Percentile analysis is a way of determining how unusual a rainfall or temperature event is. To determine whether a temperature or rainfall observation is unusual for a particular month, it is compared with all other days in the same month in the climate record. All the observations for that month are ranked in order, from coolest to hottest for temperature, and driest to wettest for rainfall and then broken into 100 equal groups. The first group is the 1st percentile, the second group the 2nd percentile and so on. The 0th and 100th percentiles are the lowest and highest on record daily values for those calendar months. The 97th percentile value is approximately a once-a-month high value, while the 3rd percentile amounts to approximately a once-a-month low value.
Sometimes it can be easier to think of percentiles in qualitative terms, using words to describe how unusual the conditions on a particular day were (see example key on the right). If you read a temperature map using the worded key on the right, any area shaded red on the map has experienced a new record high temperature; this temperature falls into the 100th percentile, which means that it was hotter than 100% of all the other temperatures recorded on all the days in that month in the climate record. Likewise, any area shaded dark blue has experienced a new record low temperature.
Temperature or rainfall that is ranked in the top or bottom 1% of ranked observations fall into the >99th or <1st percentile. A value that is very close to the highest or lowest on record and can be thought of as infrequent event. The top 3% of ranked observations are in the 97th percentile, while the bottom 3% of ranked observations are in the 3rd percentile and can be thought of as events that happen occasionaly (approximately once-a-month). Any areas not coloured on the maps have experienced normal or typical conditions - nothing extreme or unusual, events that occur frequently.
The rainfall maps can be read in a similar way, the 100th percentile is a record high rainfall event, an event that falls into the 99th percentile is an infrequent rainfall event and an event that falls into the 97th percentile for a particular month occurs occasionally.
The key on the right is not based on a recognised standard; it's simply a rough guide to help you interpret the maps.
Read more about climate extremes maps.
