Seasonal lag

MANKATO, Minn. (KEYC) - December 21st was our winter solstice, the calendar start to our winter, and also the shortest day of the year. Since then daylight hours have slowly been increasing across the United States. Unfortunately, even with the extended daylight hours, many, including Mankato, MN, are still headed for the coldest days of the season.

In this graph you can see when the winter solstice is and also see the dip when the average... — In this graph you can see when the winter solstice is and also see the dip when the average coldest temperatures are for Mankato, MN.(Applied Climate Information System)

This phenomenon is known as seasonal lag or seasonal delay. The lag is due to the time it takes our atmosphere to both heat up and cool down. The troposphere is the lowest layer of the Earth’s atmosphere and contains 99% of the water vapor in the entire atmosphere. Water is a slow conductor of heat. 1 Kg of water must lose 4200 Joules of energy to drop 1-degree Celsius. Conversely, it takes 4200 Joules of energy to heat up 1 Kg of water to raise the temperatures by 1-degree Celsius.

So since water is a slow conductor of heat, our earths atmosphere doesn’t start to warm up as soon as we add more daylight (energy). Based on climatological data, coldest time of the year for Mankato, MN is January 12th through the 17th when the average air temperature is 13.9º Fahrenheit. From December 21st to January 12 the average temperature in Mankato, MN drops 3.4º Fahrenheit. Mankato, MN won’t gain that 3.4º back until February 7th.

Even though on average our coldest temperatures for Mankato, MN are between January 12th and the 17th doesn’t mean it will only be cold during that period of time and also doesn’t mean things will start to warm up going into the end of January, there are other factors to consider. Most of Earth’s temperature swings are caused by convection, the movement of energy from one place to another. In weather this is in the form of fronts. Also, during the winter months, the Sun is lower on the horizon meaning less direct sunlight. With less direct sunlight some of the Suns radiation is lost. Finally, during the winter, snow has a high albedo meaning it reflects high amount of solar energy back into space, which in turn limits conductive heating caused by the Earth’s surface.