Defining Seasons

Now that winter has arrived in full force, I’m sure many people are wondering when warmer temperatures will be gracing us with their presence again. Don’t get me wrong, there are plenty of fun outdoor activities to enjoy in the winter like sledding, skiing, and winter hiking, but when temperatures are as cold as they have been, I can’t help but look forward to spring. That led me to wonder: When does spring officially start? After doing some research, I realized that the answer is not as straightforward as it might seem.

Seasons can be tricky because there are two popular ways of defining them: the astronomical way and the meteorological way. Astronomical seasons (think astronomy; the movement of planets) are distinguished based on where the Earth is on its orbit around the sun. This method uses the dates of equinoxes (when day and night are equal lengths) and solstices (the longest and shortest days of the year) as transitions between the four seasons. Any time after the winter solstice (December 21st or 22nd) but before the spring equinox (March 19th, 20th, or 21st) is considered winter, and so on.

using the astronomical definition, each season begins with a solstice or equinox, meaning the calendar date of the season’s beginning may vary each year.

Since the Earth’s orbit around the sun is not a perfect circle, there is some variation in the length of the seasons and on which calendar day the seasons will begin. This can make it difficult for scientists and weather forecasters to compare weather and climate information between different years. Weather from some day in 1990 might not actually be comparable to that same calendar date in 2024 if the Earth was in a slightly different place in its orbit. Thus we needed a different way to track the seasons that is more relevant to our human activities. Enter: meteorological seasons!

Meteorological seasons (think meteorology; forecasting weather) are grouped together in three-month blocks based on average temperatures. Instead of including only the last ten-ish days of December in the official definition of winter like the astronomical method would, meteorological winter includes all of December, January, and February, since those tend to be the coldest months of the year.

This way of classifying seasons is much easier to work with when predicting things like weather and crop schedules because there is less year-to-year variation in the length of seasons. The length of seasons always stays the same, with the only exception being leap years, when winter is one day longer. Meteorological seasons also match better with our standard calendar since it’s able to be broken down into full months, allowing us to say things like, “students don’t have classes in the summer” instead of, “students don’t have classes during the last third of spring and the first two-thirds of summer.” That would be quite the mouthful!

A wheel of the year showing the four seasons of the meteorological definition. The background contains blue sky and fluffy white clouds

Using the meteorological definition, each season begins on the first of the month.

Additionally, meteorological seasons surround the solstice and equinox, including some time before those astronomically significant days and some time after them. So while the astronomical seasons would consider the warm days of mid-June to be spring (because the summer solstice doesn’t occur until around June 20th), the meteorological seasons recognize all of June as a summer month because it has some of the warmest yearly temperatures.

Both of these ways of defining seasons have value, which is why we keep them both around! For the average person, the meteorological seasons are more comprehensible because they follow the calendar that we are familiar with and are distinguished by something we all experience: temperature. Astronomical seasons, on the other hand, give an important context for why our temperatures vary throughout the year the way they do. Most ancient calendars were based on the astronomical seasons because there were clear and measurable events (namely the solstices and equinoxes) that signaled the change of seasons.

A child in snowpants and a coat sledding down a snowy hill. A large oak tree and some snow-covered prairie grasses stand in the background.

Whether you want to use the meteorological definition or the astronomical definition of spring, we still have a few weeks of winter before we should start expecting warmer temperatures and ephemeral flowers popping up. So instead of spending our time longing for the green leaves of spring, let’s choose to take advantage of the short-lived beauty of snow lining the trees! There are plenty of ways to stay active and connected to nature, even when it’s cold. Be sure to check out our events calendar to see how you can stay connected to people and nature this winter!

The Science of Fall: Changing Leaves

Gorgeous Fall colors on a Maple tree. Photo by Ashlyn Rogers.

Halloween scene. Photo by Ashlyn Rogers.

What does Fall mean to you? Do you think of pumpkin patches and corn mazes? Of dressing up for Halloween? Do you think of going apple picking or baking an apple pie? Or maybe pumpkin spice-flavored everything? I think when we hear the word “Fall” or even “Autumn”, we often think of the leaves changing color. There are astounding scenes that can result from the trees turning shades of red, yellow, orange, and brown. My favorite views have often been those where the landscape is a palette of color, as if someone stopped and painted the sight before me. But not many people think of or know about why the leaves change.

Pumpkin patch at sunset. Photo by Ashlyn Rogers.

Let’s start with the “status quo”: why are leaves green? Chlorophyll is a green pigment found in the cells of plants. This pigment is essential for plants to undergo photosynthesis, the process that plants use to turn light energy (the sun) into chemical energy or sugars that the plant needs to perform its necessary functions. Since most plants need to photosynthesize, most plants are green. During the spring and summer, when plants are growing, chloroplasts and chlorophyll are being continually produced, keeping the green color. However, when it starts to get colder and nights get longer, the plant reduces and eventually stops the production of chlorophyll and the plant loses all of its green color.

Yellow Black Walnut leaves. Photo by Jessie Crow Mermel.

So where do all the colors come from? In leaves, there are other pigments, such as carotenoids and anthocyanins. Carotenoids produce yellows, oranges, and browns and are found in corn, carrots, daffodils, buttercups, bananas, and more. This pigment is found all year long in the leaves of trees. Anthocyanins produce reds and purples such as in cranberries, red apples, concord grapes, plums, strawberries, and more. Unlike the carotenoids, anthocyanins are only produced in the Fall in response to bright light and excess sugars present in the leaves. When all of the chlorophyll is gone in the fall, these are the pigments that are left, producing the gorgeous colors we associate with the season.

View of many Fall colors. Photo by Ashlyn Rogers.

Now you might find yourself wondering why the trees aren’t all the same color. This is mainly due to different combinations of the pigments found in the leaves. For example, a brown leaf is usually produced from a combination of anthocyanin and chlorophyll pigments. Whereas an orange leaf may result from a mixture of anthocyanins and carotenoids. Oak trees are usually a red or brown, Hickory trees a golden bronze, Dogwoods a purple-red, and different species of Maples ranging from yellow all the way to a brilliant red. Because of the variation of color by species, forests that are more diverse will have a longer and more colorful Fall leaves display. The predetermined chemical make-up of the leaf is important, but there are other factors that affect the color produced as well.

Back road lined with colorful trees. Photo by Ashlyn Rogers.

Weather is another contributing factor to leaf color. If you ever found yourself thinking that one Fall season was more colorful than another, you may have been right. Drought, an early frost, or warm Fall nights can mute leaf colors or cause the leaves to fall off before changing color. On the other hand, a stretch of warm, sunny days and cool, but not freezing, nights can allow for an increased production of anthocyanins and result in a spectacular color display. However, this is only true for the red and some orange leaves. Since carotenoids are always present, the yellow and orange leaves remain fairly constant. The best color display is believed to come from a warm and wet Spring, good Summer weather, and those ideal Fall days.

Fall colors over a bridge. Photo by Ashlyn Rogers.

Some of you may have heard of “Leaf Watch” or seen Fall Foliage Prediction Maps. Tools such as these exist because no two falls are exactly alike, in color or in timing. Previous seasons, temperature, and geography are the main factors behind when leaves change. Late summer drought can cause leaves to fall early and without changing color, earlier arrival of cooler temperatures will cause change to begin earlier, and persistent warmer weather may cause a delay in the change of the leaves. Overall, Fall comes earlier the farther North you go. But this is also true for higher elevations which may have cooler temperatures than the surrounding area. The main predecessor to changing leaves is the shortening of days, but it is just one of many factors affecting the timing of Fall colors.

Make sure you get out there and observe nature in action! We are nearing the peak for leaf change and I am looking forward to the beautiful colors of the trees. I hope that the science of Fall inspires you. Visit a local park or forest preserve and enjoy!

The Quickening

In the middle of April, between snowfalls, with the new leaves of this year’s early wildflowers just starting to unfold, I posted on social media a couple of plant photos (newly emerging leaves) with the following commentary:  “Hang on, kids, here it comes: the quickening exultation of floristic fever as our local vegetation awakens from the deep sleep of winter and races into full form to dazzle and enchant plant geeks across the land.”

I call it the quickening, a term also used to describe the first movements of an unborn child felt by an expectant mother. I use it to describe the first movements we perceive on the part of growing plants each spring, the exceptional acceleration of botanical activity that takes place here April-to-May, that frenzied state of hastening changes, barreling headlong into the growing season as wildflowers and trees alike press forward in an eruption of fecundity that takes our collective breath away. Blink and you’ll miss it.

Every day brings new leaves, new blooms, new discoveries along the trail. Before the trees put forth their leaves, wildflowers—we call them spring ephemerals for the fleeting nature of their appearance—bloom in profusion across our timbered lands. Generally speaking, prairies and wetlands bloom later; springtime is time for the woods to shine.

Bloodroot (Sanguinaria canadensis) flowers may persist for only a few days. Photo from Severson Dells, April 24, 2018.

Bloodroot (Sanguinaria canadensis) flowers may persist for only a few days. Photo from Severson Dells, April 24, 2018.

Spring ephemerals appear in the woods early in the season, before the trees put forth their leaves. Around here, a few weeks after the vernal equinox, sunlight reaches the surface of the earth at a fairly direct angle, fueling the process of photosynthesis in woodland wildflowers. Once the trees come into leaf, shade is more prevalent in the woods and spring ephemerals scale back their metabolic activity; some go dormant by midsummer.

Food produced by these wildflowers during the brief period of active spring growth may be stored underground in a root organ called a corm. Unlike a bulb, which is layered (like an onion), a corm is a solid storage device. It holds enough food for the plant to survive the lengthy period of dormancy, summer to spring, and feed the plant’s growth until it can refuel, so to speak, by conducting photosynthesis.

Sharp-lobed hepatica (Anemone acutiloba) is a delicate flower of wooded slopes. Photo from Severson Dells April 24, 2018.

Sharp-lobed hepatica (Anemone acutiloba) is a delicate flower of wooded slopes. Photo from Severson Dells April 24, 2018.

Woodland wildflowers are among the native plants most favored in this region, arriving as they do to grace the land with color and vitality after the long sleep of dull dormancy. We love our spring ephemerals not only for their sudden beauty, delicate and subtle, but also for their role as harbingers, heralding the commencement of the growing season, with all its promise of delights and discoveries to come.

Our early blooms signal the release of winter’s hold upon the land, offering cheerful relief from the cold and dark months that came before. And these first flowers of the year entice us to explore our natural areas, seeking out each successive species as it expresses itself in the environment, taking its place in a grand botanical pageant that spans the seasons. It starts with the quickening; don’t blink, or you’ll miss it.

Light in the Sky

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Recent weather systems have ushered in some glorious skyscapes, with a delightful play of light and shadow, color and texture, gracing the space above us. Cool days and crisp nights herald the coming of autumn, weeks before the equinox arrives. Sunsets come earlier and sunrises, later. Sunlight glances in on a slant distinctly different from that of high summer. We can feel the change in the air.

Of all these seasonal changes, one that tickles me most is the light in the sky. Clouds, lit from beyond, cast shadows of blue or gray that seem to support the white billows above. As gentle winds send undulating waves through the prairie grasses and set the tall, yellow Silphium and Helianthus to wave rollicking across the landscape, breezes usher low clouds across the sky in a procession of festive forms.

Any time of day might offer a show or feature a surprise, like the red moon that shone down upon us in September, just past full, ruddy with the residue of western wildfires. For those of us fortunate enough to be under a cloudless sky, August’s solar eclipse was something rare and wonderful to experience. Those even more fortunate caught a glimpse of the elusive aurora borealis in September.

No matter where you are, in city or town or out and away, you might catch a glimpse of the parade in the sky. For my part, I prefer to see it framed in nature, cast in a setting of leafy trees or adorned by the nodding spikelets of tall grasses.

Pause to breathe deeply; capture a moment of quiet; see the seasonal skyscape.  Go ahead. Take a look. Raise your gaze and smile back at the sky.