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Near-surface temperatures are related to a variety of processes:

• cloud cover and , cloud optical properties and aerosol 
• albedo and radiative transfer
• precipitation
• surface fluxes
• turbulent diffusion in the atmosphere
• strength of land-atmosphere coupling
• soil moisture and temperature

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Differences between observed and first guess values such as these may lead to very low weight being given to the observation, or to it even being rejected. In many cases the analysed temperatures remain similar to first guess values despite the observations. Users beware!

Effects of aerosol and dust

Aerosol advected across a region can reduce incoming radiation.  Aerosol Optical Depth (AOD) measures the extinction of a ray of light as it passes through the atmosphere.  This can be due to advection of dust etc.  A very crude rule of thumb is that an anomaly (with respect to climatology) of 1 AOD unit corresponds to a 0.5-1.5 °C day-time temperature decrease under otherwise clear skies.  Cloud cover has a much stronger effect upon surface temperature   and mask any signal from the aerosols.  The radiative impact of the forecast aerosol value is more distinct for shorter lead-times (12 or 24 hours).  At longer lead times, the evolving differences in flow patterns and clouds may become more important for the surface temperature differences than the reduced solar radiation.  

Miscellaneous

• Forecast maximum 2m temperatures can be too low particularly during anomalously hot weather.
• If the predicted humidity is too low then maximum temperatures can be forecast to be too high.
• Post-processing (e.g. using a calibrated statistical technique) usually improves 2m temperature forecasts, sometimes substantially.
• Model 2m temperature output corresponds to short grass cover, because by meteorological convention observations are ordinarily made over such a surface.  This strategy may not work so well in:
  • complex terrain - e.g. forests with clearings.
  • over snow areas.  The algorithm which derives 2m temperature uses the model surface temperature while the snow surface is above the earth's surface.  This can be an important consideration where there is deep snow.  See section on snow effects.  

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