Temperature related variables

Air frost


Figure 22 - Days of air frost for Scottish regions each year, from 1961/62 to 2004/05, with smoothed curves showing a running average.

Table 12 - Change in days of air frost (percentage), from 1961/62 to 2004/05. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.


North Scotland East Scotland West Scotland Scotland
Spring -30.5 -29.0 -29.2 -29.7
Autumn -33.5 -31.3 -33.7 -32.8
Winter -20.1 -21.3 -24.7 -21.7
Annual -25.7 -25.1 -27.7 -26.0


Please note, days of air frost are calculated according to seasons rather than a full calendar year; they are calculated from September to the end of the following May.


Regional trends


  • Since 1961 there has been more than a 25% reduction in the number of days each year of air frost. This is part of a downward trend that is clear in all three regions and nationally.
  • Although changes in winter are the largest in terms of total number of days (a reduction of 10 days) it is the spring and autumn seasons that have seen the largest percentage changes and which show a clear downward trend. This is consistent with the warming trend we discussed earlier.
  • The time-series of annual days of air frost shows that the years with the highest number of days of air frost coincide with years of low average temperature.

Spatial trends


  • The largest changes are usually in areas close to the coast or the Scottish islands. The closeness of an area to the sea has a moderating effect of temperatures in these regions, so days of frost will be less common than regions further inland.
  • Some areas show an increase in the number of days of frost, particularly in northern mainland Scotland and Orkney and Shetland.

Future trends


  • The UKCIP02 report does not mention air frost, but the reductions in expected minimum temperatures should mean some reduction in the number of air frosts also takes place.

Figure 23 - Pattern of change in annual days of air frost (in days), from 1961 to 2004.