Grass embryos possess structures that do not occur in any other flowering plants. Due to the specific embryo structure and position, grass embryo surfaces may be exposed to surrounding air under partial caryopsis–soil contact conditions, but whether caryopses of the grass family (Poaceae) can sense soil air humidity to initiate successful germination under partial caryopsis–soil contact conditions remain unknown. Here, we found that grass embryos have the unique ability to absorb water from atmospheric water vapor under partial caryopsis–soil contact conditions. To absorb atmospheric moisture, grass embryos developed profuse and highly elongated hairs on the embryo surface. These hairs, classically known as coleorhiza hairs, developed only on the embryo surface exposed to humid air, and submergence of the embryo surface inhibited their development. In addition to humid air-dependent development, almost all other developmental features of coleorhiza hairs were substantially different from root hairs. However, coleorhiza hair development was regulated by ROOTHAIRLESS 1. Besides the genetic control of coleorhiza hair development, we also identified how caryopses manage to keep the hairs turgid in natural open environments as the hairs were highly sensitive to dry air exposure. Moreover, we video-documented the regulation of developmental processes. The unique humid air-dependent coleorhiza hair development and their ability to absorb water from water vapor present in microsites or soil air give grasses advantages in germination and seedling establishment. Ultimately, coleorhiza hairs may have contributed to the ecological success of the grass family.