How glaciers shape the land

Weekly Weather Almanac

• Week's warmest temperature: 100 degrees, Aug. 13

• Week's coldest temperature: 52 degrees, Aug. 16

• Weekly precipitation: 0.00 inches

• Precipitation month to date: trace

• Normal precipitation month to date: 0.63 inches

• Precipitation month to date last year: 0.79 inches

• Precipitation year to date: 14.73 inches

• Normal precipitation year to date: 15.54 inches

• Precipitation last year to date: 22.56 inches

• Normal annual precipitation: 26.77 inches

• Total precipitation last year: 33.67 inches

• Precipitation predicted this year: 31.12 inches

• Wettest month on record (since 1895): 9.91 inches in December 1933

• Wettest year on record (since 1895): 43.27 inches, 2012

• Driest month on record (since 1895): 0.00 inches (14 times)

• Driest year on record (since 1895): 15.18 inches, 1929

Readings taken week ending 3 p.m. Sunday, Aug. 16

CliffHarris

Weather Gems

Dr. Billy R. Caldwell, Ph.D., a prominent geologist that I met on my recent voyage to southeastern Alaska, was kind enough to provide me with much of the following information on glaciers. Thanks again, Billy.

What are glaciers?

An alpine glacier is an impressive sight. From its source on a mountainside to its lower end, or snout, it may be miles long, and it may be hundreds of feet thick. Filling a valley from wall to wall with ice, it resembles an immense frozen river. And, like a river, a valley glacier of this sort may be swollen along its descent by tributary glaciers flowing into the main ice stream. Valley glaciers are typical of the Alps and other high mountains around the world.

A second type, the piedmont glacier, forms when a valley glacier descends all the way to the lowlands at the foot of a mountain. With no valley walls to confine it, the ice spreads out to form a broad lobe, sometimes of gigantic size.

Ice sheets, in turn, are immense domes of ice that can cover entire mountain ranges, islands, or even continents. Ice sheets once covered much of North America and northern Europe, and still cover most of Greenland and Antarctica.

Why do glaciers form?

Whenever more snow falls in winter than is lost by summer melting or evaporation, conditions are right the birth of a glacier. Year after year more snow and ice build up in the accumulation zone at the head of the valley. Eventually the weight and pressure become so great that the ice begins to ooze slowly downslope, or, in the case of an ice sheet, to spread outward on all sides.

Where does the ice come from?

The snow that falls on a glacier's accumulation zone is just like snow everywhere else. But over the course of time, natural processes convert it into ice. Repeating thawing and refreezing first transform the snowflakes into a mass of small ice granules called firn or neve. As more snow piles on top of it, the firn becomes more and more tightly packed. Finally, when the accumulation is about 150 feet deep, weight and pressure cause the lowest layers to recrystallize into ice.

How does a glacier move?

Despite their seeming stability, glaciers can and do move, sometimes with surprising speed. Generally the movement is barely perceptible, with the ice advancing less than a foot a day, but some glaciers maintain a steady daily pace of 50 feet or more. And occasionally, for complex reasons, a glacier may undergo a 'surge,' sliding forward for several miles at a rate of 300 feet or more a day.

A glacier does not begin to move until the ice is about 200 feet deep. Then, under tremendous pressure, the normally brittle ice begins to flow somewhat like a mass of frigid molasses. Some of the movement is due to the pull of gravity. Some is due to the slippage of ice crystals over one another. Melting along the bottom of the glacier also plays a role by 'oiling' the track. This happens because ice melts if it is under enough pressure, and the pressure at the bottom of a glacier may be as much as 28 tons per square foot. (In much the same way, the weight of an ice skater causes a film of water to form beneath his or her blades, easily under stress.) When the glacier passes over an obstruction or begins an abrupt descent, for example, its surface may break open into gigantic fissures, known as crevasses, sometimes more than 150 feet deep.

Can glaciers carve solid rock?

A slowly moving glacier has tremendous cutting power. In a process called plucking, the ice actually losses huge chunks of bedrock and carries them along as it creeps downslope.

Embedded in the glacier, the rocks and boulders, sometimes as big as houses, rasp away like the teeth of a file. Grinding against the bedrock, they smooth down projections and carve deep grooves and hollows. In time, many of the embedded rock are themselves worn down to bits of sand that further polish the bedrock.

Eventually the debris reaches the snout of the glacier, where it is dropped in long ridgelike moraines by the melting ice. Streams flowing from glaciers are so heavily laden with rock flour, finely ground particles of once solid rock, that glacier-fed lakes have a characteristic milky look.

Are glacial valleys distinctive?

While stream-carved mountain valleys have a more or less V-shaped cross section, glacier-carved valleys have an unmistakable U-shape. Sheer walls often rise abruptly from the valley floor. In California's glacier-carved Yosemite Valley, the walls in places are more than 3,000 feet high.

Far above the main valley, moreover, 'hanging valleys' may open abruptly on the cliffsides. These were formed by tributary glaciers that flowed into the main glacier, which carved a deeper trench. Waterfalls often spill from hanging valleys, adding to the beauty of the scene.

Can a glacier grow indefinitely?

As a glacier descends a mountainside, it eventually reaches elevations where the ice at its lower end begins to melt and evaporate. When the annual loss of ice at the snout equals the amount accumulated at its head, the glacier stops advancing. If the rate of loss exceeds the glacier's rate of flow, its snout begins to retreat upslope, much like what we saw on our trip to Alaska in late July.

Moraines, accumulations of rubble, build up around a glacier's melting snout. A terminal moraine marks the site of a glacier's maximum advance. Recessional moraines are deposited along the path of a retreating glacier.

North Idaho weather outlook

It was another week of record heat and dryness across the Inland Empire, including North Idaho. The wildfire season accelerated across Idaho late in the week forcing evacuations in some areas of the state.

We set a new record maximum reading on Thursday of 100 degrees in Coeur d'Alene, which broke the previous mark for Aug. 13 of 99 degrees set in 1981.

It was 102 degrees at Post Falls, Rathdrum and Fernan Lake on Thursday. Hayden posted a record high for the date of 101 degrees. Thursday was our 33rd 'Sholeh Day' this blistering summer of 2015 in Coeur d'Alene at or above 90 degrees. The normal number of such hot afternoons is 25 for an entire summer season.

As of this writing on Friday, Aug. 14, we were still awaiting our first measurable precipitation of the month. The first two bone-dry weeks of August tied four other such periods for extreme drought since local Coeur d'Alene weather records began on a daily basis in 1895.

Our total precipitation in town since the current drought began in earnest in early June has been less than a third of normal. It's possible that we could see a stray shower or thunderstorm in the next week to 10 days, but the long-range weather outlook still calls for warmer and drier than normal conditions past the North Idaho Fair and Rodeo at the end of this month into at least early to mid September.

Longer-term, as I predicted last week, we will probably see far less snowfall this winter of 2015-16 than usual in the Inland Northwest due to a still strengthening MONSTER EL NINO in the waters of the east-central Pacific Ocean.

Things could change weatherwise, but I doubt it.