Seasonal Lag: The relationship between incoming sunlight and temperatures on earth

Weather Blog

Have you ever noticed that the coldest, or even hottest temperatures of the year don’t occur right on the solstices? For example, we typically don’t feel the coldest temperatures of the winter season in Rochester until we get into January and February. In the summer, we typically see our hottest temperatures in July and August, but not in June.

Note the trend in average temperatures for Rochester in the top portion of the graph below:

Image courtesy: weather.gov

The green shading represents where “normal” temperatures fall for Rochester, NY in a given year. Notice how the green peaks sometime around the end of July, and reaches its minimum sometime around the end of January.

You would think that we experience our hottest temperatures of the year once we reach our maximum tilt towards the sun on the summer solstice (June 20th/21st), and our coldest temperatures once we reach maximum tilt away from the sun on the winter solstice (December 21st/22nd).

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On these specific dates, we’re receiving the most insolation, or direct sunlight from the sun, but we experience a delay in feeling the effects of the changing air temperatures.

Water absorbs a large amount of energy before it actually gets warmer due to it having a higher specific heat than land does. In other words, it physically heats up slower than the land. Water has a major influence on regulating air temperatures around the globe, especially around coastal areas. As a result, some of the coldest or warmest temperatures of the year will lag by about a month here in the U.S.

We call this phenomenon, seasonal lag.

Try this analogy at home!

Think of seasonal lag like the opening and closing of a freezer door. Open the freezer door slowly. Notice how it takes some time to feel the full effect of the colder air spilling out. The winter solstice represents the point at which the freezer door is open all the way it can go. At this point the earth is tilted as far away from the sun as you can get. Now slowly start to close the freezer door. Notice how you’re still feeling the effects of the colder air, and it may even feel colder to you since that air took a few seconds to hit your skin as you started to close it. The door is still open even as you’re closing it, which means you’re still feeling all that colder air spilling out. As you close the freezer door, this represents the earth slowly tilting towards the sun once again. There’s still plenty of “winter” left, and plenty of cold air in this situation to get through before we get close enough to the sun that we start warming again. It’s not an immediate reaction.  

This is similar to how lake effect works in the winter. Due to the specific heat of water being higher than it is for land, water is able to absorb a large amount of energy, but heats up slower than land. This is the case during the summer. Once the colder months hit, it takes much longer for the lake water to cool down than it does the land. This results in large temperature differences between the air above the lake and the lake water itself when colder air moves in. This difference in temperature creates enough instability for lake-effect snow showers in the winter!

Did you know? Seasonal lag varies between climates. For our region of the world known as the mid-latitudes, our seasonal lag occurs roughly about a month after the solstice. For other areas of the world, the summer lag is much longer than the winter, and across the polar regions the winter lag is longer. 

All in all, the maximum insolation, or direct sunlight received does not fall in line with the maximum temperatures felt on earth.

~Meteorologist Christine Gregory

Copyright 2021 Nexstar Media Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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