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Process and Conditions of Condensation

Condensation has been defined earlier as the process in which water vapor is changed to liquid water. It should be noted that when this process occurs at subfreezing temperatures, the vapor often turns directly into the solid form-ice-with no intervening liquid stage. Technically, the transition from vapor to solid is known as sublimation, but we shall use the term condensation to cover either situation, for they are both a part if the same major process.

In the process of condensation in the atmosphere, three general conditions must be satisfied: (1) a sufficient water vapor content must be present, (2) cooling of the air to and below the dew-point temperature is usually required, and (3) nuclei of condensation must exist. These conditions are interrelated in theirs effects.

Forms of Condensation

The forms assumed by condensed moisture in nature are varied and often beautiful in appearance. Depending on the differing conditions under which cooling occurs, the resulting condensation types may be classified under the common headings of dew, frost, clouds and fog.

The Fog

The type of fog that forms depends on exiting conditions and falls into four recognizes categories: radiation fog, advection fog, frontal fog, and upslope fog. The thickness of the fog depends on various factors of humidity, temperature, wind, nuclei, etc.

Dense fog… object not visible at 50 yds
Thick fog… object not visible at 200 yds
Fog… object not visible at 500 yds
Moderate fog…object not visible at ½ nautical mile
Thin fog… object not visible at 1 nautical mile

Forms of Precipitation

All forms of precipitation, regardless of appearance, are often termed collectively hydrometeors. The U.S. Weather Bureau has classified those hydrometers into about fifty specific types.

Rain. Rain is obviously the most familiar example of precipitation. It is so common that in ordinary conversation it usually replaces the more general word. Although the definition of rain is simple, the explanation of its origin is complex, as just the source of rain is simple, the explanation is complex, as just noted. Much rain is the direct result of condensation of water droplets in clouds followed by growth to a size necessary to overcome the buoyant effect of the air. However, a considerable amount of rain is actually melted snow, especially in high latitudes or during the winter in the mid-latitudes.

Snow. When condensation occurs in rising air that has cooled to subfreezing temperatures, typical hexagonal ice crystals tend to form, instead of liquid droplets. It will be remembered that condensation at the earth’s surface yields frost when the dew point is below 32^oF. Snow crystals may exist in isolated form or may coalesce to form snowflakes of varying sizes and shapes.

Sleet. Sleet is often true frozen rain. If the waterdrops falling from the clouds encounter a layer of air with freezing temperature, they solidify into small, hard, clear ice pallets. Sleet thus indicates a temperature inversion even though it may be a very slight one. The temperature in the clouds may be very close to freezing and if the air below the clouds is somewhat colder sleet may result. Sleet also develop fro freezing of melted snow as the later falls though a colder layer of air near the ground.

Hail. Hail is a product of the violent convection found in a thunderstorm and occurs only in connection with a thunderstorm. In the thundercloud the strong vertical air swirls the raindrops above and below the freezing level. As a result, the drop freezes when it is carried to a height above the freezing line and grows by the accumulation of snow and water at the different levels. Consequently, when a hailstone is cut apart, it shows a series of concentric shells formed by the successive passages above and below the freezing level.

Glaze. When rain falls on object or on ground having subfreezing temperatures, it freezes into a sheet or coating of ice, known as glaze or freezing rain. If this coating becomes thick, if often has a destructive effect owing to its heavy weight.

Rime. Rime is a freezing fog. It forms as a thick, frosty deposit when objects with subfreezing temperatures encounter a fog. In such a case the minute fog droplets freeze and adhere to the cold surface. Rime is thicker on the windward sides of objects, particularly when forming on hulls, masts, or bulkheads of vessels sailing through fog in cold weather. It also occurs on leading edges of airplanes flying through certain types of clouds.

(Donn, William L.: Meteorology)

Temperature is a fundamental weather element. In response to the irregular disposal of the sun’s energy (insolation) the air temperatures show variations between wide extremes. These variations in turn cause other significant instruments and methods of measuring temperature variations, examining the periodic, the horizontal, and the extremely important temperature variations that result whenever a mass or masses of air engage in vertical motion will be studied.

Temperature Instruments

Thermometers

The temperature of an object is measured by a thermometer, which is a sealed glass tube having a very small opening –the bore –running through the center of it from the top to bottom. The bore of the tube is greatly enlarged into a bulb-shaped opening within the bottom of the tube.

Maximum Thermometers

It is often necessary to determine not only the current temperature, but also the highest temperature reached during a given period. For this purpose a type of registering thermometer known as a maximum thermometer is used. This is almost identical with the standard thermometer describe above, with one exception. Just above the bulb of the thermometer, the bore narrows very abruptly for a short space this leaves a constriction of the bore to a very thin channel, much thinner than that in the rest of the tube.

Minimum Thermometers

Minimum thermometers are used to register the lowest temperature reached during a given period. The minimum thermometer reached a given period. The minimum thermometer resembles the regular thermometer in appearance, except that it always contains a liquid of low density such as alcohol, instead of mercury. In addition, within the liquid in the bore of the tube, there is a thin glass rod shaped somewhat like a dumbbell, called the index.

Six’s Thermometers

Since Six’s thermometer is so common a board ship, it will be explained in detail. It is a combination maximum-minimum thermometer commonly used because of its convenience.

The Thermograph

The last instrument to be considered is the thermograph. This is a purely mechanical device consisting essentially of a graph. This is purely mechanical device consisting essentially of a metallic element whose curvature varies with the temperature. One end of the curved sensitive element is connected to a long movable laver arm which contains an inked pen at its end. The arm, in turn, rest on a cylindrical drum which rotates by means of an inner clockwork. A sheet of graph paper is wrapped around the drum and is divided into days and hours, horizontally, and temperature in degrees, vertically.

(Donn, William L.: Meteorology)

Heat and Temperature

Heat and temperature are closely related properties that often become confused with each other. Thus, a hot object is one with high temperature rather than high heat content. Temperature can be both measured and defined rather easily. Heat measurement is more involved and tends to evade simple definition.

Processes of heat transmission

Heat energy is transmitted from place to place by the processes of radiation, conduction, and convection. Each of these three processes has an important function in determining processes affecting weather changes.

Radiation. This is the process of heat transfer in wave form, without the use or necessity of a transmitting medium. For example, the insolation (radiant) received by the earth comes from the sun by radiation in wave form through the emptiness of space. When we stand near a very hot object, the intense heat felt is mostly the result of heat rays radiated by the hot object. By the use of infrared (or heat-sensitive) film in a camera in a dark room, it’s possible to obtain an excellent photograph of an ordinary hot pressing iron merely by the heat rays radiated from the iron.

Conduction. This is the process of heat transmission through a medium by contact of the minute particles of which the medium is composed. For instance, if one end of a metal rod is heated, the other end will soon become hot. This is accomplished by conduction, in which the heat energy is passed (or conducted) along the rod from the heated to the cold end by the molecules within the rod.

Convection. This is the process of heat transmission by the actual motion of the heated material. When the air over a radiator is heated, it expands and rises to the ceiling. This motion is evident by the distortion of the light coming through the window and pasting through the rising air. Similarly, images seen across an airport runway often waver owing to the disturbance caused by the warmed air rising from heated runway. In the same way, when water or any liquid is heated, the warm liquid at the bottom rises to the top and is replaced by cooler descending water. Gases exhibits convection as a means of heat transfer more so than liquids and liquids more so than solids. In fact, convection, or the rising and falling of warm and cold air masses respectively, is one of the most important and fundamental processes in the atmosphere.

(Donn, William L.: Meteorology)

Clouds indicate the prevailing and past conditions in the air and, more important, the probable future atmospheric conditions. For making short-period forecasts from local observations alone, clouds are one of the two most important criteria available. We will therefore examine the observation features and then the physical causes of clouds.

Classification of Clouds
In accordance with the 1956 International Cloud Atlas of the World Meteorological Organization, clouds are classified into 10 characteristic forms or general which are mutually exclusive. Variations within most general are recognized, but will be omitted from the descriptions that are given here.

Basic Clouds Forms
There are basic forms are recognized: cirrus, cumulus, and stratus. If a basic cloud form (with exception of cirrus) occurs above its normal level, the cloud will be thin and the prefix alto precedes the name. If any cloud is associated with precipitation, the word nimbus (Latin for rain) is often introduced in combination with the name.

Cirrus. This form embraces very high, thin, separated, or detached clouds that develop delicate patches or long extended fibers, frequently with a feathery appearance and always white in color.

Cumulus. This form always exhibits flat-based individual cloud masses, with a pronounced vertical doming, and frequently a cauliflower-like structure.

Stratus. This form implies an extended sheet or layerlike cloud covering all or large portions of the sky. The stratus type is usually a continuous cloud deck and may show minor rifts, but no definite individual cloud units.

Cloud types:

• Cirrus (Ci)
• Cirrostratus (Cs)
• Cirrocumulus (Cc)
• Altostratus (As)
• Altocumulus (Ac)
• Stratus (St)
• Nimbostratus (Ns)
• Stratocumulus (Sc)
• Cumulus (Cu)
• Cumulonimbus (Cb)

To be continue…

(Donn, William L.: Meteorology)