In considering the role of cirriform clouds in climate change, it is important to distinguish among the relationships of different high cloud types to large-scale atmospheric dynamics. While cirrostratus and cirrocumulus (CsCc) have a clear relation to deep convective sources, the ensemble behavior of cirrus is more subtle. An empirical relation is found between cirrus fraction and deep cloud top temperature that points to detrainment temperature as a dominant factor governing tropical and subtropical cirrus. This cirrus-detrainment-temperature (CDT) relation provides a target for modelers, and suggests an additional cloud-climate feedback. As surface temperatures warm, detrainment temperatures cool as deep cloud top height increases. The CDT relation implies that cirrus fraction increases. Because cirrus are optically thinner than CsCc, the competition between longwave feedbacks and cloud albedo feedbacks leads to a hypothesized positive climate feedback by cirrus fraction. ¿ 1999 American Geophysical Union