This morning, at 9:03 AM Central Time, the winter solstice occurs in the Northern Hemisphere. As a result, this will be the shortest day of the year and, from now until the summer solstice in late June, the day length will gradually increase.
Since almost all life on Earth is dependent on light and heat from the sun, this is the beginning of nature's year across our Hemisphere. Within a few weeks, we will notice the lengthening sunlight but the low sun angle will negate significant solar heating for a couple of months; during that time, heat waves will develop only when atmospheric conditions allow the movement of air from south to north (or from high to low elevations along mountain ranges).
All of this has nothing to do with the sun itself. Rather, it reflects the fact that our planet rotates on a tilted axis that causes each Hemisphere to lean toward the sun during the months surrounding its summer solstice and away from the sun during the months surrounding its winter solstice. Of course, global warming will increase average temperatures during all phases of this annual cycle.
