34. Appearance of Ice on Lake Windermere
DG George

Number of days on which ice was recorded each year in a sheltered bay on the west side of Lake Windermere shown in relation to average air temperatures recorded at Ambleside, Cumbria during the first ten weeks of each year

Specific Indicator
Daily records of ice-cover are available for Loch Leven (Kinross) from 1968 to1995 and Lake Windermere (Cumbria) from 1936 to the present. Loch Leven is a relatively shallow lake that frequently becomes completely covered with ice during the winter. Lake Windermere is seldom completely covered with ice, but a sheltered bay near the Institute of Freshwater Ecology usually freezes for several days every winter. The specific indicator chosen is the number of days in each year when ice was recorded in a sheltered bay on the west side of Lake Windermere.
[Source: Institute of Freshwater Ecology, The Ferry House, Far Sawrey, Ambleside, Cumbria, LA22 0LP]

Relevance
A long-term trend towards shorter periods of ice cover has been reported in a number of waters in Scandinavia, North America and the Alps. There, the lakes are covered with ice for several weeks every year, and the best measure of change is the date at which the ice starts to break up in spring. In the UK, where lakes freeze for a short period, a better measure is the number of days of partial or complete ice-cover recorded in any one year.

Sensitivity to climatic factors
Obviously, sub-zero mean daily air temperatures are required to produce ice, but lakes freeze at different rates depending on their size and exposure. There is a strong negative relationship between the number of days of ice-cover during January-March on Lake Windermere and the average air temperature for the first ten weeks of the year, recorded nearby at Ambleside.
In northwest Europe, the most important factor influencing the winter air temperature is the magnitude of the North Atlantic Oscillation (see Indicator 4). When the North Atlantic Oscillation index is high there is a strong westerly flow, winters over much of Europe are relatively mild and the zone of maximum moisture transport extends further east.

Changes over time
In recent years, high values of the North Atlantic Oscillation index have been associated with mild winters which have been reflected in few days with ice cover on Lake Windermere. Indeed, there has been a sequence of years when the lake has not frozen at all. It remains to be seen whether this is an aberration or the start of a persistent feature.