Recent summerlike warmth won't make up for the impact of shorter days

With the end of Daylight-Saving Time, sunsets in Boston now fall between 4:11 and 4:30 p.m., which can feel like a shock. This time change aligns with the start of "solar winter"—the period from November 7 to February 5, when days are shortest, and the sun sits lowest in the sky.

During summer months, the sun peaks in the sky around 70 degrees above the horizon in Boston, but by the start of winter, the sun angle peaks around 25 degrees above the horizon. This limits human exposure to sunlight and, in turn, the body's ability to produce vitamin D. North of the 37th parallel (Virginia, North Carlina state line) sunlight to vitamin D processes in the body are extremely limited if at all. Low vitamin D levels can contribute to immune system weakness, brain fog, and symptoms of Seasonal Affective Disorder (SAD).

Late fall normal/average high temperatures drop quickly during this time due to the lack of sunlight and cold air advection (cold airmasses moving in from Canada). Although New England has seen recent record warmth, it's not because the sun has gotten stronger. It's due in part to the extreme dry conditions across New England.

Drier ground heats up faster than saturated soil, even with short days and a less direct sun angle. This also allows for larger temperature swings from cold mornings to above normal afternoons. In order for temperatures to stay cooler for the long haul, cold fronts will need to continue ushering in air masses from farther north and keep that cool air in New England.

The shortest day and lowest sun angle will arrive on December 21, the winter solstice, and the body's ability to absorb the sun's rays and produce Vitamin D will start to pick back up in March.