Component Definitions

There are six individual indices available which have been chosen for the impact and risk they pose to people and the economy.  The indices are described in Table 1 below.



High Temperature

Monthly frequency of maximum and minimum temperatures above the 99thpercentile.

Low Temperature

Monthly frequency of maximum and minimum temperatures above the 1stpercentile.


Monthly frequency of 5-day consecutive rainfall above the 99thpercentile.


Monthly frequency of daily wind gust above the 99thpercentile.

Consecutive Dry Days

Annual maximum consecutive dry days.

Sea Level

Monthly maximum sea level.

For the high temperature, the AACI measures the change in the proportion of days in a month on which the maximum and minimum temperatures exceed the 99th percentile of the reference period distribution for the relevant day.  The results for the maximum and minimum temperatures are then averaged. The low temperature measure is calculated using a similar process by looking at the frequency above the 1st percentile. Both the high and low temperature components therefore record if there is a warming in temperature.

Extreme precipitation over short periods is a key driver of flood and storm damage.  The rainfall component of the index focuses on extreme rainfall over a consecutive 5-day window.

During severe wind weather events such as storms and cyclones, it is the maximum wind gusts that are likely to lead to higher risk and damage. The wind metric measures the monthly frequency of daily maximum wind gust above the 99th percentile.

Consecutive Dry Days
Consecutive dry days is included in the index as an indication of drought conditions. It is defined as the number of consecutive days with less than 1mm of rain up to a maximum of 365 days. The cap is included for practical purposes and has no material impact on the index.

Sea Level
The sea level measure tracks movements in the monthly maximum observed sea level, an important risk factor for coastal inundation. This measure uses sea level movements relative to land. Using this measure doesn’t allow for land movements which can be caused by a mixture of climate and non-climate (tectonic movements) related factors, but this is the most readily available measure and is measured accurately by tide gauges.

Further information on component definitions is available in the Design Documentation.