After I wrote about the winter solstice back in December, I got to wondering why the solstice isn’t typically the coldest day of the year. Since we get our heat from the sun and on the winter solstice we receive the fewest hours of sunlight, why does the coldest day on average occur almost a month later (at least here in Minnesota – this map shows average coldest day across the US.)? On January 21st, we receive almost forty more minutes of sunlight then we do on December 21st, so why is January 21st on average the colder of the two days?
The answer is actually quite straight forward. It takes awhile to warm up! Technically, this phenomenon is known as the lag of the seasons. There are a few forces at work. First, the oceans and ground absorb heat from the sun. They are still hanging onto (and releasing) some of the heat they gathered up over the hot summer months into December. As we move past the solstice, deeper into winter, the amount of residual heat has decreased, so even though we are getting more sunlight, the temperatures still continue to drop.
One apt analogy is a tea kettle on a stove top. When you place the tea kettle on the stove and light the burner, it takes awhile for the water to absorb the heat. The water won’t be hottest thirty seconds after you turn in on, even if you’ve turned the burner all the way up; it takes some time for the water to warm up. Inversely, if after you have heated the water all the way up (this would be July or August on Earth) and then you turn the burner down to low, the water won’t be immediately cool. It will take some time (until January) to cool down to the minimum temperature.
Since the ocean is efficient at retaining heat, this is why some places along the ocean experience less seasonal change than one might expect given their latitudes. For example, London (51 degrees N) is farther North than Minneapolis (44 degrees N), but the average temperature of London in the winter is much warmer than in land-locked Minneapolis. Additionally, the average monthly temperatures in Minneapolis vary by 61.7F, while in London monthly temperatures vary by only 24.3F. The North Atlantic Current brings warm water up to London from the Gulf of Mexico. This water retains much of its warmth during its long Tranatlantic voyage into northerly latitudes, thus keeping temperatures in London much warmer than one would expect given the city’s latitude.
Another factor that slows down warming in the winter is snow cover. When the solar heat emitted by the sun is retained by the Earth, it leads to greater warming. However, abundant snow cover reflects the solar heat back into space, so that we don’t feel its full warming effects. This is like wearing a white t-shirt on a hot day in an attempt to stay cool. So, in the months with the most snow cover (December – February), solar heat retention is reduced.
This year in Minneapolis, we may already have our coldest day behind us. We had a stretch of several sub-zero days at the beginning of January, with a low of -11F on January 5th. Now, we are in the midst of a warming trend. We enjoyed a balmy 37F, sunny day today and we broke out the sleds and skis. It can’t last though, February will bring more cold weather, but hopefully we’ve seen the bottom and are on a (slow) path toward spring weather.