U.S. climate normals: hourly-derived products

New in this installment of the normals product is the use of hourly data to compute selected hourly, daily, and monthly normals. The hourly data are obtained from a subset of NCDC's Integrated Surface Data known as ISD-Lite. Climate normals will be computed for approximately 250 stations whose observations are reported at the hourly timescale. These stations are primarily located at airports. Note that many of these stations changed from manual observations to Automated Surface Observing System (ASOS) measurements in the 1990s, resulting in inhomogeneities in the station time series. At present, procedures for accounting for these inhomogeneities have not been established. Hourly, daily, and monthly normals of temperature, dew point, sea level pressure, and wind have been computed. Among the insights obtained from computing daily values from the hourly observations, this second computation of temperature normals will facilitate comparisons between the customary definition of ‘mean temperature' – the average of the daily maximum and minimum temperature – with a mean daily temperature derived as the average of 24 hourly values. Considering the aforementioned inhomogeneity issue as well as the limited quality control applied to ISD-Lite, care should be taken when interpreting climate normals derived from hourly observations.