Rain and drizzle are common forms of liquid precipitation, but they are scientifically distinct phenomena resulting from different atmospheric conditions. Understanding the specific physical properties and origins of each reveals why they feel and look so different when they fall. The fundamental distinctions between rain and drizzle center on the size of the water droplets and the rate at which they accumulate on the ground.
Defining Characteristics: Drop Size and Intensity
Meteorologists distinguish between rain and drizzle primarily by measuring the diameter of the individual water droplets. Rain is formally defined as liquid precipitation with droplets having a diameter of \(0.5\) millimeters or greater. Droplets smaller than this threshold are classified as drizzle, typically ranging between \(0.2\) and \(0.5\) millimeters in size.
This size difference directly affects the intensity, which is the rate at which the precipitation falls. Drizzle droplets are small and light, descending slowly and often appearing to float like a fine mist. Drizzle typically moistens surfaces without causing significant runoff or forming puddles. In contrast, larger, heavier raindrops fall faster, creating a much higher accumulation rate that wets surfaces quickly.
Atmospheric Origin: How Rain and Drizzle Form
The contrasting drop sizes are a direct result of the clouds and processes where they originate. Drizzle almost exclusively forms in thin, low-lying stratus clouds close to the Earth’s surface. These clouds are generally stable and lack the strong vertical air currents, or updrafts, necessary to loft the droplets to great heights.
Without strong updrafts, the tiny cloud droplets have limited opportunity to collide and merge with other droplets, a process called collision-coalescence. They fall out quickly, remaining small and uniform before they can grow into full-sized raindrops. Rain, however, typically forms in much thicker, taller cloud structures, such as nimbostratus or cumulonimbus clouds. These clouds feature powerful updrafts that suspend water droplets and ice crystals for longer periods. This extended suspension allows the droplets to undergo numerous cycles of collision and coalescence, or growth via the Bergeron process. The droplets become large and heavy enough to overcome the updraft and fall as rain.
Meteorological Classification and Visibility
Meteorologists classify precipitation events by the rate of accumulation, measured in millimeters per hour. Drizzle is typically reported in the lowest intensity categories, often falling at a rate less than \(1.0\) mm per hour. Light rain is classified as falling between \(1.0\) and \(2.5\) mm per hour, with moderate and heavy rain defined by increasingly higher rates.
The difference between rain and drizzle also affects visibility. Drizzle, due to the immense number of small droplets suspended in the air, creates a misty or hazy environment that significantly reduces visibility. While heavy rain also reduces visibility, this is due to the density and speed of the large drops obscuring the view. Official weather reports often use the two-letter codes DZ for drizzle and RA for rain to differentiate between the two types of liquid precipitation.