Land Surface Temperature (LST) is the radiative skin temperature of the land derived from infrared radiation. In the SLSTR project, "skin" temperature refers to the temperature of the top surface when in bare soil conditions, and to the effective emitting temperature of vegetation "canopies" as determined from a view of the top of a canopy.

A simplified definition would be how hot the "surface" of the Earth would feel to the touch in a particular location. From a satellite's point of view, the "surface" is whatever it sees when it looks through the atmosphere to the ground. It could be snow and ice, the grass on a lawn, the roof of a building or the leaves in the canopy of a forest. LST is not the same as the air temperature that is included in the daily weather report.

Global LST map (September 2016) derived from Sentinel-3A/SLSTR daytime data. (Credit: University of Leicester)

LST is a basic determinant of the terrestrial thermal behaviour, as it controls the effective radiating temperature of the Earth's surface. However, because of the extreme heterogeneity of most natural land surface, this parameter is difficult to estimate and validate. Several factors can fundamentally influence the derivation of LST including:

• temperature variations with viewing angles
• sub-pixel in-homogeneities in temperature and cover
• surface spectral emissivity at the channel wavelengths
• atmospheric temperature and humidity variations
• clouds and large aerosol particles such as dust.

The SLSTR thermal bands used for SST retrieval (the three infra-red channels S7, S8 and S9 at 3.74 µm, 10.85 µm and 12 µm) are also used to retrieve LST in the SLSTR Level-2 products. Algorithms for deriving LST using split-window radiances are sufficiently advanced that an accuracy of 1 K is possible, especially at night when differential surface heating is absent.

SLSTR also includes two low-gain, wide-dynamic range IR (fire and high temperature) channels (F1 and F2) designed to deliver the radiometric data necessary for the generation of quantitative active fire products (FRP products). This prevents saturation of the thermal channels and applies to targets with an upper limit of 500°C.

For further information about the principles of measurements of the SLSTR , see the Technical Guide SLSTR Instrument Description.