Abstract Canopy temperature, which may be estimated by infrared thermometry (IRT), can serve as an indicator of plant water status. Idso et al. (1981a, 1986) proposed the nowadays much used concept of the crop water stress index, which relates observed canopy surface temperature ( T s ) to maxima and minima temperature bounds. Jackson et al. (1981) defined those bounds on the basis of the energy balance. Those bounds vary with the meteorological situation. In this paper a chart is offered for general use with a fixed frame for the upper and lower bound. It relates canopy surface temperatures with r 1(=1 + r c / r a )-values ( r c the canopy resistance and r a the aerodynamic resistance) as a function of a specifically defined temperature sum ( S). It links the curved lower bound with the straight upper bound by a bundle of r 1-curves (the T s – S– r 1-chart). The lower bound can be expressed by an equation, which approximates the energy balance solution with high accuracy. The sensitivity of the upper bound is also discussed. A comparison was made between bounds following Jackson et al. (1981) and the proposed alternative method, which, however, is limited by the short data-set available for this paper.