Natural Resources In Snow Climates

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Snow falls instead of rain at places of high elevation and high latitude (Figure 3.6). For instance, there is snow every year above about 1,500 m in south-east Australia, and there were nine brief occasions between 1900 and 1979 of snow on the inland edge of Sydney, which is beside the sea at 34°S. But there is no permanent snow even on Australia's highest mountain (Mt Kosciusko at 2,228 m, 37°S) because it is too low (Figure 3.6). Likewise, there is no permanent snow even on the several peaks over 3,000 m in Lesotho, because of a latitude of only 30°S.

Rates of snowfall are low because (i) snowflakes descend at only about 1 m/s, no matter what size, about a quarter the rate for typical raindrops or a tenth the rate of the large drops in heavy rainfall (Table 9.1), and (ii) almost all snow comes from non-convective clouds (Section 8.1).

The depth of settled snow is initially about 5-16 times that of an equal weight of rain, becoming about 2.5 times the depth by the end of winter, i.e. the density becomes 40 per cent that of water. Where there is permanent snow it solidifies after some months as firn, which is granular and has a density which is 40-80 per cent that of water, and then eventually it may become glacier ice with a density of about 90 per cent.

The greatest depth of snow likely on a roof in the Snowy Mountains (which straddle New South Wales and Victoria) increases linearly with elevation (Figure 10.20). Years of deepest snow in a valley in the Snowy Mountains were 1956, 1958, 1960, (1962), 1964, (1966), 1968, (1970), (1972), 1974, (1977), 1981, (1984), (1986), 1990, 1994. (The bracketed years were less notable.) These dates are consistent with the idea of a quasi-biennial variation (Section 10.4), i.e. a tendency for a good year for skiers to be followed by a poor one. In fact, records from 1954-87 show that any year of abnormal snowfall is followed by another above-average year on only 20 per cent of occasions. This is an example of anti-persistence.

Most years with above-average snowfall in the Snowy Mountains coincide with below-average rainfall on the eastern slopes because that rain is due to onshore east winds which are too warm to produce snow, whereas snow comes from outbreaks of polar air from the south-west.

Trends this century indicate a rise in snowfalls during the period 1930-60, and since then a decline (Figure 10.21). Figure 10.21

Max Depth Riser
Figure 10.20 Variation with elevation in the Australian Alps of the average maximum depth of snow each year.

describes the amount of snowfall in a year in terms of the integrated snow depth in metres, i.e. the sum of all the daily depths during a year, so that the figure combines depth and duration, and the maximum value is reached with the last snow of the season. The fluctuations shown in Figure 10.21 happen to correspond approximately to opposite trends in rainfalls at Darwin at the other end of Australia (Figure 10.10).

The duration of the snow season is about sixteen days more for each 100 m elevation in Britain, Switzerland and Australia, though this varies from year to year. The ski season in the Snowy Mountains was almost six months in 1968 but only one month in 1979. Global warming will reduce the period, and preliminary calculations for New Zealand suggest that the snowline will rise 100 m for each 1 K of warming.

Large accumulations of snow can lead to avalanches on slopes of 35-45 degrees, in early spring. Avalanches are most likely when there is a fall of at least 50 mm one day, then a week or two of daily maximum temperatures below -5°C, followed by two or three days with maxima over +2°C.

The melting of snow depends on the net

Figure 10.21 The variation of the integrated snow depth in the Australian Alps. The values shown by circles were measured at Spencer's Creek (at 1,475 m), about 5 km from Mt Kosciusko. Measurements at Kiandra (at 1,395 m, some 40 km to the north) were used prior to 1954 until 1968, as measurements at Spencer's Creek started only in 1954; crosses show the measurements at Kiandra, multiplied by the ratio of the mean snowfall at Spencer's Creek to that at Kiandra between 1954 and 1968. Solid squares show 11-year running means, e.g. the value for 1958 is the average of cross or circle values during the period 1953-68, inclusive.

Figure 10.21 The variation of the integrated snow depth in the Australian Alps. The values shown by circles were measured at Spencer's Creek (at 1,475 m), about 5 km from Mt Kosciusko. Measurements at Kiandra (at 1,395 m, some 40 km to the north) were used prior to 1954 until 1968, as measurements at Spencer's Creek started only in 1954; crosses show the measurements at Kiandra, multiplied by the ratio of the mean snowfall at Spencer's Creek to that at Kiandra between 1954 and 1968. Solid squares show 11-year running means, e.g. the value for 1958 is the average of cross or circle values during the period 1953-68, inclusive.

radiation, heat from the wind, and latent heat released when water vapour deposits onto the snow. Which is most important depends on the circumstances. Heat from the wind is the major cause of melting in the case of a New Zealand glacier. Cloud accelerates melting when temperatures are above freezing, since snow accepts the longwave radiation from clouds but reflects away shortwave radiation from the Sun (Table 2.3).

The melted snow contributes to rivers, combining with the runoff of rain from the land and with groundwater to flow down to the sea. So the next stage of the hydrologic cycle which is the topic of Part III of the book involves the oceans. These are discussed in the following chapter.

NOTES

10.A Typical effects of rainfall in agriculture 10.B Rain gauges

10.C Remote rainfall measurement 10.D Indication of seasonal rainfall by tree rings

10.E Acidity and alkalinity 10.F Soil erosion

10.G Estimation of rainfalls for long recurrence intervals

10.H Defining the 'typical' rainfall

10.I The Bradfield Scheme

10 J The possible effect of forests on rainfall

10.K Dependable rainfall

10.L Indices of rainfall variability

10.N The chance of a second dry day after a first

10.0 Desert runoff

10.P Water budgets of soil moisture

10.Q Flooding of the Brisbane River in 1893

10.R Droughts in New South Wales

10.S Droughts and sunspots

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  • abel
    Is snow a natural resource?
    1 year ago

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