Rising Cloud Base Over Northeast Could Disrupt Appalachian Forests

The base height of clouds that form over the Northeastern states has been rising for 30 years, and this could disrupt forests at the north end of the Appalachian Mountains, according to researchers at the Yale School of Forestry & Environmental Studies (F&ES).

Their study will appear June 15 in the Journal of Climate.

“This has implications for the high-elevation forests of the White Mountains, Green Mountains and Adirondacks, where the transition from deciduous hardwood forest to coniferous spruce-fir forest is thought to be controlled by the cloud base,” said Andrew Richardson, who recently obtained his Ph.D. at F&ES and is a member of the Yale research team. The other members of the team are Xuhui Lee, associate professor of forest and micrometeorology, Thomas Siccama, professor in the practice of forest ecology, and Ellen Denny, a research assistant.

The rising cloud ceiling, if it continues, could enable broad-leafed deciduous trees, such as sugar maple and yellow birch, to move up the mountainside, replacing the red spruce and balsam fir which grow at the highest elevations. Some bird species, such as the blackpoll warbler, specialize in coniferous forest habitat and may be indirectly affected by this change in forest composition. In addition, at high elevations, a significant amount of water comes annually from cloud-water deposition – a rising cloud base should result in drier forests and soils, which could have negative consequences for moisture-loving, forest-floor amphibians, such as toads and salamanders.

The researchers examined data from 24 airports, which routinely measure the cloud ceiling because it is important to pilots. The team discovered that in the 18 most northerly airports, the cloud ceiling has climbed an average of 20 feet per year, or 600 feet, since 1973. “It is pretty stunning,” said Richardson.

The study does not suggest what may be causing the upward shift in the cloud base, but Richardson speculates that a variety of factors may be at work. One possibility is cleaner air – in particular, reductions in sulfate aerosols – which has resulted from the implementation of the Clean Air Act. Small, airborne particles can act as condensation nuclei around which cloud droplets form – fewer nuclei would make it harder for clouds to form at lower altitudes.

Other studies have documented greater reductions in sulfate aerosols in the Northeast compared to the Southeast. This fits in with the pattern that Richardson observed: changes in cloud base height at the northern airports were larger at the more northern airports and negligible at the most southern airports. Other factors may also include urbanization or changes in atmospheric profiles related to larger-scale global climate change.

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Dave DeFusco: david.defusco@yale.edu, 203-436-4842