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Seasonal Lag

In the previous article, we learned multiple ways exist to demarcate seasons on Earth. The traditional method of observing the change from one season to the next depends on the interplay between our planet’s axial tilt and the sun. One of the interesting aspects of this method presents at the solstices. The longest day of the year ends spring and begins summer; the shortest day of the year puts an end to autumn and births winter.

One self-evident aspect of life mixes the length of the day with the air temperature. Things are warmer in summer than in winter.

Perhaps you noticed something intriguing in the chart we used to demonstrate the periods of meteorological seasons in the aforementioned episode:

Average high and low temps in Ohio - graphic by weatherspark; highlights by TMAC

The graph above highlights the meteorological seasons, defined strictly by temperature. The three warmest months in the Northern Hemisphere tend to be June, July, and August. Therefore, those months represent summer.

But let’s add another data point to this graph:

Solstices marked in pink

Imposed into the graph is the approximate date of the solstices, the longest and shortest days of the year. What do we notice? The hottest and coldest days do not correspond to the longest and shortest days!

The warmest temperatures appear sometime in July and the coldest in January. If the length of sunlight compares to air temps, what’s going on? Why isn’t the longest day also the hottest day?

The answer is seasonal lag!

The Pacific Ocean captured by Apollo 11 crew - NASA

The image above displays just how much water covers Earth. We’re blue for a reason. Oceans blanket more than 70% of the surface of the planet.

Water is a magical substance. Obviously, it sustains life, but its molecular structure and chemical composition include a slew of other notable attributes. One such characteristic is its heat capacity. Water has a rather high latent heat of freezing and condensation. In short, the amount of energy required to raise the temperature of water is high, especially relative to land. You can think of the oceans as massive energy sinks. As the length of daylight increases after the winter solstice, the planet’s water starts to heat but it takes time to raise the temperature. The process works in reverse, too. Water loses energy more slowly than land. So, even though sunlight decreases after the summer solstice, the oceans hang onto some of their heat. Air temperatures are tied not only to the sun’s radiation pouring in on any given day but also to the radiation emitting from the land and sea. As daylight starts to get shorter, the oceans are still able to radiate a significant amount of heat away from themselves, which causes higher air temperatures.

This phenomenon causes seasonal lag. The oceans reach their maximum summer energies and begin to shed that heat, even after the longest day of the year. Usually, the hottest day of the year occurs approximately three weeks after the summer solstice. The reverse also occurs in the winter. The coldest day transpires several weeks after the winter solstice. You can see this effect on a small scale each day. The hottest part of the day usually happens around 3 PM, despite the sun being the highest around noon. Though the heat capacity of land is lower than oceans, it still functions the same way. The land holds some of the radiation from the sun, which starts to radiate after the zenith. The spread between the solstices and the top temperature would be lower if Earth’s ocean composition were lower, but there would still be some lag.

The video above demonstrates the effect of water’s heat capacity and a lowering of radiation input. Heat a pot of water, replicating the sun adding energy, then take it off the stove, which somewhat approximates the shortening days. For a short period, the temperature of the pot of water continues to rise!

Seasonal lag can also explain another quirk of the calendar. Why is the autumnal equinox at the end of September typically warmer than the vernal equinox at the end of March? After all, they both get the same amount of daylight. In terms of temperature, the fall equinox is just two months removed from the warmest days, while the spring equinox is two months after the coldest days. Because of the trends – it takes time to change the temperatures of matter – one side of the calendar is warmer than the other, even though they’re both receiving the same amount of radiation from the sun.

The influence is so strong that the months after the summer solstice are warmer than their respective counterparts after the winter solstice even though the latter receives more sunlight. For example, February has more sunlight than November, but November is usually much warmer than February. The same applies to August and May. More sunlight appears in May than in August, but August is often far warmer.

Humans love regularity – just look at our calendars – but the more we look into the science behind seasons and celestial periodicity the more we see the universe, physics, and matter love to throw wrenches into our desire for neatness.

Further Reading and Exploration

Seasonal Temperature Lag – Weather Works

Why is March colder than September in Northern Hemisphere? – Earth Science Stack Exchange

Temperature Extremes & Seasonal Lag – Weather Works

Summer solstice and seasonal lag – Royal Meteorological Society

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