Why is moisture necessary for precipitation to develop

With moisture available, winds blowing up the mountain will provide the lifting of air since air can't go through the mountain, it must go up and over , and this combination creates a favorable environment for the development of precipitation. The latest position is roughly west of here. Will precipitation arrive within 24 hours? Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW Because of the different sizes, cloud droplets move about at different speeds.

As they collide, some of them stick together to form larger drops. The larger cloud droplets grow at the expense of smaller ones, and actually become more effective in the collecting process as they become larger. As larger drops begin to fall, they tend to sweep out the smaller drops ahead of them. The coalescence process takes place in clouds of both above-freezing and below-freezing temperatures. Snowflakes coalesce with other snowflakes as they fall to form the large clumps which we sometimes observe.

They may also coalesce with supercooled water droplets to form snow pellets. In order to recognize and identify clouds it is necessary to classify and name them. Clouds are identified by their development, content and appearance, and their altitude. They are classified into many types and subtypes, but we need be concerned only with the more basic types. We will consider four families of clouds distinguished by their height of occurrence: High clouds , middle clouds , low clouds , and clouds with vertical development.

In addition, the word nimbus is used as a prefix or suffix to indicate clouds producing precipitation—resulting in such names as nimbostratus or cumulonimbus. The word fractus is used to identify clouds broken into fragments by strong winds—such as stratus fractus and cumulus fractus. Air stability has an important effect on the type of cloud formation. A stable layer which remains stable through forced lifting will develop stratiform clouds.

Cumuliform clouds develop in air that is initially unstable or becomes unstable when it is lifted. A layer of conditionally unstable air which is forced to ascend may first develop stratiform clouds and then develop cumuliform clouds as the layer becomes unstable.

The cumuliform clouds project upward from a stratiform cloud layer. Thus, the type of cloud formation can be used as an indicator of the stability of the atmospheric layer in which the clouds are formed. High clouds have bases ranging from 16, to 45, feet. Cirrus , cirrocumulus , and cirrostratus clouds are included in this family. They are usually composed entirely of ice crystals, and this is their most distinguishing characteristic.

Cirrus are isolated wisps of thin, feathery cloud up near the top of the troposphere. Cirrocumulus clouds consist of patches of small, white cloud elements. They may form definite patterns at times, showing small but firm waves or ripples. Cirrostratus clouds are thin, whitish veils, sometimes covering the entire sky. Halos around the sun or moon, caused by their ice-crystal composition, frequently identify this cloud type.

Cirrus clouds are thin, white, feathery clouds in patches or narrow bands. They are composed of ice crystals of varying size. Cirrocumulus clouds contain small, white individual puffs. They may contain some supercooled water droplets mixed in with the ice crystals.

Cirrostratus is a thin, whitish, transparent cloud layer appearing as a sheet or veil, It generally produces a halo around the sun or moon. Cirrus-type clouds may be produced in several ways. Often they are the forerunner of warm-front activity and give advance warning.

Sometimes they are associated with the jet stream and usually are found on the south side of the jet. They may also be produced from the anvil tops of thunderstorms. The value of cirrus clouds in fire weather is their advance warning of warm-front activity and their use in indicating high-altitude moisture and wind direction and speed. Middle clouds have bases ranging from 6, feet up to 20, feet.

Altocumulus and altostratus clouds fall into this group. Middle clouds are most generally formed by either frontal or orographic lifting, but may be formed in other ways. Often they are associated with lifting by convergence in upper-air troughs and sometimes develop with thunderstorms. Altocumulus are white or gray patches, with each individual component having a rounded appearance. Altocumulus may appear as irregular cloudlets or in definite patterns such as bands or rows parallel or at right angles to the wind.

Usually, the stronger the wind, the more distinct the pattern. Altocumulus are usually composed of water droplets and often are supercooled. Altocumulus are white or gray patches or rolls of solid cloud. They are distinguished from cirrocumulus by the larger size of the cloud elements.

Altocumulus clouds are usually composed of water droplets, often supercooled, but may contain some ice crystals at very low temperatures. Altostratus appears as a gray or bluish layer or veil with a sort of fibrous texture. It may be made up of supercooled water droplets or a mixture of water droplets and ice crystals. It tends to cover the entire sky, and the sun will shine through dimly as through a frosted glass. As altostratus becomes thicker and lower, the sun becomes obscured.

If it becomes dense and low enough, it becomes nimbostratus and takes on a wet and rainy appearance due to widespread precipitation. If the precipitation evaporates before reaching the ground, it is called virga. Three special types of middle clouds are of considerable importance in identifying weather conditions. The lens-shaped lenticular cloud appears over the ridge and to the lee of mountain ranges. Lenticular clouds indicate waves in the air flow caused by strong winds blowing across the range.

The clouds form in the rising current on the upwind side of the wave crest and dissipate in the downward flow on the other side. The castellanus type of cloud consists of cumuliform masses in the form of turrets, usually arranged in lines. They indicate marked instability at high levels, and their occurrence in the forenoon is a warning of possible thunderstorm activity in the afternoon.

Clouds with rounded lower surfaces in the form of pouches or udders are called mamma. They are most common on the underside of cumulonimbus anvils. The pendulous blobs of cloud are sinking into the clear air immediately below because they contain droplets and ice crystals. As the droplets and crystals evaporate, they chill the air in the pendants. By this chilling, they keep the air denser than the surrounding clear air. The process ends when all cloud particles have evaporated.

Pilots have reported that the downdrafts within mamma are quite weak. However, any adjacent thunderstorms will have zones of marked turbulence. Altostratus appears as a gray or bluish layer having a fibrous appearance, often associated with altocumulus. Frequently, it is composed of a mixture of supercooled water droplets and ice crystals. Light rain or snow often falls from it.

Nimbostratus is a gray or dark massive cloud layer diffused by more-or-less continuous rain or snow which usually reaches the ground. It is thick enough to blot out the sun. Lower ragged clouds often appear beneath the nimbostratus layer. The bases of low clouds range from the surface to 6, feet. Low clouds include stratus , stratocumulus , and nimbostratus. Nimbostratus clouds form a gray, often dark, layer usually accompanied by continuously falling rain or snow.

The precipitation usually reaches the ground, but occasionally only virga appears. Nimbostratus usually develops from thickening and lowering alto-stratus. Stratus fractus or scud clouds often appear beneath the nimbostratus layer. Stratus and stratocumulus are very common and widespread. They usually occur beneath an inversion and are fairly thin, ranging from a few hundred to a few thousand feet thick.

Stratus forms a low, uniform sheet, dull gray in appearance. It is composed of water droplets and does not produce rain, although it may produce drizzle. Fog is simply a stratus cloud lying on the surface. When a fog layer lifts, as it frequently does during the forenoon, it becomes a stratus layer. In some localities, particularly the west coast, stratus is referred to as high fog. Stratus is a low, gray cloud layer with a fairly uniform base and top.

Usually it does not produce precipitation, although it may cause some drizzle or snow grains. Stratus often forms by the lifting of a layer of fog. Stratocumulus clouds consist of gray or bluish patches or layers with individual tolls or rounded masses. They are generally composed of small water droplets and may produce light drizzle. Fog is important in fire weather because of its effect on the moisture content of forest fuels.

While fog is forming or persisting, conditions are favorable for fuels to absorb moisture. Fog occurs during calm or light-wind conditions in a stable atmosphere and is formed in several ways. Radiation fog is formed when moist air cools to its dew point at night over a strongly radiating surface. Some vertical mixing is necessary to produce a layer of fog of significant thickness. Advection fog forms when warm, moist air passes over a cool surface and its temperature is reduced to the dew point.

Many fogs are a combination of these two types. Upslope fog forms when moist, stable air is forced to rise along a sloping land surface. This type occurs especially along the western edge of the Great Plains when mT air moves northwestward from the Gulf of Mexico.

Fog may also occur in connection with fronts, particularly in advance of warm fronts where evaporating rain falling through a layer of cold air near the surface saturates the cold air.

The distinction between stratus and stratocumulus is not particularly important. Stratocumulus shows individual rolls or rounded masses, usually soft and gray. It forms when the air is somewhat unstable, whereas stratus forms in stable air.

Like stratus, it is composed of small water droplets and may produce light drizzle. Clouds with vertical development include cumulus and cumulonimbus. These are irregularly shaped masses with domes or turrets and have a cauliflower appearance. They usually appear in groups, and individual cloud bases are at about the same altitude.

The height of the bases, which is the condensation level described in chapter 4 , depends upon the air temperature and the amount of moisture in the atmosphere. Cumulus clouds are formed near the top of rising convection columns, and their bases may range in height from a few thousand feet to 15, feet or more. Their presence is of special interest in fire weather as an alert to possible convection in the surface layer.

They are a common type during the fire season, particularly in mountainous regions. Cumulus clouds are detached clouds in the form of rising mounds or domes.

They are dense, have sharp outlines, and the upper portion often resembles a cauliflower. They are composed of a great density of small water droplets; ice crystals may appear in the tops of larger cumulus.

Cumulonimbus clouds are heavy and dense with considerable vertical development sometimes reaching the tropopause. The top often takes on the shape of an anvil. The most common type of cumulus is a small, puffy type occurring during fair weather, called cumulus humili or fair weather cumulus. They appear after local surface heating becomes sufficiently intense to support convection, and dissipate in the late afternoon as surface heating decreases and convection ceases.

These clouds have relatively flat bases, rounded or cone-shaped tops, and are usually isolated or in small groups. Their vertical growth is usually restricted by a temperature inversion which makes the tops fairly uniform. Occasionally a single cloud element will develop vertically to some height. If you wear glasses and go from a cold, air-conditioned room to outside on a humid day, the lenses fog up as small water droplets coat the surface via condensation.

People buy coasters to keep condensed water from dripping off their chilled drink glass onto their coffee tables. Condensation is responsible for the water covering the inside of a window on a cold day unless you are lucky enough to have double-paned windows that keep the inside pane relatively warm and for the moisture on the inside of car windows, especially after people have been exhaling moist air.

All of these are examples of water leaving the vapor state in the warm air and condensing into liquid as it is cools. Air, even "clear air," contains water molecules.

Clouds exist in the atmosphere because of rising air. As air rises and cools the water in it can "condense out", forming clouds. Since clouds drift over the landscape, they are one of the ways that water moves geographically around the globe in the water cycle. A common myth is that clouds form because cooler air can hold less water than warmer air—but this is not true.

As Alistair Fraser explains in his web page " Bad Meteorology ": "What appears to be cloud-free air virtually always contains sub microscopic drops, but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules. As air is cooled, the evaporation rate decreases more rapidly than does the condensation rate with the result that there comes a temperature the dew point temperature where the evaporation is less than the condensation and a droplet can grow into a cloud drop.

When the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms," accessed on Sep. You've seen the cloud-like trails that high-flying airplanes leave behind and you probably know they are called contrails.

Maybe you didn't know they were called that because they are actually condensation trails and, in fact, are not much different than natural clouds. If the exhaust from the airplane contains water vapor, and if the air is very cold which it often is at high altitudes , then the water vapor in the exhaust will condense out into what is essentially a cirrus cloud. As a matter of fact, sailors have known for some time to look specifically at the patterns and persistence of jet contrails for weather forecasting.

On days where the contrails disappear quickly or don't even form, they can expect continuing good weather, while on days where they persist, a change in the weather pattern may be expected. Contrails are a concern in climate studies as increased jet traffic may result in an increase in cloud cover.

Several scientific studies are being conducted with respect to contrail formation and their impact on climates. Cirrus clouds affect Earth's climate by reflecting incoming sunlight and inhibiting heat loss from the surface of the planet.

It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20 percent. Source: National Weather Service: What is a contrail and how does it form?

Condensation causes clouds. The Oxford English Dictionary defines a cloud as "a visible mass of condensed watery vapor floating in the air at some considerable height above the general surface of the ground. And they do "fall" on you, sometimes, when the fog rolls in. According to columnist Cecil Adams, "a modest-size cloud, one kilometer in diameter and meters thick, has a mass equivalent to one B jumbo jet. If you compressed that cloud into a trash bag, well, in that case, you would not want to be standing below it.

Even though a cloud weighs tons, it doesn't fall on you because the rising air responsible for its formation keeps the cloud floating in the air. The air below the cloud is denser than the cloud, thus the cloud floats on top of the denser air nearer the land surface. Source: Gleick, P. In Encyclopedia of Climate and Weather, ed.

Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds.

When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail What is streamflow? How do streams get their water? To learn about streamflow and its role in the water cycle, continue reading.

Perhaps you've never seen snow. Or, perhaps you built a snowman this very afternoon and perhaps you saw your snowman begin to melt. Regardless of your experience with snow and associated snowmelt, runoff from snowmelt is a major component of the global movement of water, possibly even if you live where it never snows.

For the water cycle to work, water has to get from the Earth's surface back up into the skies so it can rain back down and ruin your parade or water your crops or yard.

It is the invisible process of evaporation that changes liquid and frozen water into water-vapor gas, which then floats up into the skies to become clouds.

The atmosphere is the superhighway in the sky that moves water everywhere over the Earth. Water at the Earth's surface evaporates into water vapor which rises up into the sky to become part of a cloud which will float off with the winds, eventually releasing water back to Earth as precipitation.