Standard Reference Period in climatology is a time interval of 30 years which averages are characterising the present 'normal' climate conditions over a certain area. For example, the average temperature and rainfall of the time interval between 1971 and 2000 is used as a reference for the 'normal' climate of England and Wales.
Compare the temperature averages of the present reference period (1971 to 2000) with the previuos ones (1961 to 1990). Note how most months were cooler (beside October), particularly in winter. July and August also show an increase. This is because of the mild winters and hot summers of the 1990s. Note how April, May, June and September, show little warming. However, the average Central England Temperature rose from 9.48 °C to 9.75 °C.