*2.6. Statistical Analysis*

A Kruskal–Wallis non-parametric one-way analysis of variance on ranks was used to investigate if the GPS PWV stochastically dominates the other PWV estimations (i.e., atmospheric sounding and MODIS Aqua and Terra) during the dry and wet season [39]. A pairwise multiple comparison procedure was applied using Dunn's method for those groups having a significant difference in PWV in order to isolate the stochastic dominance of the group or groups that differ from the others [40].

**Figure 3.** Representative 72-h air mass back trajectories for the dry (red) and wet (blue) seasons in 2017 calculated using the HYSPLIT Lagrangian model [26].

We also applied a multiple linear regression (MLR) model using surface meteorological data in order to identify the major variables controlling the PWV values in the Central Valley (AACR GPS station).

The cumulative annual precipitation for AACR during 2017 was 2586 mm with a mean daily precipitation during the dry season of 2 mm (range: 0 mm–54 mm) and 11 mm (range: 0 mm–85 mm) during the wet season. In LIBE, the corresponding cumulative annual precipitation was 2161 mm with a mean daily precipitation during the dry season of 1 mm (range: 0 mm–49 mm) and 9 mm (range: 0 mm–247 mm) during the wet season. At both sites, maximum daily precipitation values were recorded at the end of October (Figure 4A). Despite the differences in the cumulative annual precipitation, the relative humidity and air temperature variations were similar in the two regions. The mean relative humidity was 80.8 ± 5.6 % (range: 39.0%–92.6%) and 82.4 ± 8.0% (range: 34.6%–93.6%) in AACR and LIBE, respectively (Figure 4B). The mean air temperatures were 21.0 ± 0.9◦C (range: 16.2 ◦C–23.2 ◦C) and 26.7 ± 1.3 ◦C (range: 22.5 ◦C–30.5 ◦C) in AACR and LIBE, respectively (Figure 4C). However, on a seasonal basis, the mean daily air temperatures and mean daily relative humidity were 6.3 ◦C and 2.1% greater in LIBE than in AACR during the dry season, respectively. During the wet season, the corresponding values were 5.2 ◦C and 1.2% greater in LIBE than in AACR, in that order.

**Figure 4.** The time series of (**A**) the daily precipitation (mm/day) recorded in Heredia (blue bars) and in Liberia (red bars) during 2017: The left y-axis corresponds to Heredia, whereas the right y-axis shows the data of Liberia. (**B**,**C**) The average daily relative humidity (%) and air temperature (◦C) for Heredia (blue circles) and Liberia (red circles).
