North American Climate Extremes Monitoring

Indices

The current NACEM indices are based on daily minimum and maximum temperature values. Some indices are computed using a fixed reference temperatures—these reference temperatures are the same for all stations across North America. Other indices are computed using reference temperatures that vary from station to station—in these cases, reference temperatures are typically defined as a percentile of the relevant station data series. Using percentiles allows for the selection of a reference temperature for each station used in the analysis, resulting in a unique reference temperature for each station.

Index Definition
Number of Frost Days Count of days with daily minimum temperature less than
the reference temperature of 0°C.
Number of Summer Days Count of days with daily maximum temperature greater than
the reference temperature of 25°C.
Number of Icing Days Count of days with daily maximum temperature less than
the reference temperature of 0°C.
Number of Tropical Nights Count of days with daily minimum temperature greater than
the reference temperature of 20°C.
Much Below Average Minimums Percentage of days when daily minimum temperature was
less than the 10th percentile.
Much Below Average Maximums Percentage of days when daily maximum temperature was
less than the 10th percentile.
Much Above Average Minimums Percentage of days when daily minimum temperature was
greater than the 90th percentile.
Much Above Average Maximums Percentage of days when daily maximum temperature was
greater than the 90th percentile.