In tropical areas, PV production can drastically decrease with a passing cloud. In this context, to increase the amount of PV power injected into the insular electricity network, it is necessary to obtain accurate forecasts to react to such brutal changes. All-sky cameras are suitable solutions for observing short-term sky state evolutions. However, such instruments do not provide directly ceiling cloud altitude. Moreover, mechanical features and data transmission features involve a costly maintenance for a long-term sky state monitoring. Cloud infrared radiometer (CIR-4) is a low-cost and autonomous instrument providing three dimensional cloud time-series properties. We collected such parameters in Reunion Island with collocated global horizontal irradiance (GHI) measurements. We built a 4-month continuous measurement time-series illustrating the complex relationships between solar irradiance variability and cloud regime transitions in a tropical area. The possibility to distinguish several clear-sky index distributions in function of typical variational cloud regimes is shown. Our results permit to consider CIR-4 observations as analogues usable for very short-term solar irradiance forecasting algorithms.