In this work we discuss the depressions often preceding the most important Forbush decreases by less than 24 h, and certain features of a Forbush decrease such as the rarity of onsets during morning hours, the greater amplitude when they happen during the evening as well as the small increases that sometimes happen during the main decrease. We bring forward the fact that the Forbush decrease is always associated to a train of diurnal variations and vice-versa a train of diurnal variations often contains a more or less evident Forbush type decrease. Through an analysis of the energy dependence of the above pre-decreases, and of a separation of local variations from world-wide variations in a Forbush decrease, we show that the forementioned characteristics associated with a decrease, are simply due to the superposition of diurnal variations to the world-wide Forbush decrease. The train of diurnal variations, with the Forbush effect removed, has a variable length that apparently does not exceed about ten days; it is reinforced and is anticipated in phase during a period of two or three days containing the onset of the Forbush storm.