Final answer:
Warm, humid, and unstable air masses are typically referred to as 'maritime tropical' and are denoted as 'mT,' which is relevant to meteorology but not a part of the Köppen climate classification.
Step-by-step explanation:
The classification of air masses is a part of physical geography which deals with the various climate classifications across the globe. The letters that represent an air mass that is warm, humid, and unstable are not specifically outlined in the standard Köppen climate classification, which tends to focus on regional climates rather than specific air masses. However, in the context of air mass classification, warm and humid air masses that tend to be unstable are often referred to as 'maritime tropical air masses' and can be denoted with the acronym 'mT' (marine Tropical) which is not directly a part of the Köppen classification but is relevant to meteorological studies.