Online climate change projections report Box 1.2
Mean daily temperature (often referred to as simply mean temperature) is the average of the daily maximum and daily minimum temperatures.
(sometime shortened in this report to just maximum temperature) is the average of the daily maximum temperatures over the temporal averaging period (for example, a season).
(sometime shortened in this report to just minimum temperature) is the average of the daily maximum temperatures over the temporal averaging period.
Precipitation is given as a rate, in millimetres per day; however, when discussing monthly, seasonal or annual average changes to this we refer to it for convenience as simply precipitation. Note also that it is a total of precipitation of all types - rain, snow and hail.
Relative humidity (RH) and cloud
Just as a change in precipitation from 50 mm/day to 60 mm/day would represent a proportional increase of 20%, so a change of from 50% in the baseline climate to 60% in the future climate represents a proportional increase of 20% (rather than 10%). The same comment applies to changes in total cloud,
Extremes of temperatures and precipitation
These refer to changes in the 1st and 99th percentiles of the daily distribution of that particular variable during a season, over the complete 30-year period (that is, about 2700 days). However, because a season has roughly 100 days, changes in the 1st and 99th percentiles of the distribution can be thought of as roughly equivalent to changes in the extreme value of the season, giving a more user-friendly name. Thus the change in the 99th percentile of the daily maximum temperature of the summer season can be thought of as the change in temperature of the warmest day of the summer and will be referred to as such in this report. The change in the 1st percentile of daily maximum temperature will be referred to as that of the coolest day of the season. The change in the 99th percentile of minimum temperature will be referred to as that of the warmest night of the season, that in the 1st percentile as that of the coldest night of the season - whilst recognising that the daily minimum temperature does not always occur at night. The change in the 99th percentile of daily precipitation will be referred to as the change in the wettest day of the season.