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This paper describes a class of strategies for reducing persistent contrail formation with the capability of trading off between contrails and aircraft-induced emissions. The concept of contrail-frequency index is defined and used to quantify the contrail activities. The contrail-reduction strategies reduce the contrail-frequency index by altering aircraft's cruising altitude with consideration to extra emissions. The strategies use a user-defined factor to trade off between contrail reduction and extra emissions. The analysis shows that contrails can be reduced with extra emissions and without adding congestion to airspace. For a day with high contrail activities, the results show that the maximal contrail-reduction strategy can achieve a contrail reduction of 88%. When a tradeoff factor is used, the strategy can achieve less contrail reduction while emitting less emissions compared to the maximal contrail-reduction strategy. The user-defined tradeoff factor provides a flexible way to trade off between contrail reduction and extra emissions. Better understanding of the tradeoffs between contrails and emissions and their impact on the climate need to be developed to fully use this class of contrail-reduction strategies. The strategies provide a starting point for developing operational policies to reduce the impact of aviation on climate. [ABSTRACT FROM AUTHOR]