Precipitation related variables



Precipitation includes rain, snow, hail and sleet. This section presents the analysis for:



Calculated Precipitation Variables


As with temperature, there are a number of other measures that we can calculate from the information we have on precipitation. These usually describe something about the nature of precipitation within a year and relate to frequency (in other words how often it happens), the intensity and the volume of rainfall happening over a period


  • Snow cover. Meteorological stations record the state of the ground at 0900 hrs, that is to say 9 o'clock Greenwich Mean Time, and from this we can measure the number of days with snow lying on the ground. It is recorded that snow is lying on the ground if more than 50% of the ground is covered with snow. This index is not the same as the length of the snow season but it gives us a good idea of it.
  • Days of intense or heavy rainfall, in other words we count of the number of days with rainfall of more than 10 millimetres.
  • Consecutive dry days (CDD). The maximum number of dry days in a row, where a dry day is a day with no more than 0.2 millimetres of rainfall. This does not really give us an idea of drought. Even in drought conditions there may be an occasional day of rain.
  • Rainfall intensity. This is the average amount of rainfall that falls in a day. This only includes days when rainfall is greater than or equal to one millimetre and represents the average rainfall on that day.
  • Maximum five-day precipitation amount. This is a measure of the heaviest rainfall in a five-day period for a year.

Figure 27 shows the rainfall climatology and figure 2 (in chapter 1) shows the network of rainfall stations from which we gather this information. The rainfall climatology is the pattern of average rainfall for Scotland over the 30-year period 1961 to 1990 (the standard length of time over which we assess patterns in climate). As the map shows, North and West Scotland have a wetter climate than eastern areas. This is to be expected given that the Highlands are in this area and they provide a rain-shadow effect for eastern areas, because of the prevailing westerly winds which pass over Scotland. The map allows you to compare the patterns of change presented on the following pages.


Figure 27- Climatology of average amount of rainfall in a year (millimetres) for Scotland between 1961and 1990.