Precipitation change indicator: data sources and methods, chapter 4

4. Methods

The Precipitation Change in Canada indicator is based on Environment and Climate Change Canada's gridded precipitation anomalies data (Canadian Gridded Temperature and Precipitation Anomalies [CANGRD]), which in turn is based on the Adjusted and Homogenized Canadian Climate Data (AHCCD) for historical climate observations and on near real-time data in the national climate archives for the current year.

Total precipitation considered in this indicator includes rainfall and snowfall amounts. Rainfall amounts were adjusted to take into consideration wind undercatch, evaporation and wetting losses of the rain gauge, while snowfalls were converted to snow water using density corrections based on coincident ruler and Nipher measurements.

The annual mean precipitation departures are computed at each observing station and for each year by subtracting the reference value (defined as the average over 1961-1990 reference period) from the relevant annual values. They are then normalized by dividing by the reference value and expressed in percentage to produce normalized precipitation departures.

The 1948-2014 period is used because nationwide recording of consistent and comparable climate observations began in 1948 in Canada. A nine-year moving average for precipitation is used because precipitation is non-linear (i.e., there is no inherent link in precipitation amounts from one year to the next). The nine-year moving average smooths the variation in the year-to-year fluctuations to help show the trends in the data over the longer nine-year period.

More information about the calculation method can be found in the Climate Trends and Variations Bulletin documentation.