The UAH satellite temperature dataset was created by the University of Alabama in Huntsville. It estimates the temperature of different layers in the atmosphere by using satellite data that measures how oxygen emits microwave signals. This data comes from Microwave Sounding Unit measurements.
This was the first global temperature dataset made using satellite information. Scientists have used it to study changes in surface and atmospheric temperatures. The dataset is published by John Christy and others. Earlier, it was also published together with Roy Spencer.
Satellite temperature measurements
Satellites do not measure temperature directly. Instead, they measure light in different parts of the spectrum, and scientists use this information to calculate temperature. The temperature data scientists get depends on the specific methods they use to turn light measurements into temperature readings. Because of this, different research groups have created different temperature records from the same satellite data (see Microwave Sounding Unit temperature measurements). Examples of these groups include Remote Sensing Systems (RSS) and the University of Alabama in Huntsville (UAH). The satellite data series is not completely consistent. It comes from many satellites, starting with the 1978 TIROS-N, and each satellite had similar but not exactly the same equipment. Over time, the sensors on the satellites wear out, and adjustments are needed to account for changes in the satellites' orbits and movement. Large differences in temperature records happen when there is little overlap in the time periods covered by different satellites, making it hard to match the data accurately.
Description of the data
The UAH dataset is created by a group that measures temperature using satellite data.
UAH provides temperature information for three levels of the atmosphere:
– The lower troposphere (TLT, originally called T2LT).
– The mid troposphere (TMT).
– The lower stratosphere (TLS).
The data show temperature differences compared to a seasonal average from a past time period, as well as exact temperature values. The reference period for temperature differences was updated in January 2021, changing from 1981-2010 to 1991-2020.
All data can be downloaded from the UAH website.
To compare with surface temperature trends (which show an increase of +0.161±0.033 °C per decade from 1979 to 2012, according to NASA GISS), it is best to use data from the part of the atmosphere closest to Earth’s surface, the lower troposphere. From December 2019, UAH data show a warming trend of +0.13 °C per decade for the lower troposphere between 1979 and 2019.
Another group, Remote Sensing Systems (RSS), also studies the same satellite data. Their data show a warming trend of +0.208 °C per decade.
Data are available as global, hemispheric, zonal, and gridded averages. The global average covers 97-98% of Earth’s surface, excluding areas near the poles (above +85 degrees and below -85 degrees) and, for TLT and TMT, some high-altitude land areas. Hemispheric averages cover the northern and southern hemispheres from 0 to ±85 degrees. Gridded data create nearly complete temperature maps. Some sources note that global trends are calculated between 82.5S and 82.5N (70S to 82.5N for TLT).
Daily global, hemispheric, and zonal data are available. Monthly averages are also available in gridded format, as well as for each hemisphere and globally.
All datasets include information starting from December 1978.
Comparison with other data and models
When comparing these measurements to surface temperature models, it is important to remember that the lower troposphere temperatures measured by the MSU are an average that considers temperatures at different altitudes (about 0 to 12 km), not just the temperature at Earth's surface (see figure in the Microwave Sounding Unit temperature measurements article). Because of this, the results are not exactly the same as surface temperature records or models.
Before 1998, results from UAH showed no warming in the atmosphere. In a 1998 study, Wentz and Schabel found that this (along with other differences) was caused by the NOAA satellites moving closer to Earth over time. After correcting these errors, the UAH data showed a temperature increase of 0.07 °C per decade in the lower troposphere.
Some differences still exist between UAH measurements and those from other groups. As of 2019, UAH calculated the lower troposphere temperature trend from 1979 to 2019 as +0.13 °C per decade, while RSS calculated it as +0.208 °C per decade.
A more detailed explanation can be found in the "Comparison with surface trends" section of the Microwave Sounding Unit temperature measurements article.
Corrections made
The table below shows the changes made to the UAH TLT dataset. The "trend correction" describes how the global average temperature trend, measured in degrees Celsius per decade, changed after the correction was applied.
In a 2005 study by Mears et al., NOAA-11 played an important part in finding an error in the correction for daily temperature changes. This error caused a 40% increase in the temperature trend from version 5.1 to 5.2 of the dataset.
In a 2007 paper, Christy et al. said that the temperature trends in the tropics, measured by radiosondes, are more similar to the v5.2 UAH-TLT dataset than to the RSS v2.1 dataset.
Many differences between the UAH and RSS datasets, especially in the Lower troposphere global average temperature trend, were reduced when RSS released version 3.3 in January 2011. At that time, the RSS and UAH TLT datasets were very close, differing by less than 0.003 K per decade. However, differences remained in the Mid Troposphere (TMT) temperature trends. In June 2017, RSS released version 4, which raised the temperature trend from 0.136 to 0.184 K per decade, making the difference between the datasets much larger again.
A test version of dataset 6.0 was released on April 28, 2015, through a blog post. This version has better spatial resolution and uses new methods to average temperature data across grid points.