De Wit 1672 [Osher Collection]

Current Exhibition:

American Treasures
October 18, 2009 - August 21, 2010

American Treasures celebrates the reopening of the newly renovated and expanded Osher Map Library and Smith Center for Cartographic Education at the University of Southern Maine. It explores the library’s rich and varied collections and its mission to preserve those collections and make them accessible. Beginning with the foundational gifts by the Smith and Osher families, the library’s collections emphasize Maine and New England, followed in order by the United States, the Americas, and the (continued)

Wind and Wave

(click on image to view in greater detail)

Oceanweather Inc. Significant Wave Height and Directions http://www.oceanweather.com/data/

Wind and Wave

The earth's seasons are produced by its annual orbit around the sun, while the rotation of the earth on its axis is responsible for the alternation of night and day; both influence wind regimes of the earth. Within the major general flow of air about the planet are numerous smaller masses of air, all in constant motion, both horizontally and vertically. This flow, at the same time creates changes in pressure, with concomitant low pressure and high pressure cells.

Everything is in motion, everything flowing, trying to reach some state of equilibrium. Wind, pressure, temperature, and the amount of moisture in the air, are all interrelated and part of one large cyclical pattern. To understand that basic pattern is to understand its variations, which, in turn, is to understand weather.

Charted here is one moment (midnight, GMT of Aug. 31, 1996) in this continually shifting pattern of pressure cells and winds. The Bermuda/Azores high pressure system that dominates weather patterns in the North Atlantic is clearly visible as a broad band extending from the Azores to Ireland. In its center the winds are light and variable. Winds move outward and clockwise from high pressure cells. Since Maine lies in the northwest quadrant of this system, the predominant summertime winds are from the southwest.

Three migratory lows--a large, weak low just off the coast of Africa, another east of Puerto Rico, and a third, deeper low off the coast of Florida--will move westward with the Northeast Trade Winds. Another low of 998 millibars, southeast off the tip of Greenland, will move eastward with the prevailing Westerlies. In low pressure cells, winds move inward toward the center, and counterclockwise. Note the tightly compressed isobars between the low off Greenland and the Bermuda/Azores high, where winds reach 45 knots. Similar wind speeds are present around the low pressure cell east of Florida. On the Beaufort Wind Scale, 45 knots is Force 9, and called a strong gale, producing wave heights of 23 to 32 feet.