Heat Stress Index

• Definition
• Climatology
• Data Characteristics
• Datasets and Documentation
• References

Addressing Changes in Data Characteristics

NCEI's objective is to extend the Gaffen and Ross dataset and periodically update it. However, as with many climatological datasets, changes in data characteristics posed a challenge to constructing a homogeneous time series. Gaffen and Ross used the National Solar Radiation Data Base (NSRDB), a data base developed by the National Renewable Energy Laboratory (NREL) (NREL 1992) that was derived from surface airways observations. The NSRDB data have not been extended past 1995, so it was necessary to recalculate the base-period climatology using a data set available at NCEI that is continually updated on an operational basis.

We selected the TD3280 dataset of hourly and 3-hourly synoptic observations from first-order National Weather Service stations, which was the basic dataset used in constructing the NSRDB. Currently TD3280 data are routinely quality controlled and updated, and we expect that to continue in the foreseeable future. The highest and lowest daily temperatures were extracted and used to calculate the maximum, minimum and mean daily apparent temperatures. (Note: Because these data were taken from hourly and 3-hourly observations, they are not necessarily the absolute daily maximum and minimum values.) Although relative humidity values were generally available, the calculation of vapor pressure involved an estimation of station level pressure. With the implementation of the Automated Surface Observing System (ASOS), station level air pressure has not been reported in TD3280 since 1998, so mean sea level pressures were extracted and adjusted to station level with the formula P(z) = P(msl)*exp(-z/H), where P(z) is station level pressure, P(msl) is mean sea level pressure, z is station elevation and H is set at 8 km (the assumption of a constant barometer height may be a source of error). To ensure consistency across the entire period of record, this process was used for all years, even though surface pressure data were available prior to 1998.

Results for the 1961 - 1990 base period were consistent with those obtained by Gaffen and Ross, with the 85th percentile values generally within half a degree Celsius. However, because the TD3280 dataset had more missing or erroneous data, complete records could not be constructed for some stations. A total of 187 stations were used in the reconstruction. Time series of the number of days that exceeded the re-computed 85th percentile values can be found below.