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Why does the coldest time of the year align with the darkest time of the year?
Category: Earth Science
Published: December 27, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
In general, the coldest time of the year does not align with the darkest time of the year. The basic reason for this discrepancy is the fact that objects take time to cool down and heat up.
The earth rotates daily about its rotational axis, which is an imaginary line connecting the North and South geographic poles. In addition, the earth orbits once a year in a nearly circular path around the sun. The axis of earth's rotational motion is tilted 23.4° away from the axis of its orbital motion around the sun. Due to the conservation of angular momentum, earth's rotational axis always points roughly to the same region in the starry sky (generally towards Polaris). This means that as the earth orbits the sun, at one point in the year, the North Pole is tilted 23.4° directly towards the sun (the northern summer solstice); at another point in the year, the North Pole is tilted 23.4° directly away from the sun (the northern winter solstice); and at two other points in the year, the North Pole is titled 23.4° exactly to the side with respect to the sun (spring equinox and autumn equinox).
The more a spot on earth's surface is tilted away from the sun, the less time that spot spends each day in daylight. The day that the North Pole tilts the most away from the sun is therefore the shortest day of the year (plus or minus a day because of the way we define time zones), and the day that the North Pole tilts the most towards the sun is the longest day of the year (plus or minus a day). The northern winter solstice occurs every year some time between Dec. 20 and Dec. 22. The exact day changes because of the way we as humans have designed our calendar and time zones and not because the actual timing of the solstice physically changes every year. As soon as Dec. 20-22 has come and gone on the Northern Hemisphere, the days begin getting longer and brighter.
Since sunlight is the principal agent that warms the earth, and since the Northern Hemisphere starts getting more light every day after Dec. 20-22, you may be led to believe that Dec. 20-22 is not only the darkest time of the year, but also the coldest time of the year. But this assumption is false. In the Northern Hemisphere, the coldest time of year is generally the middle of January. The exact day that is the coldest varies from town to town, and from year to year because the weather is a very complex phenomenon shaped by many factors. But averaged over many years and over many locations, the coldest time of year is the middle of January. The coldest time of year is therefore typically 3 to 4 weeks after the darkest time of year. Why?
While it is true that the earth is mostly warmed by sunlight, it takes time for the earth to cool down and heat back up after the sunlight has reached the minimum in its cycle. Summer heat is stored in the ground, plants, and water on earth's surface. When the low-light conditions of December come, the earth is losing a lot of heat but has not lost all of it compared to the coldest day. Even though the end of December and beginning of January has more light warming the earth than on winter solstice, there is not enough heat being inputted by the light to offset the massive amounts of heat still being lost by the earth because of its previously high temperatures. The average coldest day of the year in middle January is only reached when the heat inputted by the increasing levels of sunlight overcomes the amount of heat lost by the earth each day. The situation is a bit like putting hot soup that is at 200° F into an oven that is at 100° F, but is steadily increasing to 300° F. The soup will at first cool down even though more and more heat is being pumped into the soup every minute. Eventually, the oven's temperature surpasses the temperature of the cooling soup, and the soup begins heating up again.