CHAPTER #7
Precipitation



Precipitation: the water in solid or liquid form that falls from clouds to the earth surface. Precipitation from nimbostratus is longer but lighter than precipitation from cumulonimbus.

Terminal velocity: the constant downward-directed speed of a particle within a fluid due to a balance between gravity (downward) and fluid resistance (upward). It increases with the size of the particle. The cloud droplets and ice crystals, which have relatively low terminal velocities, suspended in the atmosphere indefinitely unless they undergo significant growth or vaporize. The terminal velocity of raindrop is given as

terminal_velocity (m/sec) = 9.65 -10.3 exp(-0.6 D)

where D is the drop diameter in millimeter.

Table 7.1, page #172 (Ahrens)

Figure 7.1, page 170 (Ahrens)


Cloud droplet growth by condensation cannot produce raindrops alone. The Collision-coalescence process is necessary to grow the cloud droplets into raindrops. It requires a family of droplets at different sizes. The growth becomes more frequent as a large droplet undergo repeated collision and coalescence with smaller droplets followed by droplet break-up. Not all collisions result coalescence, small droplets can bounce off large droplet.

Figure 7.4, page #172 (Ahrens)

Ice crystal growth by deposition cannot produce precipitation particles alone. The Bergeron process is necessary to grow the ice crystals into precipitation particles. The Bergeron process requires co-existence of water vapor, supercooled water droplets and ice crystals.

Figure 7.10, page #177 (Ahrens)

Figure 7.13, page #179 (Ahrens)


Virga: A shaft of precipitation that falls from a cloud but evaporates before reaching the ground.

Precipitable water: the amount of water produced when all the water vapor in a column of air condenses. An average precipitable water within the troposphere is about 25 mm (1.0 in.). It decreases with latitude from more than 40 mm (1.6 in.) in the humid tropics to less than 5 mm (0.2 in.) near the poles. The precipitable water in an atmopsheric column can be calculated as

precipitable water = specific humidity x vertical pressure gradient / (gravitational constant)

Drizzle: the small water droplets between 0.2 and 0.5 mm in diameter. It is associated with fog (stratus cloud) and poor visibility, but never with convective clouds.


Rain: the precipitating water drops mostly between 1 and 6 mm in diameter. Raindrops produced in warm clouds are usually smaller (d < 2 mm) than those falling from the cold clouds. Ironically, raindrops in warm Hawaiian showers reach 4 to 5 mm as large as 8 mm. Recently, the largest raindrop at 9.7 mm diameter was reported in Oklahoma thindersorms. Raindrops are circular when they are below 1 mm in diameter and their bottom surface flatten with increasing the size (tear-drop).

Table 7.2, page #176 (Ahrens)

Freezing rain (or freezing drizzle): the supercooled raindrops or drizzle-drops that freeze on contact with cold surfaces. It can bring down tree limbs, snap power lines, and totally disrupt traffic.

Figure 7.24, page #186 (Ahrens)


Snow: an assemblage of ice crystals in the form of flakes. All snowflakes are hexagonal (six-sided). Snowflakes may consists of needles, dendrites, plates, or columns depending on the temperature. The size of a snowflake depends on the water vapor concentration and collision efficiency. An aggregate snowflake may exceed 5 to 10 cm (2 to 4 in.) in diameter.

Table 7.4, page #184 (Ahrens)


It is more favorable at relatively warmer temperatures regarding the availability of water vapor. In fact, snow can fall even the surface temperature is as high as 10 C (50 F) if the wet-bulb temperature is below 0 C (32 F).

Table 7.3, page #184 (Ahrens)

Figure 7.19, page #184 (Ahrens)

Baltimore, MD receives 20.1 inches of snowfall annually.


Snow grains: the opaque grains of ice crystals (frozen drizzles) with diameters generally less than 1 mm.

Snow pellets: soft conical or spherical ice crystals with diameters of 1 to 5 mm. They form when supercooled cloud droplets collide and freeze together. They usually fall as showers, especially from cumulus congestus clouds. SNow pellets are larger and softer than snow grains.

Ice pellets (sleet): the frozen raindrops that bounce on impact with the ground. It differs from the freezing rain since the latter does not bounce when it hits the surface.

Figure 7.25, page #187 (Ahrens)

Hail: the rounded or jagged lumps of precipitating ice particles. It is associated with thunderstorms that have a strong updrafts and relatively great moisture content. The largest hailstone ever reported in the United States fell on Aurora, Nebraska in June 2003. It had a diameter of 17.8 cm (7 in.) and circumference of over 47.6 cm (18.7 in.). Hail is usually a spring and summer phenomenon that destroys the crops in particular.


Snow flurries: light snow showers that fall intermittently for short durations. They may produce light accumulations.

Snow squall: intense snow showers, usually fall from cumuliform clouds. It reduces visibility and may produce moderate accumulation.

Aircraft icing: the large supercooled drops strike the leading edge of the airplane wing and rapidly freeze forming thin sheet of ice. Since the supercooled drops are abudant at temperatures between 0 C and -10 C, the icing occurs during takeoff and landing phases. The prediction of aircraft icing has four categories: trace, light, moderate,and heavy. The deicing is advised in moderate and heavy icing cases.


Rime: a deposit of ice formed by rapid freezing of supercooled water drops as they come in contact with an object in below-freezing air.

Graupel: ice particles that form by the Bergeron process (riming) with diameters of 2 to 5 mm. It reaches to ground as raindrops if it melts, snow pellets, or hail.

Table 7.5, page #191 (Ahrens)


Rain gauge: a cylindrical container which collects and measures the rainfall. It resolves the rainfall to increments of 0.01 in. and the total rainfall is less than 0.005 in. is recorded as a trace. Rainfall is measured once every 24 hours and the gauge is emptied.

With regard to snow, the depth of snow at each observation time and every 24 hours are measured with a yardstick. The meltwater equivalent of snow is determined by melting the snow collected in a rain gauge. As a general rule, 10 cm of fresh snow melts down to 1 cm of water. The ratio of snowfall to meltwater may vary from 3 to 1 for very wet snow to 30 to 1 for dry fluffy snow.

Weighing bucket rain gauge: a device that records the rainfall directly, calibrating the weight of accumulating rainwater as water depth. It is superior in snow measurements.

Tipping bucket rain gauge: a device that collects rainfall in equivalent of 0.01 in. It is more accurate than the weighing bucket gauge, but it does not perform well in subfreezing temperatures. It is superior for rain measruements.

Figures 7.32 page #191

Figure 7.33 page #192 (Ahrens)


Disdrometer: a device that measures the raindrop size distribution. The optical devices can measure fall velocity and shape of raindrops as well as properties of snowflakes. Rainfall and radar reflectivity are an integral product of randrop size distribution.


Weather modification: any change in weather that is induced by human activity, either intentionally (e.g. cloud seeding, fog dispersal, hail suppression) or unintentionally (e.g. rain enhancement by air pollution).

Cloud seeding: a type of weather modification intended to increase rainfall or snowfall by stimulating Bergeron process in clouds that are deficient in ice crystals. The seeding (nucleating) agent is either silver iodide (AgI), or dry ice, solid carbon dioxide (CO2). Statistically significant rainfall enhancement was observed in Israeli cloud seeding project, but not in Florida from early 60's to mid - 70's.

Fog dispersal: a type of weather modification intended to clear the radiation fog either by increasing the air temperature (thereby lowering the relative humidity) in warm fog or by cloud seeding in cold fog. Today, warm fog dispersal systems operated routinely are Orly and De Gaulle airports outside the Paris.

Hail suppression: a type of weather modification intended to suppress the Hail by stimulating the Bergeron process by producing a large number of small hailstones that melt long before reaching ground (Soviet hypothesis). In the United States, annual agricultural losses due to hail exceeds $1 billion.