Statistical and climatologic analyses were performed for long-term (1979-2004) runs of diurnal saturation deficit variability measured at Wrocław-Swojec Agricultural Observatory. The values of 3 standard time-limit measures (6, 12, 18 UTC) and readings taken from thermo-hygrographs (0 UTC) were used. For purposes of comparisons, equal parts of day were taken for considerations: 'night' half-day (18-6 UTC) and 'day' half-day (6-18 UTC), and also 'a.m.' quarter-day (6-12 UTC) and 'p.m.' quarter-day (12-18 UTC). The changes were averaged as monthly, semi-annual and yearly average values of saturation deficit for analysed parts of day. The annual and long-term runs of saturation deficit defined in that manner, with a basic division into 'day' and 'night' saturation deficits, are characterized by considerable variability. A distinct increase of 'night' and 'day' saturation deficit averages, particularly strong in the last decades, is the most important attribute of the analysed 26-year period. The relative participation of night saturation deficit in the diurnal averages increases as well. Within the annual run, beside a dominant tendency of strong increase of saturation deficit, especially in February, April, June and August, there is also a symptomatic occurrence of its stabilization or decrease (in October for 'day' and November for 'night' saturation deficit). The strongest year-to-year variability of the diurnal saturation deficit is characteristic of the winter period (February). Increase of saturation deficit fluctuations and the occurring humidity trends are linked with intensification of zonal flows (NAO positive phase).