Hayden's students chart microclimates in pavilion gardens

Stephanie Gross

Environmental sciences professor Bruce Hayden (left) and other students watch while Tara Zimet (center) points an infrared thermometer in the air to get a temperature reading. The students are looking at how, even in the same garden, the climate varies from spot to spot, depending on aspects of the landscaping, from trees and plants, to terraced or sloping ground, to brick walls.

By Nancy Hurrelbrinck

In gardens where the University's first students grew vegetables, environmental sciences majors taking a new class, "Climate of Engineered Landscapes," are studying how plantings create microclimates.

"Since it's a student research-oriented course, I'm using the Grounds," said Bruce Hayden, environmental sciences professor. "The Pavilion gardens are very specific landscapes, walled off from one another, each with a different design" -- and a slightly different climate.

The gardens on the East Range, for instance, are steeply terraced, except one that is sloped, and they only get morning sun, so they tend to be cooler.

"We know that cold air runs downhill," said Hayden, whose research focuses on climate and vegetation interactions. "If you sit in a lower garden just after sundown, you can feel that little river of cold air come down and rush past you."

The students make sky maps of the gardens using an infrared thermometer that allows them to take several kinds of measurements, he said, adding that each of the 10 students in the class has a different garden to study. Hayden concedes that the students could just as easily learn the physical principles in the classroom and then go look at the gardens. But he prefers a hands-on approach. "When we're taking measurements, it's hard to put it all together into one picture, to answer questions like, 'Do gates and cross walls make a difference?'

"It takes hours and hours to take these kinds of measurements; but this is an empirical course where they're trying to discover something. You can sit in a room and read about the productivity of others, or you can go out and [do your own research]."

His students will point a thermometer directly above them at the sky or the trees' canopy, measuring the temperature, he said. On a clear October day when the temperature is in the upper 60s, the sky is about ­22°F, while the canopy is about 77°F.

"The three-dimensional architecture of the garden plays a large role in determining where it's warm and where it's cold," Hayden said. "Plants don't produce heat, but they do give off energy; there's always a balance between the energy the plant [releases] and the energy it gains from its surroundings."

In mapping the temperature of the sky and trees all over the garden, students gain a better understanding of the balance of energy.

"Gardens are designed and placed on the landscape to make life more comfortable, so they tend to reduce temperature extremes. In some cases they're designed to increase or decrease the humidity as well," he said, adding that gardens in the desert often have fountains in them.

Plants keep a garden cool during the day by using sunlight that would otherwise be heating the air to evaporate water, he explained. The coolest garden on the Lawn is the evergreen one on the East Range, which has a canopy that's nearly closed. The warmest gardens are the most open ones on the West Range, which get plenty of afternoon sun.

In these flat gardens, without cold air runoff, Hayden said, it's the materials in the garden that make the difference in temperature.

A brick walk with sand between the bricks is 15° cooler than one with cement between the bricks, because the sand holds moisture that evaporates during the day, keeping the bricks cool. The use of certain building materials to influence air temperature has broad implications for cities, he said, noting that some are looking for ways this kind of technology can "make the whole city more comfortable. There's been a movement in the last 15 years to increase the shading of streets. Chicago is proposing rooftop gardens, and cities like Phoenix and Los Angeles are trying to use more reflective surface materials that retard heat."

His students are surprised by the effects different materials can have in engineered landscapes, he said, as well as by the presence of cold air drainage, which they typically associate with river valleys.

Hayden, who has taught at U.Va. since 1970, said he also learns a great deal from his students in courses like this. "Empirical work involves generating theories as well as testing them; it's hard to have one without the other. The students are critical to the capacity to generate ideas. Questions we wouldn't dream of asking, the novice asks."