The days are getting longer so why is it getting colder?
Learn how the annual seasonal temperature lag behind increasing daylight hours is a result of the Earth’s tilt!
January 27, 2015 - Author: Tracy D’Augustino, Michigan State University Extension
The New Year has begun and the days are getting longer (meaning there are increasingly more daylight hours each day), so why is it getting colder? Each year, many people look forward to the winter solstice, which usually falls on December 21 and is the shortest day of the year, because they know after that, there will be progressively more daylight each day. From the winter solstice until the summer solstice (the longest day of the year, usually June 21), the days in Michigan get longer. So why, if we are getting more sunshine each day after December 21, are we still getting colder? Unfortunately, the winter solstice is also the beginning of winter and the coldest days in Michigan are normally in January and February.
In order to further explain the annual seasonal temperature lag behind increasing daylight hours, we need to address the reason for the seasons. The seasons are a result of the tilt of the Earth’s axis. This 23.5-degree tilt defines the area of the Earth which receives direct insolation (sunlight), the surface where the sun’s rays hit the Earth at a 90-degree angle. This area of direct insolation varies throughout the year but is confined within 23.5 degrees north and south of the equator. We label these areas the Tropic of Cancer, which is 23.5 degrees north of the equator, and the Tropic of Capricorn, which is 23.5 degrees south of the equator. During a Michigan summer, the area from the equator to the Tropic of Cancer gets direct sun, whereas in our winter, we do not. This is because during our winter, it’s the Tropic of Capricorn’s turn to get direct sun and thus experience their summer.
A second reason for the temperature lag behind increasing daylight hours is accumulated heat energy in the Earth. Let’s start with summer: think about rocks around a fire pit. These rocks stay warm long after the fire has gone out. Well, the Earth is a giant rock being heated by the sun. Just as the rocks around the fire pit slowly release their heat after the fire is out, so does the Earth slowly release the heat it accumulated over the summer. So even though the days are getting shorter after the summer solstice, the Earth cools slowly, giving us a summer heat lag and consequently hot Julys and Augusts.
An opposite heat lag occurs after the shortest day of the year, resulting in cold winters, as heating of the Earth slowly ramps up as the days lengthen. Again, think about the rocks around the fire pit. Initially, they are cool to the touch but as the fire roars, they become hot. So even though our days are currently getting longer as Michigan receiving more insolation each day, we are cold during the delay before the Earth warms and air temperatures increase, eventually bringing us spring.