The Agatelady: Adventures and Events: October 2013

For today's posting I searched the webpage http://earthobservatory.nasa.gov for images and information about Lake Superior.

The above image shows the vantage point of the International Space Station. Astronauts frequently observe atmospheric and surface phenomena in ways that are impossible to view from the ground. Two such phenomena—gravity waves and sunglint—are illustrated in this photograph of northeastern Lake Superior.

At the top of the image, the Canadian Shield of southern Ontario is covered by an extensive forest canopy typical of early summer. Offshore and to the west and southwest of Pukaskwa National Park, several distinct sets of parallel cloud bands are visible. Gravity waves are produced when moisture-laden air encounters imbalances in air density, such as might be expected when cool air flows over warmer air. This can cause the flowing air to oscillate up and down as it moves, causing clouds to condense as the air rises and cools and to evaporate away as the air sinks and warms. This produces parallel bands of clouds oriented perpendicular to the wind direction. The orientation of the cloud bands in this image, parallel to the coastlines, suggests that air flowing off of the land surfaces to the north is interacting with moist, stable air over the lake surface, creating gravity waves.

The second phenomenon—sunglint—affects the water surface around and to the northeast of Isle Royale. Sunglint is caused by light reflection off a water surface; some of the reflected light travels directly back towards the observer, resulting in a bright mirror-like appearance over large expanses of water. Water currents and changes in surface tension—typically caused by presence of oils or surfactants—alter the reflective properties of the water and can be highlighted by sunglint. For example, surface water currents are visible to the east of Isle Royale that are oriented similarly to the gravity waves, suggesting that they too are the product of winds moving off of the land surface.

A relatively rare blanket of ice rested on the surface of Lake Superior in early March 2009. A NASA satellite captured this image on March 3, 2009.

In this true-color, photo-like image, ice floating on the surface of Lake Superior ranges in color from white to pale gray-blue. The ice appears most solid along the southern shore of the western half of the lake. North of that solid band of ice, cracks reveal deep blue lake water. Dark lake water also appears in the eastern part of the lake, especially along its northern shore. As it does in the west, ice cover appears relatively unbroken at the extreme southeast end of the lake.

Although a completely frozen surface of Lake Superior is unusual, ice lingering on the lake in early March has happened before. As has been reported before on this blog, ice coverage on Lake Superior is down 80 percent since the 1970s.

The calendar may have set September 23 as the first day of autumn in 2007, but the forests that line the eastern shore of Lake Superior had already started to mark the turning of the season.The forests of northern Michigan and southern Ontario flamed orange as the first trees of the season—maples—began to display their brilliant red and orange fall colors. Veins of green run through the sea of orange where the deciduous forest gives way to deep green pine trees.

It was a typical summer day on Lake Superior on August 2, 2007, when the NASA’s Terra satellite recorded the lake’s temperature. Warm waters fringed the southern shores, while cooler waters gather in the center of the lake. Cold waters were welling up from the depth of the lake along the northern shore.

On August 2, it was clear that westerly winds had pushed the warmer surface water on the northwestern part of the lake away from shore, allowing deep, cold water to rush to the surface (dark blue strip along the shore). Unlike the cooler water in the center of the lake, there is a distinct line between the cold upwelling water along the shore and the warmer water to its south.

The upwelling seen here may become a more frequent feature of Lake Superior’s circulation. A recent study of temperature and wind trends as measured by buoys on the lake found that both summer wind speeds and water temperatures increased between 1979 and 2006. An increase in wind speeds would lead to more upwelling. In their analysis of the measurements, Experts have theorized that Lake Superior was able to warm up more because the spring melt is happening earlier. The water has more time to heat up, and so naturally, becomes warmer. If wind speed is increasing at the same time, the lake may be warming more than surface temperature measurements alone indicate. Increased wind-driven upwelling essentially stirs the waters of the lake, tucking heated water in deeper layers of the lake.

The lake effect is particularly clear in this true-color image of the North American Great Lakes region, acquired December 5, 2000. Lakes Nipigon, Superior, and Michigan show striking contrasts between clear and cloudy air as the wind blows from the northwest across the lakes.

As it flows across the relatively warm lakes, the cold dry air gathers heat and moisture from the surface. The warm moist air rises into the atmosphere and mixes vigorously with the cold dry air above. The layer of warm moist air deepens as it travels across the lake. Some of the evaporated water from the lake condenses into streamers of fog rising from the surface, while much of the moisture condenses to form a stratocumulus cloud in the upper half of the mixed layer.

The cloud-forming water droplets may freeze into ice crystals and, due to accumulated water deposition over time, grow into snowflakes. This process can generate snowstorms that produce significant amounts of snowfall downwind. It is not uncommon for lake effect snowstorms to produce as much as two feet of snow within a 24-hour period in northwestern parts of New York and Pennsylvania.

Late August 2010 provided a rare satellite view of a cloudless summer day over the entire Great Lakes region. North Americans trying to sneak in a Labor Day weekend getaway on the lakes were hoping for more of the same.

The Great Lakes comprise the largest collective body of fresh water on the planet, containing roughly 18 percent of Earth's supply. Only the polar ice caps contain more fresh water. The region around the Great Lakes basin is home to more than 10 percent of the population of the United States and 25 percent of the population of Canada.

Isle Royale National Park includes a group of islands and the surrounding waters of Lake Superior. The entire park takes in 2,200 square kilometers (850 square miles), including the territory up to 4 kilometers (2.5 miles) out from shore. Access to the park is via seaplane or ferry service, and from November through mid-April each year, the park is completely closed to visitors. This isolation has allowed the park to remain relatively pristine, one of the reasons it was designated a U.S. Biosphere Reserve in 1980. All but a tiny fraction of the park is federally designated wilderness.

This natural-color Landsat 7 satellite image shows Isle Royale National Park in Lake Superior. The most striking aspects of this image are the deep blue waters of the lake around the main island and the long parallel ridges running the 72-kilometer (45-mile) length of Isle Royale. These ridges formed from lava flows some 1.2 billion years ago. They have eroded and weathered in the eons since, and most recently, they were scoured by an ice sheet during the last ice age. A subtler feature is what you do not see: the entire park is roadless. Visitors to the park either use the network of hiking trails or travel around in small boats. Ferries from the U.S. mainland also offer cruises around parts of the island.

The isolation from the mainland is one key reason that the area was designated a biosphere reserve. Among other kinds of wildlife, the island has populations of moose and wolves. The isolation of the island allows field biologists to track predator-prey relationships with fewer potential sources of confusion and to observe the impact of sudden change on the relationship. As recently as 1900, there were no moose on the island at all. On the other hand, lynx, caribou, and coyotes, all now absent, were observed by visitors.

Biologists believe that moose swam from the mainland at some point early in the twentieth century. In a land with abundant food (at least initially) and no predators, their population grew unchecked until they overgrazed. Eastern timber wolves were first recorded on the island in 1949, having traveled to the island over 15 miles of water during the exceptionally cold winter, when an ice bridge linked the island to the mainland. The wolf and moose monitoring program has been running continuously since 1958. It tracks the influence of weather patterns on populations, inter-pack warfare, survival rates of wolf cubs and moose calves, and grazing and predation patterns. Beaver, fox, hares, and a host of other woodland creatures also make their home on the island archipelago, where their complex interactions are easier to track than they would be on the mainland. Confusing factors such as predation by domestic animals are absent since pets are banned.

The park offers its wilderness experience for visiting humans too, with 36 campgrounds and 265 kilometers (165 miles) of hiking trails connecting them. Nor are park visitors the first humans in the area. Archaeological evidence points to seasonal presence of Native Americans who came and, among other things, mined and smelted copper for trade and tools. Settlers used the area during the 1800s and early 1900s before the park was established in 1940. Lake Superior was and still is a major seaway, and historical lighthouses dot the island where they once warned ships of dangerous shallows. Shipwrecks also testify to the dangers, and are sites for diving for the more intrepid visitors to the park.

This satellite image is a merged pair of scenes obtained with the Landsat 7 Enhanced Thematic Mapper Plus instrument. The northeastern section of the scene was obtained on July 4, 2000; the southwestern section on September 25, 2001.

CITES:
http://earthobservatory.nasa.gov/Search/index.php?q=Lake+Superior