The HadAT data are global radiosonde gridded temperature anomalies at standard levels (850, 700, 500, 300, 200, 150, 100, 50, and 30hPa) in the troposphere and in the lower stratosphere from 1958 to December 2012. This monthly timeseries are available on a 10 degree longitude by 5 degree latitude basis. This dataset supersedes the HadRT dataset. All values are anomalies relative to the monthly 1966-95 climatology.

The gridded product is derived from 676 individual radiosonde stations with long-term records. Because of the criteria of data longevity the resulting dataset is limited to land areas and primarily Northern Hemisphere locations. Radiosondes are single launch instruments and there have been many changes in instruments and observing practices with time. HadAT has used a neighbour-based approach to attempt to adjust for these effects and produce a homogeneous product suitable for climate applications.

Zonal averages were created by averaging all available gridbox values in each 5 degree latitude band. Large scale mean timeseries for the globe and the tropics (defined here as 20N to 20S) are simply cos(lat) weighted zonal mean field values. This reduces the spatial sampling bias towards Northern Hemisphere mid-latitudes. Other plausible large-scale averaging techniques would impact the timeseries values calculated.

Cautionary note from the Met Office Hadley Centre

It is important to note that significant uncertainty exists in radiosonde datasets reflecting the large number of choices available to researchers in their construction and the many heterogeneities in the data. To this end we strongly recommend that users consider, in addition to HadAT, the use of one or more of the following products to ensure their research results are robust. Currently, other radiosonde products of climate quality available from other centres for bona fide research purposes are:

- Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC)
- RAdiosonde OBservation COrrection using REanalyses and Radiosonde Innovation Composite Homogenization )
- IUK (Iterative Universal Kriging) Radiosonde Analysis Project

Also see a comparison with Microwave Sounding Unit (MSU) data by Remote Sensing Systems (see links under Docs tab below) .