This paper describes some new approaches for enhancing infrared thermometry (IRT) as a technique for detecting stomatal closure as a measure of plant water stress in humid environments. Although infrared thermometry has been widely used in arid climates for detecting plant stress (as indicated by stomatal closure) and for irrigation scheduling it has been found to be less reliable in more cooler humid climates. The use of wet and dry reference surfaces to reduce the method's sensitivity to environmental variation is described and indexes based on IRT measurements of the temperatures of individual leaves and of reference surfaces in the same environment are evaluated. Both an index that corresponds to Idso's original crop water stress index, but based on `wet' and `dry' reference leaves, and an index that is linearly related to leaf conductance were derived and shown to be closely related to measured leaf conductance in runner bean crops under a range of conditions. Various types of reference surface were evaluated and the use of non-transpiring and wet real leaves was found to be particularly convenient.