The demand for electric power over transmission circuits is increasing at a faster rate than the construction of new transmission facilities. This trend is pushing the capacity of many transmission circuits to their design limits. The power capacity (i.e., the rating) of most overhead transmission lines is prescribed by the so-called "static rating" based on both the conductor configurations and the environmental conditions. Typically, very conservative worst-case assumptions about environmental conditions were used when developing these "static ratings." Due to this conservative approach, significant extra power capacity exists beyond the design margin on most lines most of the time.