With common UTC effects across longitude groups, morning six-hour gains of at least 10 kt were more frequent in an eastern Pacific sample in 1982-2024

We tested a morning-to-evening contrast in large six-hour cyclone wind gains after control for UTC reporting time. We used fixed IBTrACS v04r01 records for 1982-2024. Complete offshore days gave four non-overlapping wind changes and one start in each solar-time bin. The samples had 1,539 North Atlantic days and 2,833 eastern Pacific days. With day and UTC effects, the morning minus evening probability of a gain of at least 10 kt was -1.11 percentage points in the Atlantic and +2.81 in the Pacific. Nominal 97.5% season-bootstrap intervals were -4.09 to 1.97 and 0.78 to 5.17. Three-season blocks gave the same primary interval signs. The Pacific estimate stayed positive at 150 and 500 km land-distance cuts. Gains of at least 15 kt, any positive gain, and mean wind change gave uncertain adjusted Pacific contrasts. The short 2018-2024 subset also gave an uncertain contrast. Only two longitude-to-clock mappings occurred in each basin. Separate UTC effects for those groups made the solar effect unidentifiable. Thus the positive result requires a common additive UTC effect across longitude groups. It does not establish a physical solar effect, operational forecast skill, or a change in standard 24-hour rapid intensification. Discrete revised winds, selected complete days, sparse clock support, and missing physical fields limit inference. Source subsets, full source hashes, code, all bootstrap contrasts, a separate audit, and a figure are included.

tropical cyclone · diurnal cycle · UTC reporting time · local solar time · IBTrACS · intensification · identification · threshold sensitivity