Australia is the driest continent in the world. Much of its agricultural land is prone to unpredictable rainfall which has an annoying tendency to fall as soon as the spray rig enters the paddock! Even then it is often not a good soak but an intermittent drizzle, which leaves us confused about whether or not a product sprayed will be washed off wasting time and money.
Most foliar applied, agricultural chemical products are designed to either remain on the surface of leaves or to penetrate into them; this does not happen instantaneously. For example, glyphosate takes many hours before the active is safely inside the plant and protected from the elements. While chemicals remain on a leaf surface they are vulnerable to being washed off by rain.