Currently, the ET recommends 27 core indices (the 'ETCCDI' indices) that are calculated for the globe using the RClimDex/FClimDex software.

**Definitions of the 27 core indices**

- FD,
*Number of frost days*: Annual count of days when TN (daily minimum temperature) < 0^{o}C.Let*TN*be daily minimum temperature on day_{ij}*i*in year*j*. Count the number of days where:*TN*< 0_{ij}^{o}C. - SU,
*Number of summer days*: Annual count of days when TX (daily maximum temperature) > 25^{o}C.Let*TX*be daily maximum temperature on day_{ij}*i*in year*j*. Count the number of days where:*TX*> 25_{ij}^{o}C. - ID,
*Number of icing days*: Annual count of days when TX (daily maximum temperature) < 0^{o}C.Let*TX*be daily maximum temperature on day_{ij}*i*in year*j*. Count the number of days where:*TX*< 0_{ij}^{o}C. - TR,
*Number of tropical nights*: Annual count of days when TN (daily minimum temperature) > 20^{o}C.Let*TN*be daily minimum temperature on day_{ij}*i*in year*j*. Count the number of days where:*TN*> 20_{ij}^{o}C. - GSL,
*Growing season length*: Annual (1^{st}Jan to 31^{st}Dec in Northern Hemisphere (NH), 1^{st}July to 30^{th}June in Southern Hemisphere (SH)) count between first span of at least 6 days with daily mean temperature TG>5^{o}C and first span after July 1^{st}(Jan 1^{st}in SH) of 6 days with TG<5^{o}C.Let*TG*be daily mean temperature on day_{ij}*i*in year*j*. Count the number of days between the first occurrence of at least 6 consecutive days with:*TG*> 5_{ij}^{o}C.and the first occurrence after 1^{st}July (1^{st}Jan. in SH) of at least 6 consecutive days with:*TG*< 5_{ij}^{o}C. - TX
_{x},*Monthly maximum value of daily maximum temperature*:Let*TX*be the daily maximum temperatures in month_{x}*k*, period*j*. The maximum daily maximum temperature each month is then:*TX*=max(_{xkj}*TX*)_{xkj} - TN
_{x},*Monthly maximum value of daily minimum temperature*:Let*TN*be the daily minimum temperatures in month_{x}*k*, period*j*. The maximum daily minimum temperature each month is then:*TN*=max(_{xkj}*TN*)_{xkj} - TX
_{n},*Monthly minimum value of daily maximum temperature*:Let*TX*be the daily maximum temperatures in month_{n}*k*, period*j*. The minimum daily maximum temperature each month is then:*TX*=min(_{nkj}*TX*)_{nkj} - TN
_{n},*Monthly minimum value of daily minimum temperature*:Let*TN*be the daily minimum temperatures in month_{n}*k*, period*j*. The minimum daily minimum temperature each month is then:*TN*=min(_{nkj}*TN*)_{nkj} - TN10p,
*Percentage of days when TN < 10*:Let TN^{th}percentile_{ij}be the daily minimum temperature on day*i*in period*j*and let TN_{in}10 be the calendar day 10^{th}percentile centred on a 5-day window for the base period 1961-1990. The percentage of time for the base period is determined where:TN_{ij}< TN_{in}10To avoid possible inhomogeneity across the in-base and out-base periods, the calculation for the base period (1961-1990) requires the use of a bootstrap procedure. Details are described in Zhang*et al.*(2005). - TX10p,
*Percentage of days when TX < 10*:Let TX^{th}percentile_{ij}be the daily maximum temperature on day*i*in period*j*and let TX_{in}10 be the calendar day 10^{th}percentile centred on a 5-day window for the base period 1961-1990. The percentage of time for the base period is determined where:TX_{ij}< TX_{in}10To avoid possible inhomogeneity across the in-base and out-base periods, the calculation for the base period (1961-1990) requires the use of a bootstrap procedure. Details are described in Zhang*et al.*(2005). - TN90p,
*Percentage of days when TN > 90*:Let TN^{th}percentile_{ij}be the daily minimum temperature on day*i*in period*j*and let TN_{in}90 be the calendar day 90^{th}percentile centred on a 5-day window for the base period 1961-1990. The percentage of time for the base period is determined where:TN_{ij}> TN_{in}90To avoid possible inhomogeneity across the in-base and out-base periods, the calculation for the base period (1961-1990) requires the use of a bootstrap procedure. Details are described in Zhang*et al.*(2005). - TX90p,
*Percentage of days when TX > 90*:Let TX^{th}percentile_{ij}be the daily maximum temperature on day*i*in period*j*and let TX_{in}90 be the calendar day 90^{th}percentile centred on a 5-day window for the base period 1961-1990. The percentage of time for the base period is determined where:TX_{ij}> TX_{in}90To avoid possible inhomogeneity across the in-base and out-base periods, the calculation for the base period (1961-1990) requires the use of a bootstrap procedure. Details are described in Zhang*et al.*(2005). - WSDI,
*Warm spell duration index*: Annual count of days with at least 6 consecutive days when TX > 90^{th}percentileLet TX_{ij}be the daily maximum temperature on day*i*in period*j*and let TX_{in}90 be the calendar day 90^{th}percentile centred on a 5-day window for the base period 1961-1990. Then the number of days per period is summed where in intervals of at least 6 consecutive days:TX_{ij}> TX_{in}90 - CSDI,
*Cold spell duration index*: Annual count of days with at least 6 consecutive days when TN < 10^{th}percentileLet TN_{ij}be the daily maximum temperature on day*i*in period*j*and let TN_{in}10 be the calendar day 10^{th}percentile centred on a 5-day window for the base period 1961-1990. Then the number of days per period is summed where, in intervals of at least 6 consecutive days:TN_{ij}< TN_{in}10 - DTR,
*Daily temperature range*: Monthly mean difference between TX and TNLet TX_{ij}and TN_{ij}be the daily maximum and minimum temperature respectively on day*i*in period*j*. If*I*represents the number of days in*j*, then:

- Rx1day,
*Monthly maximum 1-day precipitation*:Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. The maximum 1-day value for period*j*are:*Rx1day*= max (_{j}*RR*)_{ij} - Rx5day,
*Monthly maximum consecutive 5-day precipitation*:Let*RR*be the precipitation amount for the 5-day interval ending_{kj}_{k}, period*j*. Then maximum 5-day values for period*j*are:*Rx5day*= max (_{j}*RR*)_{kj} - SDII,
*Simple precipitation intensity index*: Let*RR*be the daily precipitation amount on wet days,_{wj}*w (RR ≥ 1mm)*in period*j*. If W represents number of wet days in*j*, then:

- R10mm,
*Annual count of days when PRCP≥ 10mm*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. Count the number of days where:*RR*_{ij}≥ 10mm - R20mm,
*Annual count of days when PRCP≥ 20mm*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. Count the number of days where:*RR*_{ij}≥ 20mm - Rnnmm,
*Annual count of days when PRCP≥ nnmm, nn is a user defined threshold*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. Count the number of days where:*RR*_{ij}≥ nnmm - CDD,
*Maximum length of dry spell, maximum number of consecutive days with RR < 1mm*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. Count the largest number of consecutive days where:*RR*_{ij}< 1mm - CWD,
*Maximum length of wet spell, maximum number of consecutive days with RR ≥ 1mm*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. Count the largest number of consecutive days where:*RR*_{ij}≥ 1mm - R95pTOT,
*Annual total PRCP when RR > 95p*. Let*RR*be the daily precipitation amount on a wet day_{wj}*w (RR ≥ 1.0mm)*in period*i*and let*RR*be the 95_{wn}95^{th}percentile of precipitation on wet days in the 1961-1990 period. If*W*represents the number of wet days in the period, then:

- R99pTOT,
*Annual total PRCP when RR > 99p*: Let*RR*be the daily precipitation amount on a wet day_{wj}*w (RR ≥ 1.0mm)*in period*i*and let*RR*be the 99_{wn}99^{th}percentile of precipitation on wet days in the 1961-1990 period. If*W*represents the number of wet days in the period, then:

- PRCPTOT,
*Annual total precipitation in wet days*: Let*RR*be the daily precipitation amount on day_{ij}*i*in period*j*. If*I*represents the number of days in*j*, then

**References**

- Karl, T.R., N. Nicholls, and A. Ghazi, 1999: CLIVAR/GCOS/WMO workshop on indices and indicators for climate extremes: Workshop summary.
*Climatic Change*,**42**, 3-7. - Peterson, T.C., and Coauthors: Report on the Activities of the Working Group on Climate Change Detection and Related Rapporteurs 1998-2001. WMO, Rep. WCDMP-47, WMO-TD 1071, Geneve, Switzerland, 143pp. (Available as: wgccd.2001.pdf)
- Zhang, X.,
*et al.*(2005): Avoiding Inhomogeneity in Percentile-Based Indices of Temperature Extremes.*J. Climate*,**18**, 1641-1651. (Available as: JCLI3366.1.pdf)

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