hemisphere arrived at 3:44 AM, Mountain Time. Last night was the longest of the year (about 15 hours at Denver's latitude) and today will bring the shortest period of daylight (about 9 hours). From now until the summer solstice (on or about June 21), our days will gradually lengthen.
The variable daylight results from the tilt of Earth's axis. On our annual journey around the sun, any given area on the planet receives more or less solar radiation as it leans toward or away from the sun. At the winter solstice of the Northern Hemisphere, the sun is shining directly on the Tropic of Capricorn while, at our summer solstice, maximum radiation reaches the Tropic of Cancer. Those who live near the Equator experience minimal variation in daylight throughout the year while polar regions go dark over two months surrounding their winter solstice and receive perpetual light for a month on either side of their summer solstice. In effect, the variation of sunlight between summer and winter increases as one moves farther north or south of the Equator and we notice this phenomenon as the sun moves higher or lower in the sky (highest at the summer solstice and lowest at the winter solstice).
While the winter solstice is the astronomical start of winter, winter weather generally arrives well before it occurs and the coldest weather of the year most often occurs in late January, a month after the solstice (the cumulative effect of low solar radiation). In reality, our calendar months of winter (December through February) more closely match the period of winter weather than do the astronomical months of winter (late December to late March).