Air humidity in Zagreb (A contribution to the knowledge of climate of the city of Zagreb, I)

(in Croatian)

Abstract

The paper analyzes homogeneous data of vapour pressure and relative humidity recorded at the Zagreb-Grič observatory between 1944 and 1978.

The mean annual course of vapour pressure resembles that of air temperature. Relative humidity in the mean annual course decreases from January to April, increases from April to June, slightly decreases from June to July, and then increases until December; the major minimum in April is due to sudden heating of air, and the major maximum in December is due to the prevalence nightly air cooling. Singularities of vapour pressure (August 13-16) and of relative humidity (January 21-27, May 4-9, June 8-15, December 21-27) have also been determined.

The mean diurnal course of vapour pressure has its major minimum early in the morning, and its maximum is in the afternoon; from June until October unpronounced secondary afternoon minimum appears. Relative humidity mean diurnal course has its maximum about sunrise, and its minimum is in the afternoon.

In the 1944 – 1978 period, vapour pressure does not show a pronounced trend, while relative humidity tends to increase in August and September.

Numerous atmospheric processes are reflected in the vapour pressure data and the relative humidity data recorded on Grič. The processes are air advection, daily heating and nightly cooling of air, evapotranspiration, condensation, sublimation, vapour diffusion, vertical air mixing, subsidence, cyclonal activity (precipitation, increased cloudiness), European monsoon, foehn influence of the Dinaric mountains and the Alps, and, finally, the urban influence. A combination of these processes often increases or decreases the vapour pressure or relative humidity. The influence of these processes is generally not constant throughout the year.