Rain falling on a field, in southern
Air contains water vapor, and the amount of water in a given mass of dry air, known as the mixing ratio, is measured in grams of water per kilogram of dry air (g/kg).
 The amount of moisture in air is also commonly reported as
relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.
 How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a
cloud (a group of visible and tiny water and ice particles suspended above the Earth's surface)
 depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The
dew point is the temperature to which a parcel must be cooled in order to become saturated.
There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling.
Adiabatic cooling occurs when air rises and expands.
 The air can rise due to
convection, large-scale atmospheric motions, or a physical barrier such as a mountain (
orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface,
 usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of
infrared radiation, either by the air or by the surface underneath.
 Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its
wet-bulb temperature, or until it reaches saturation.
The main ways water vapor is added to the air are: wind convergence into areas of upward motion,
 precipitation or virga falling from above,
 daytime heating evaporating water from the surface of oceans, water bodies or wet land,
 transpiration from plants,
 cool or dry air moving over warmer water,
 and lifting air over mountains.
 Water vapor normally begins to condense on
condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature)
 force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as
Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of
advection fog during breezy conditions.
Coalescence and fragmentation
The shape of rain drops depending upon their size
Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets.
As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing, and is also known as the warm rain process.
 In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.
Raindrops have sizes ranging from 0.1 to 9 mm (0.0039 to 0.3543 in) mean diameter, above which they tend to break up. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become increasingly flattened on the bottom, like
hamburger buns; very large ones are shaped like
 Contrary to popular belief, their shape does not resemble a teardrop.
 The biggest raindrops on Earth were recorded over
Brazil and the
Marshall Islands in 2004 — some of them were as large as 10 mm (0.39 in). The large size is explained by condensation on large
smoke particles or by collisions between drops in small regions with particularly high content of liquid water.
Rain drops associated with melting hail tend to be larger than other rain drops.
Intensity and duration of rainfall are usually inversely related, i.e., high intensity storms are likely to be of short duration and low intensity storms can have a long duration.
Droplet size distribution
The final droplet size distribution is an
exponential distribution. The number of droplets with diameter between and per unit volume of space is . This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it.
 The parameters are somewhat temperature-dependent,
 and the slope also scales with the rate of rainfall (d in centimeters and R in millimetres per hour).
Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as
 The distribution has an upper limit due to droplet fragmentation.
Raindrops impact at their
terminal velocity, which is greater for larger drops due to their larger mass to drag ratio. At sea level and without wind, 0.5 mm (0.020 in)
drizzle impacts at 2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while large 5 mm (0.20 in) drops impact at around 9 m/s (30 ft/s) or 32 km/h (20 mph).
Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized.
 The air density dependence of the maximum raindrop diameter together with fossil raindrop imprints has been used to constrain the density of the air 2.7 billion years ago.
sound of raindrops hitting water is caused by bubbles of air
METAR code for rain is RA, while the coding for rain showers is SHRA.
In certain conditions precipitation may fall from a cloud but then evaporate or
sublime before reaching the ground. This is termed
virga and is more often seen in hot and dry climates.