Chapter 5--
Forms of Condensation and Precipitation


Condensation occurs when water vapor changes to a liquid. For condensation to take place, the air must be saturated and there must be a surface on which the vapor can condense. In the air above the ground, tiny particles known as (Cloud)condensation nuclei serve as the surfaces on which water vapor can condense. Hydroscopic nuclei, water seeking Salt is an example.

Clouds, visible aggregates of minute droplets of water or tiny crystals of ice, are one form of condensation. Clouds are classified on the basis of two criteria: form and height.

Chapter 5 power point

Clouds Out My Window

The three basic cloud forms are cirrus (high, white, and thin), cumulus (globular, individual cloud masses), and stratus (sheets or layers).

Cloud heights can be either
High, with bases above 6000 meters (20,000 feet) -- Cirrus
Middle, from 2000 to 6000 meters -- Alto
Low, below 2000 meters (6500 feet) -- Stratus
Based on the two criteria, ten basic cloud types, including cirrostratus, altocumulus, and stratocumulus, are recognized.

Stratocumulus D.
Cumulus B.
Nimbostratus F.
Cumulonimbus E.
Altostratus C.
Cirrocumulus A.

Nimbo/Nimbus -- rain cloud, Dark gray

Clouds of Vertical Development - Cumulus and Cumulonimbus. They develop into towers or domes.

For precipitation to form, millions of cloud droplets must somehow coalesce into drops large enough to sustain themselves during their descent.
The two mechanisms that have been proposed to explain this phenomenon are:
Bergeron process, which produces precipitation from cold clouds primarily in the middle latitudes
1. Supercooled - water in a liquid state below 0C
2. Freezing Nuclei - Supercooled droplets freezing on contact with solid materials. -10C or below
The warm cloud process most associated with the tropics called the collision-coalescence process. Large droplets fall more rapidly than smaller droplets,
they are able to sweep up the smaller ons in therepath and grow.

The two most common and familiar forms of precipitation:
Rain (drops of water that fall from a cloud and have a diameter of at least 0.5 millimeter) --Tempertures Above 32F/0C
Snow (precipitation in the form of ice crystals or, more often, aggregates of ice crystals). Temperatures Below 32F/0C
Other forms include sleet (falling small particles of ice that are clear to translucent), glaze (formed when supercooled raindrops turn to ice on colliding with solid objects), hail (hard, rounded pellets or irregular lumps of ice produced in large cumulonimbus clouds), and rime (a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing).



Islip, NY January 21st 2012 3.6 inches of snow

Rain, the most common form of precipitation, is probably the easiest to measure. The most common instruments used to measure rain are the standard rain gauge, which is read directly, and the tipping bucket gauge and weighing gauge, both of which record the amount and intensity of rain. The two most common measurements of snow are depth and water equivalent. Although the quantity of water in a given volume of snow is not constant, a general ratio of 10 units of snow to 1 unit of water is often used when exact information is not available.

Intentional weather modification is deliberate human intervention to influence atmospheric processes that constitute the weather.
Weather modification falls into three categories
(1) the use of energy to forcefully alter the weather
(2) modifying land and water surfaces to change their natural interaction with the lower atmosphere
(3) triggering, intensifying, or redirecting atmospheric processes. The focus of intentional weather modification using modern weather technology is on cloud seeding, fog and cloud dispersal, hail suppression, and frost prevention.

Fog, generally considered an atmospheric hazard, is a cloud with its base at or very near the ground.
Fogs formed by cooling include radiation fog (from radiation cooling of the ground and adjacent air),
Advection fog (when warm and moist air is blown over a cold surface)
Upslope fog (created when relatively humid air moves up a slope and cools adiabatically).
Those formed by evaporation are steam fog (when rising water vapor over warm water condenses in cool air) and frontal fog (when warm air that is lifted over colder air generates precipitation that evaporates as it descends and saturates the air near the surface).

Dew is the condensation of water vapor on objects that have radiated sufficient heat to lower their temperature below the dew point of the surrounding air. White frost (hoar frost) forms when the dew point of the air is below freezing.