Q: December 21st is the beginning of winter, and the shortest day of the year. Why are the 60 days following this day so much colder than the 60 days preceding this day?
A: This is known as seasonal temperature lag. When winter begins the amount of incoming solar radiation is at a minimum, and for the start of summer (late June) at a maximum. Yet coldest temperatures occur in January and warmest temperatures occur in August.
In Buffalo, temperatures begin to improve during February. The average coldest high temp is 31 with the average low temp of 17 (occurring in January/early February). This is because it takes awhile for the atmosphere, or most things for that matter, to cool down and warm up.
The Earth’s bodies of water assist this lag. Oceans take up 70% of the Earth’s surface. With a heat capacity three times that of land they can’t just warm up and cool down immediately, they take time. Think of a swimming pool versus a sidewalk. The sun hits both all day long but the sidewalk generally gets much hotter. However, by late summer your swimming pool is generally much warmer than at the start of the season. In WNY, many watch for the lake to freeze. Even though winter begins in December, it isn’t generally until mid-January when we start to see significant freezing; seasonal temperature lag!
On another note, this is why hurricane season lasts so long, into autumn! The oceans continue to warm once we pass the start of summer. Warm water is a key ingredient to these massive storms.