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Chip heating and nonuniform distribution of hot and cool zones on the die negatively affect reliability and robustness to failures of nanometer integrated circuits. In fact, signal propagation on interconnects slows down as temperature rises; for long wires crossing regions at different temperatures, such as the clock network, thermally induced delay and skew get altered and may result in timing faults. Failures of this kind are difficult to face due to their transient nature. This paper focuses on clock tree design for the class of embedded systems-on-chip with spatially nonuniform but temporally stationary thermal profiles. We contribute two algorithms for the thermal-aware clock network design that take into account on-chip temperature variations of this nature. The experimental results that we have collected on a number of examples and for different thermal profiles show that, in the presence of on-chip spatial temperature gradients, clock trees designed using a standard methodology incur very significant skew violations, thus originating circuit failures. Instead, clock networks designed using the algorithms presented in this paper always satisfy the initial skew bound.