The story of Mount Desert Island's climate is told in much the same terms as its daily weather: temperature, relative humidity, precipitation, wind velocity and direction, barometric pressure, among other indicators. The major difference between climate and weather is the time span over which observations are made. Climate, then, is long-term weather, or, conversely, weather is climate here-and-now.

The significance of climate is that it is a measure of the range of environmental conditions that living systems in a given region are "used to" or "have grown to expect." Each species can survive within an envelope of variable conditions. If temperature, say, exceeds the upper or lower limit of that envelope, or stays at one extreme or the other too long, individuals become stressed and their survival is put at risk. Snowshoe hares and American bald eagles have means for adapting to Maine's year-round climate; armadillos and brown pelicans do not.

Habitats are closely related to climate. In the sense that they are the immediate surroundings in which plants and animals find the climates and other resources they need to survive, habitats are climates come to life. Watersheds are habitat regions in which water is the limiting resource. Aspects of climate affecting the amount of water in the soil during the growing season have a pronounced influence on a given watershed's productivity and well-being. Seasonal precipitation, humidity, and temperature largely determine a watershed's suitability as habitat for local plant and animal life.

Its year-round temperature moderated by the heat-holding capacity of the ocean, Mount Desert Island has a longer growing season than areas just a few miles inland. If spring arrives more slowly than some desire, fall lingers on, and on, and on. Water loss due to evaporation is lessened in summer by humid air blowing off the Gulf of Maine. And the overall amount of precipitation useful to plants, though highly variable year to year, is consistently ample for the support of such thirsty-rooted species as northern white cedar, tamarack, and black spruce. The town of Bar Harbor on Mount Desert Island receives comparatively more annual precipitation than most regions in the United States, 1.4 times as much as Chicago, for instance, 3.4 times as much as Denver, and 7.3 times as much as Phoenix.

Mark Twain knew it, and the data support it: variability is the name of the new England weather game. At Acadia National Park, though November immediately follows October, the range from lowest to highest amount of monthly precipation from 1944 through 1993 was spanned in these two adjacent months. November 1983 with 14.57 inches being 182 times wetter than October 1947 with only 0.08 inches. Variability is evident year to year, January 1979 with 11.78 inches having 16 times as much precipitation as the same month a year later with 0.73 inches. The same story unfolds even within a single calendar year, June 1968 with 4.75 inches receiving 21.6 times as much rain as July of the same year with 0.22 inches.

The notion of a "normal" or "expected" amount of rainfall on Mount Desert Island is elusive at best. If we cite the 50-year average precipitation for January as 4.78 inches, we should not be surprised if some Januaries receive less than 2 inches while others receive more than 8 or 9 inches. The same pattern of variability is true for every month of the year.

Rather than gear our expectations to some fictitious norm, we would do better to think in terms of a range within which monthly or yearly precipitation is apt to fall. We will not be far off if we expect precipitation in January to be greater than the 0.53 inches that fell in January 1944, and less than the 11.78 inches of January 1979. Or to narrow the gap, to say that January precipitation will likely range somewhere between 3 and 6 inches (a range that includes roughly half the Januaries between 1944 and 1993.

Generalizing, mean monthly precipitation at Acadia ranges by a factor of 2, from 3 to 6 inches, climbing from minimum to maximum in three months (September through November) and declining more gradually over the next 9 months (December through August).

Annual precipitation of 37 inches deposits a million gallons of water on every acre of ground. Only 3 years between 1944 and 1993 have fallen just short of that level of precipitation on Mount Desert Island. Local watersheds typically receive between 1 and 1.7 million gallons of water per acre a year from all sources (rain, snow, sleet, fog, dew, etc.).

Climate statistics are based on records of past events. Two opposing trends seem to be wrestling to control Mount Desert Island's climate in the future. The first of these is the 100,000-year Milankovitch cycle governing alternating periods of glaciation (lasting 90,000 years) and interglacial warming periods (lasting 10,000 years). We are now on the cooling downside of the most recent warming period. The peak of the current interglacial interval occurred 10,000 years in the past. A resurgence of glaciation has been predicted to occur within the next few thousand years.

At the same time, the accumulation of carbon dioxide in the atmosphere has been on the increase from natural factors, accompanied by a global warming trend resulting from the buildup of carbon dioxide and other so-called greenhouse gases resulting from human activities on an ever-increasing scale. Glacial nurseries in the Alps, Himalayas, and Andes are ice-free for the first time in 10,000 years. Is the local climate on Mount Desert Island cooling or warming? It is changing, certainly. The answer will be told by hindsight from studies of records yet to come.