Rain is the most common form of precipitation and plays a vital role in the planet's hydrological cycle. It forms when water droplets within clouds grow by coalescence—the collision and merging of smaller droplets—or through the Bergeron process, where ice crystals in mixed-phase clouds grow at the expense of supercooled water droplets until they become heavy enough to fall and melt as they pass through warmer air layers below. Rainfall intensity is classified by accumulation rate: light (less than 2 mm/h), moderate (2–15 mm/h), heavy (15–30 mm/h), and intense or torrential (over 30 mm/h). In the Mediterranean region, torrential rainfall events are particularly frequent during autumn, often associated with phenomena such as DANA or cold drops, where over 200 mm can accumulate in just a few hours. Rainfall is measured using a rain gauge and is represented on maps using isohyets. The distribution of rainfall depends on factors including atmospheric pressure, relative humidity, orography, and general atmospheric circulation. Trade winds transport equatorial moisture, while fronts—cold and warm—generate precipitation bands by forcing air to rise. Globally, equatorial regions receive over 2,000 mm annually, while subtropical deserts barely exceed 250 mm.