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Climate is often a matter of inches and a little water
By Jay Stuller , photographs by Cameron Davidson
From SMITHSONIAN, December 1995
Planners ignore microclimates at their peril: mistakes can mean frozen crops, lower house values and camper vans blown off the highway
The snow crunches under the feet of Dennis Thomson as he leads an expedition up a hill on his 560-acre farm in the rolling terrain of central Pennsylvania, not far from the hamlet of Warriors Mark. The 54-year-old professor, head of the meteorology department at Penn State, points out the gnarled stumps of chestnut trees. They are beside the remains of an orchard planted in the early 1800s, now shaded by random oaks and pines.
"See the creek that runs down the hollow in that area about 100 yards away?" says Thomson, stopping to wipe the fog from his wire-rimmed glasses. "Because of the shape of the land and because the sun has already warmed Nittany Valley, cold air is still draining down the hollow. Just as water is pulled by gravity, the cooler air under these trees is heavier and sinking. The air right here is dead still, but down there in the hollow, I guarantee, the wind is blowing five to seven miles an hour."
Trudging farther up the slope, Thomson again stops, this time at the base of a living tree, around which is a small circle of bare soil. "Solar radiation is absorbed by the wood during the day," he explains. "Some of the energy absorbed by the tree is emitted as infrared radiation. That's why there's a snowless ring around the base. In fact, within 50 yards of us in any direction, we can probably find dozens of natural phenomena like these."
The soft-spoken son of two University of Wisconsin biologists, Thomson can masterfully wield the language of atmospheric physics. But at the moment he's engaging his guestsand himselfin the wonders of "microclimates." Small-scale interactions between the air, water, solar radiation, soil and vegetation, these tiny environments are often imperceptible to the untrained eye or just plain invisible.
A little way up the hill, Thomson stops again and pulls up a clump of moss from beneath the snow. From the moss hang prismatic hexagonal crystals of ice. Profound forces are at work here. Says Thomson: "The temperature difference between where the bottom of the crystal sits on soil and where the top of the moss touches the bottom of the snow layer is quite great. In fact, the difference is proportionally much greater than the difference between the temperature at the top of the snow and at our eye level. If that gradient were present in the larger atmosphere instead of confined beneath the snow, we'd be in the middle of a huge storm."
At the top of Bald Eagle Ridge, which is bare rock but for a few green thickets of mountain laurel, one looks northeast over Bald Eagle Valley. Just below the summit's northwest side are "flag trees," stunted oaks and pines with no branches near their tops and the existing branches pointing in a downwind direction. Each testifies to weather that's routinely much more severe on the windward side. "You're straddling two distinct microclimates," explains the professor, "and right down there, along the hillside, is where the state wants to build a major highway."
While highway planners took into account the route's vulnerability to snow, ice and some forms of fog, Thomson feels they missed one critical element. "The flag trees are a clue that winds up here are strong," he says. "We know that at ridgetops, wind speeds are amplified and accelerated. So in January, on a relatively windy day, I took some measurements along the proposed route. The gusts exceeded 70 mph, which is pretty close to hurricane force. I know the transportation officials didn't like my putting the testimony into the written record," says Thomson, "but those winds will tip over some tractor-trailers and recreational vehicles. Placing the highway in that microclimate will put lives at risk."
Microclimates are more often beneficent, benign or merely a minor annoyance. Relatively few of us notice that we're surrounded by these small climatic quirks. Yet microclimates can determine the kind of plants that grow best in a particular garden. They can make a house more expensive to heat or cool than another house less than a block away. The nuances of small-scale climate are critical to certain types of agriculture. And in building cities and clearing land of trees, humans induce far greater dynamic changes to microclimates than they do to the global atmospheric system.
"In the majority of places where people live, we simply overpower the small environments around us, to the point where we're not really in touch with them," says Brent Yarnal, an associate professor of geography at Penn State and a colleague of Thomson's. "We control climate by heating aid cooling homes with various forms of energy, and by the clothing we wear."
Do not feel guilty for modifying microclimates. Other species, including animals and trees, also create and even manipulate (however unintentionally) their own particular environments. Moreover, do not feel bad about failing to notice the little zones; microclimates are among the most peculiar of natural forces.
"It's relatively simple to understand weather, for it's something tangible that you can see, hear and feel right now," explains Yarnal. "With weather, we're usually looking at time scales of days, especially in the mid-latitudes, where a system rarely holds out for more than a couple of weeks." Climate is more difficult to understand; it is the statistical average of weather over decades, centuries and longer. "And just as weather series with location," Yarnal says, "so does climate. We know that Arizona has a climate that's hot and dry, that Minnesota has a climate of cold winters, that the climate of the Pacific Northwest involves a lot of rain."
Microclimates, however are manifestations of atmospheric influences that frequently prevail in areas not much larger than a square mile, and even in spaces as small as a half-inch. They're distinct anomalies within what atmospheric scientists call "local" scale climate, which ranges from about 100 yards to as much as 35 miles. The "meso" scale (and the scales obviously overlap) ranges from 10 miles to hundreds of miles. "Macro" scale climate can nearly encompass a continent.
Along with small horizontal scales, microclimates also occur vertically within the relatively thin "boundary layer" of air near the ground. This is where the atmosphere interacts with soil, rocks, water, vegetation and man-made objects. "It's the layer of air directly influenced by the nature of the underlying surface, such as its ability to absorb or reflect solar heat and thus to heat up or cool down," explains Tim Oke (pronounced OAK) a climatologist who is head of the geography department at the University of British Columbia in Vancouver. "It's also influenced by the drag on the air hitting those rough surfaces, which slows the wind and creates turbulence.''
The atmosphere is always in motion, so that heat is continually being exchanged between the surface and the air above it. Rocks, city streets, moist earth and bodies of water absorb, hold and lose heat very differently. So the air moving over a mosaic of surfaces is fed with blobs of warmer and cooler air. The resulting differences cause warm air to rise and cool air to sink, which creates pressure differences from place to place, which drive breezes.
Valleys, for example, produce their own wind systems, adds Oke. At night, air over the valley slopes becomes its coldest and heaviest and thus is carried by gravity to the bottom of the valley. As that pool of cold air along the valley's base becomes heavy enough, it begins to flow down the length of the valley, much as the water in a river does. These downward-flowing air currents are called "katabatic" winds.
Most microclimates are more complex. One of the most striking examples is found in Marin County, California, just north of San Francisco, where summers are characterized by a thick Pacific fog that butts against the coastal mountains. So dense that it frequently fails to burn off in the afternoons, the fog may linger for weeks. But due east about 50 miles, in California's Central Valley, daytime temperatures often exceed 90 degrees F. Drawn to that lighter warm air, the cooler fog streams through the Golden Gate, and tendrils of the mist may extend inland for several miles.
Some of that fog spills over the Marin County headlands. In hill-sheltered areas of Mill Valley, a few miles north of the Golden Gate, afternoon temperatures may be in the high 60s, while the fog itself is moving around the hills on winds that are about 10 miles per hour. But near the town is a gap be tween the headlands and looming Mount Tamalpais, and it leads to a spot where Highway 101 cuts through a pass between the Tiburon Peninsula and a rise known as Horse Hill. Although the fog typically stops here, cold air roars on a straight downhill path to the shallow waters of the bay fronting the town of Corte Madera. Packing air in the low 60s, gusts of up to 30 mph can rip across the flatlands near the bay.
Move inland a mere half-mile, however, to where Mount Tamalpais shelters west Corte Madera and the town of Larkspur, and there is virtually no breeze. A balmy 72 degrees suggests that you are in a completely different state. This is one of the reasons that homes in west Corte Madera cost a good $10,000 more than comparable houses near the bay. It's the price one pays for a better microclimate.
While humans can take out bank loans to buy a better climate, natural organisms must adapt to the variations. "When you get down to tiny objects like leaves, microclimates can influence their very structure," says Oke. "For example, conifer needles have to maintain themselves at ambient air temperature. So they are thin, with very little bulk but a large surface area, which enables them to rapidly exchange heat with the air. As a result, needle temperatures never stray more than 2 degrees from the air temperature." The author of a textbook called Boundary Layer Climates, the 53-year-old Oke, like Penn State's Thomson, derives as much pleasure from finding microclimate anomalies as from doing big-time atmospheric science. Born in southwestern England, Oke reflects that his native surroundings in the small town of Kingsbridge had much to do with his passion. "There you can see places where the trees, gorse and heather are bent and shaped by cold and blasting winds. But on the other side of the hill, in sheltered areas, you'll find plants that are almost subtropical."
Certain vegetation has a way of using, or even producing, microclimates. At the top of Thomson's Bald Eagle Ridge back in Pennsylvania, where those oaks and pines have a tough time growing, the lush and green mountain laurel seems a most odd thing: the soil is thin and holds little water. "But the laurel traps snow in drifts," Thomson says. "After a good storm, it will be several feet deeper around the laurel than anywhere else." Call it the Gaia Hypothesis writ small, an example of the link between living things and the atmosphere as we know it. 
California's redwoods, which live in a relatively narrow band along the Pacific coast, are another example. "These groves are almost like cathedrals," says Brent Yarnal, who studied these coastal microclimates while at Sonoma State college. "The horizontal and vertical temperature gradients in a redwood forest are flat lines. The temperature at the ground and l00 feet up is about the same, since the trees control all that's beneath them." What's more, while redwoods need the heavy rainfall that comes in the winter, the trees also trap coastal fog. "Through the drip action," says Yarnal, "the redwoods squeeze dozens of extra inches of water out of the atmosphere each year."
To further elucidate the intricacies of boundary layers and heat exchanges, Oke's book contains a chapter on the "climates" of animals. Most critters, you see, regulate their internal temperatures, as well as their energy and water balances, through movements, postures and the types of shelters they seek.
In Oke's book, one photograph shows a litter of piglets subjected to cold temperatures. The animals huddle together in a corner of their box, drawing in their limbs and covering their noses. They conserve their metabolic heat output by transferring it back and forth among their bodies while reducing the surface area of exposed skin, thus minimizing heat loss to the atmosphere. A second photograph shows the piggies in warm conditions. Spread out and sprawled, the creatures are shed ding as much body heat as possible.
Weather is, of course, critical to farmers. Once American farmers realized that the rain doesn't really follow the plow, growers tended to plant crops where they had the best chance of producing good yields, even if forced to use irrigation. But while all agricultural ventures are vulnerable to major weather events that come at the wrong time, microclimate is not a big concern with most crops. Except, that is, for citrus.
"We're always worried about freezes, which not only can destroy a crop, but the very wood in a whole grove of trees," says John Attaway, research director for the Florida Citrus Commission. When temperatures fall at critical times in the growing cycle, the difference between one grove surviving and the other dying is often a matter of microclimate. In Florida, this is most influenced by relative elevation and proximity to bodies of water.
"During the great freeze of 1895," says Attaway, "the groves around Keystone City in Polk County came through with relatively little damage. Just about everywhere else in Florida, the industry was wiped out. Keystone City is near several lakes of fairly good size. The water helped keep the area warm." That year, the people of Keystone renamed their city Frostproof. 
Elevation is just as critical as water, for cold air sinking toward low ground can turn into "frost pockets." When cold, calm air settles over Florida in a "radiation freeze," temperature gradients follow a punishingly steep line as one moves downhill. "There was an experiment in which some researchers measured the temperature at about 320 feet above sea level, the highest point around," says Attaway. "They set up thermometers every 100 yards for a mile, down to 150 feet above sea level. The temperature at the high point was 46 degrees F. At the low point, it was 21 degrees."
Another agricultural giant, the wine industry, has tried to make use of microclimates. Until the early 1960s, Californians vintners planted whatever variety they thought would sell bestno matter the soil or climate conditions. 
In the past, for example, in the Carneros region just north of San Francisco Bay's cooling summer fog, cabernet sauvignon grapes simply didn't ripen all that well. Further north, in the upper reaches of the Napa Valley near Calistoga, where the weather is warmer and sheltered from coastal effects, growers had planted thousands of acres of pinot noir grapes, which need generally cool tempera tures. Through experimentation and working with viticulturists at the University of California at Davis, winegrowers began matching varieties with prevailing temperatures. Most winegrowers now pay attention to their microclimates.
Clearly humans can work with microclimates or modify them for the worse. "The rivers of ancient Persia were much larger than they are today," says Penn State's Dennis Thomson. "There are ruins of large ancient bridges that now cross trickles. The likely cause was deforestation, and I guess you could say the impact is more on a local or meso scale than micro." But within a 15-minute drive of Thomson's office in State College, Pennsylvania, one can find the ill-advised actions of mankind upon a microclimate that wasn't so great to begin with.
Separated from Nittany Valley by Gatesburg Ridge is another vale known as Barrens Valley, or the Scotia Barrens, after a mining town that's long since disappeared. In the late 1700s, French explorers noted that this area with acidic soil was less fertile than the fruitful Nittany but still supported a forest of oak, pine and white birch. Then, in the 1800s, Andrew Carnegie opened an ironworks in Scotia, stripped the surrounding surface land for its iron and cut down the forests to make charcoal for its smelters. Today, the Barrens has recovered, but its trees are stunted and, gnarled and look different from those in other central Pennsylvania valleys.
One can't totally blame Carnegie, says Thomson. "The Barrens is unusual in that its sandy soil allows water to percolate deep into the ground," he explains. "Dry soil is less able to conduct heat but radiates it just as quickly as soil that's moist." What's more, this valley is contained by hills at either end; when cold air drains to its bottom, there's no place for the chill to go. "The minimum temperature in the Barrens is, on average, 10.5 degrees lower than at State college," says Thomson. "In that valley, on at least one day out of every month of the year, you re likely to have a freeze."
Unlike the Barrens, cities are inherently hot. In fact, the study of "urban heat islands" is of great interest to climatologists like Oke, a leading authority on the subject. "It's not a new concept,' he concedes "In 1815 Luke Howard of the Royal Society of London published a three-volume study that included a comparison of the temperature of London and its surrounding- rural areas. He didn't call it a 'heat at island,' but he defined the principles perfectly."
Oke and others have added greatly to the understanding of urban microclimates "In general, cities absorb more solar energy and store it better (in construction materials), and have fewer plants to cool the air than does underdeveloped countryside" says Oke. "As a result, some surfaces in a city can be more than 50 degrees warmer, and the air a few degrees warmer, than in rural areas. At night, the city has trouble ridding itself of stored heat because thermal radiation is trapped among the buildings and the street. So the air temperature can sometimes be more than 20 degrees higher than in the sur rounding countrysides."
Then there's Southern California. Some 150 years ago, before the Los Angeles region was developed, summer high temperatures averaged about 102 degrees. By the 1930s, when enormous orchards were planted and fed by water from the Sierras, summer highs averaged only about 96 degrees. As freeways and parking lots have supplanted groves, however, L.A.'s summer temperatures have climbed back to desertlike averages.
Cities as jumbles of physical microclimates
What Oke finds most interesting about cities is that they have so many physical microclimatesas opposed to the biological microclimates on a forested hillsidepacked into a confined space. "Each building has its own materials, colors, shapes and such that reflect or absorb energy, or keep nearby areas in perpetual shadow," he says. "Cities can also reduce wind speed by at least 25 percent. All those tall objects create drag and spin off turbulence."
The canyons between buildings can shunt that wind in unusual patterns. Oke is asked how a person could be walking down one city street with a stiff wind at his back, but move over to a parallel avenue and, while strolling in the same direction, get a blast of breeze in the face. The bearded Oke jumps up at the question and moves to a drawing board on his office wall.
"Anytime you have a building taller than 25 stories, you're going to have some problems with turbulence at ground level," he explains, sketching a building and drawing arrows to show that some wind spills easily over the top. "But much like the situation with a ridge, wind is buffeting against an object that it can't pass through, so pressure builds up ahead, with a low pressure behind," Oke explains. "At ground level, this sucks air in behind the building in the opposite direction. The edges of buildings also cause turbulence near corners, which can flatten pedestrians. Now engineers install baffles and canopies to protect people."
There is still the "problem" of summer urban heat islands. A team led by Hashem Akbari of the Lawrence Berkeley National Laboratory that includes scientists from UCLA is working on a project to cool Los Angeles. Investigating the degree to which shade trees can cool houses (Smithsonian, April 1990, their re also looking into various compositions of roofing materials. As Akbari told New Scientist magazine, "The urban heat island is a man-made effect. And if man is doing it, then man can undo it."
"Hashem and his group are on the right track," says Oke. "If you build roads and roofs of materials of light colors that reflect rather than absorb energy, then it's possible to cool a city, which in turn conserves energy and reduces air pollution. There are also ways to use trees in parks and around homes to modify neighborhood microclimates."
Windbreaks alone can produce microclimates. For centuries peasants have used little stone-walled planting enclosures on windy hillsides. "Knowledge of microclimates goes back to ancient times, and good gardening techniques were maintained through the Middle Ages in monastery gardens," adds Pam Pierce, cofounder of the San Francisco League of Urban Gardeners (SLUG). "In medieval gardens, you see fruit trees planted right next to walls, because the people knew that walls radiated heat, which led to better fruiting."
Like all microclimates, gardens are affected by hills, hollows and orientation to the sun. A fence built at a low spot on the northeast side of the property is going to impede the flow of evening drainage air, which makes it no place to plant tomatoes; in most places, says Pierce, tomatoes like to be planted next to light-colored walls, the better to absorb radiation. The thing is, the very shrubs and such that make for good gardens also make for a benign microclimate around the house.
With windows, location can be everything
And yet, like the difference between east and west Corte Madera, location is a matter of major importance. Dennis Thomson figured this out before he built his house at Hemlock Hollow Farm in the 1970s. All he needed was an anemometer, thermometers and the patience of a micrometeorologist. Thomson eventually picked a spot not far up the flank of Bald Eagle Ridge. His living room has large south-facing windows; full sun in winter obviates the need to use much heat in the house, except on the coldest of Pennsylvania winter days.
Where central Pennsylvania grows terribly hot and humid in the summer, however, Thomson and his wife, Joan, are about the only folks in the region who don't need an air conditioner. The house has windows placed near its corners, which promotes cross-ventilation. But the most savvy feature is that the house sits on a spot where a gentle afternoon drainage wind spills off the ridge and pours right through the open windows.. "If we'd located the place ten yards further up the hill, or ten yards lower," says Thomson, "we'd miss that band of wind."
Thomson might call his house-building an example of applied micrometeorology. The same goes for his warning about the ridge-accelerated gusts near the proposed highways. To others, the little wonders of tiny weather are simply the stuff of microclimate magic.