Weather... Under Ground

By: Mark Kane , 10:57 AM GMT on May 04, 2011

Snow covering tips of plantsPhoto courtesy of Mark Kane

"Weather... Under Ground" by - Mark Kane

Here on the edge of the Great Plains, the phrase “freak snow” is an oxymoron. Some years we have snow in May. Old timers say they’ve seen snow in June.

Late snows are typically wet and sticky. They build up on leaves and limb, adding weight that tears off branches. The falling branches drag down power lines and we go dark.

This March, with the trees bare and the lawns asleep, we had temperatures in the 50s for a week. Way early, but tempting to see as a sign of winter’s end. So I woke up one day at 5 a.m., peered out a window and was completely confused by what I saw.

What was wrong with the world? Everything was white -- trees, streets, roofs, driveways. Aiee, snow! At least an inch. I thought, sheesh, I’ll have to shovel the driveway.

But an hour later the street was bare and so was the driveway. Meanwhile, the evergreen trees and the lawn still wore white robes.

What melted the snow by 6 a.m.? One answer (possibly wrong): the asphalt in the street and the concrete in the driveway collected heat from the sun and air during the warm days. But they had two months of freezing weather before the warm spell.

Why did the snow linger on the evergreens and the lawn? Being less dense than asphalt and concrete, maybe they were colder. Just a theory. As the morning warmed, they too lost their white robe and returned to green. There was no sun, just a blurry disk in a gray sky. The temperature was in the 40s.

Still, I don’t think the weather above ground gives the whole answer. I think the weather under ground cleared the street and the driveway, and later the lawn. By weather “under ground,” I mean the fluctuations in ground temperature in the top six inches of soil.

Here in Iowa we have long winters and occasional outbreaks of Arctic air. The ground can freeze hard and deep. Our engineers specify that water mains and sewer lines must be buried more than four feet deep for a margin of safety. I asked a guy running a backhoe on my street about frozen ground and he said he’d seen it 56 inches deep after an especially long cold spell.

In permafrost regions the ground can be frozen many feet deep. Indeed, in some regions, to depths measured at more than two hundred feet. One theory says these strata were formed in the last Ice Age.

Back to my yard. Even frozen only two feet deep, the ground takes a long time to thaw. While the sun and air warm the top layer, the ground stays frozen below. This moment is what folks in Vermont call “mud time.” A thawed, soaked layer of dirt (mud) on top of frozen ground that no water seeps through. Cars bury their wheels in gravel roads, your boots pick up cold, sticky mud. There seems to be no firmness underfoot.

Grass emerging from the springtime snowPhoto courtesy of Mark Kane

I was looking for signs of life in the garden the day before the “freak snow” and I felt that the ground had thawed in only the first inch or so. Not enough to melt snow, I think. So why was the lawn white at 6 a.m.? Because the grass kept the snow off the ground (as did the mulch in the garden). Insulated by the gap, the snow persisted until almost noon, when the air finally warmed above freezing.

One last question. What warmed the street and the driveway? We know that heat rises a long way from the molten core of the earth. The temperature five or six feet down in the ground holds year round at 50 degrees, more or less, which is one reason that animals make burrows, that caves and basements feel cool in summer, that many people in the opal-mining town of Coober Pedy in the hot interior of Australia, live in underground homes.

So, the ground level, or street level, marks a boundary between two weather regimes. One we know well, and one we don’t (at least it’s not usually part of the weather report). Above ground, weather and sunlight warm or freeze the ground and ground warmth pushes back or piles on. The balance shifts constantly, with plenty of interesting moments, such as my groggy morning.

Minnesota’s climate demonstrates how complicated the two weathers can be. The state has snow cover a foot or more deep most winters and this blanket of snow insulates the ground, sparing it from direct contact with air that can be 30 to 40 degrees below zero.

Minnesota grows wine grapes and has a grape growers association. Based on the lore that wines of character come from vines grown in tough conditions, the motto of the association used to be “Minnesota: where the grapes can suffer.”

The vines are trained low so they are sheltered in the snow, where the climate is plenty cold but safe. Each has a short trunk and two arms pointing in opposite directions, less than a foot above ground, and supported by wire.

Light snow covering on tree branchesPhoto courtesy of Mark Kane

Plants know the weather underground. They set their alarm clocks by the temperature in the top six inches of soil, where most of their roots grow. Our ancestors knew this. Here’s one of their sayings, “Sow corn when oak leaves are the size of a squirrrel’s ear.” Oaks have studied climate for thousands of years. They’re hard to fool.

Our ancestors were the pioneers of what has come to be called phenology, the science of recording events in the life cycles of plants and animals. Today, along with the scientists who pursue phenology, there is The National Phenology Network (NPN), a non-profit that guides thousands of volunteers all over North American as they observe what NPN calls “calibration species,” plants and animals that are exceptionally widespread, making observation of them possible for many people.

Your Garden Show is helping with this effort to record life cycle events in nature in our communities. We’re tracking “America’s 20 most wanted” plant records as well as watching the bees with The Great Sunflower Project; you’ll find all 27 projects divided into 3 groups – Allergy Agents, Pollinators, and Season Spotting – on the tab “Citizen Science”. Pick the most interesting to you in your area. We think we’ve made it easy for lots of people to simultaneously learn, teach and record phenology and we hope that you’ll want to join the fun. Get a friend or family member involved and you’ll have company for your outdoor adventures this summer.

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3. Amsterdamme
12:18 PM GMT on May 24, 2011
Fascinating story, Mark. I never really thought about frozen ground, though perhaps because the ground hardly freezes where I live now (and where I grew up... Florida!) But I have appreciated the underground cool of cellars - a great escape from above ground heat. Keep up the good writing!
Member Since: December 5, 2010 Posts: 0 Comments: 9
1. takeahike
3:47 PM GMT on May 11, 2011
Who ever thinks about the weather under ground??? Fascinating stuff. Even with a cold spring here, the trees and fruit vines are way AHEAD of schedule - and now I know why! Thanks for the article.

I love this quote: "Oaks have studied climate for thousands of years. They’re hard to fool."

That says alot, doesn't it.

Thanks for the article.
Member Since: May 2, 2010 Posts: 0 Comments: 5

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