by Riehl and Simpson using the NHRP data from Hurricane Daisy concluded that hot towers were the principal mechanism by which tropical cyclones move warm air into the upper troposphere. The newfound importance of hot towers in tropical cyclones motivated the development of parametrization—the representation of small-scale phenomena and interactions, i.e. individual cumulus clouds—in early weather models. The hot tower hypothesis also inspired the development of convective instability of the second kind (CISK): a conceptual model that emphasized the feedbacks between the latent heat released by individual cumuli and the convergence associated with tropical cyclones. By the 1970s, many of the ideas and predictions put forth by the hot tower hypothesis had been validated by empirical observations. Critics of the hot tower hypothesis contended it was implausible that a cumulonimbus cloud could be free of entrainment. This facet of the hypothesis remained untested until dropsondes released into hot towers as part of the Convection and Moisture Experiment in 1998 provided the first direct measurements of the thermodynamic structure of hot towers. The data showed that the equivalent potential temperature within hot towers was virtually constant across their entire vertical extent, confirming the lack of entrainment. Other field observations have suggested that some tropical updrafts are diluted by their surrounding environments at altitudes lower than , though strong latent heat generated by ice within the cloud was sufficient to provide the requisite input energy for the Hadley circulation. Scientific research of hot towers experienced a resurgence in the 2000s with a renewed focus on their role in tropical cyclogenesis and tropical cyclone development. Effect on tropical cyclones Vortical hot towers aid in the formation of tropical cyclones by producing many small-scale positive anomalies of potential vorticity, which eventually coalesce to strengthen the broader storm. The high vorticity present in the hot towers traps the latent heat released by those clouds, while the merger of the hot towers aggregates this enhanced warmth. These processes are the major part of the initial formation of a tropical cyclone's warm core—the anomalous warmth at the center of such a system—and the increased angular momentum of the winds encircling the developing cyclone. In 2007, the National Aeronautics and Space Administration (NASA) hypothesized that the wind shear between the eye and the eyewall could enhance updraft through the center of a cyclone and generate convection. Hot towers may appear when a cyclone is about to intensify, possibly