Clouds are visible accumulations of tiny water droplets or ice crystals in the Earth’s atmosphere.They are the clumping of condensing water vapor.
Moist air near the Earth’s surface is raised from the ground to high in the atmosphere by the heat of the sun,orographic barrier(Orographic lift) or by a colder invading air mass, called a cold front, that pushes the warm, moist air upward. When the air is lifted, both the air pressure and temperature drops.As soon as the air temperature reaches the air’s dew point, the water vapor in the moist air condenses, and clouds form. Warm air can hold more water vapor than cold air, so lowering the temperature of an air mass is like squeezing a sponge. Clouds are the visible result of that squeeze of cooler, moist air.
Moist air becomes cloudy with only slight cooling.With further cooling, the water or ice particles that make up the cloud can grow into bigger particles that fall to Earth as precipitation.Clouds form when humid air cools enough for water vapor to condense into droplets or ice crystals. The altitude at which this happens depends on the humidity and the rate at which temperature drops with elevation.
Clouds differ greatly in size, shape, and color. They can appear thin and wispy, or bulky and lumpy.
Clouds usually appear white because the tiny water droplets inside them are tightly packed, reflecting most of the sunlight that hits them. White is how our eyes perceive all wavelengths of sunlight mixed together.
When it’s about to rain, clouds darken because the water vapor is clumping together into raindrops, leaving larger spaces between drops of water. Less light is reflected. The rain cloud appears black or gray.
Normally, water vapor can only condense onto condensation nuclei—tiny particles that serve as kernels around which drops can form.Condensation nuclei are often nothing but natural dust. But soot particles from automobile exhaust or other types of pollution can also serve the purpose.
Cloud Types
Clouds are classified according to their height above and appearance (texture) from the ground.
The following cloud roots and translations summarize the components of this classification system:
1) Cirro-: curl of hair, high.
2) Alto-: mid.
3) Strato-: layer.
4) Cumulo-: heap.
5) Nimbo-: rain, precipitation.
High-level clouds:
High-level clouds occur above about 20,000 feet (6,000 meters) and are given the prefix "cirro-". Due to cold tropospheric temperatures at these levels, the clouds primarily are composed of ice crystals, and often appear thin, streaky, and white (although a low sun angle, e.g., near sunset, can create an array of color on the clouds).
The three main types of high clouds are cirrus, cirrostratus, and cirrocumulus.
Unlike cirrus, cirrostratus clouds form more of a widespread, veil-like layer (similar to what stratus clouds do in low levels). When sunlight or moonlight passes through the hexagonal-shaped ice crystals of cirrostratus clouds, the light is dispersed or refracted (similar to light passing through a prism) in such a way that a familiar ring or halo may form. As a warm front approaches, cirrus clouds tend to thicken into cirrostratus, which may, in turn, thicken and lower into altostratus, stratus, and even nimbostratus.
Finally, cirrocumulus clouds are layered clouds permeated with small cumuliform lumpiness. They also may line up in streets or rows of clouds across the sky denoting localized areas of ascent (cloud axes) and descent (cloud-free channels).
Mid-level clouds:
The bases of clouds in the middle level of the troposphere, given the prefix "alto-", appear between 6,500 and 20,000 feet (2,000 - 6,000 meters ). Depending on the altitude, time of year, and vertical temperature structure of the troposphere, these clouds may be composed of liquid water droplets, ice crystals, or a combination of the two, including supercooled droplets (i.e., liquid droplets whose temperatures are below freezing).
The two main type of mid-level clouds are altostratus and altocumulus.
Altostratus clouds are "strato" type clouds that possess a flat and uniform type texture in the mid levels. They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow. However, altostratus clouds themselves do not produce significant precipitation at the surface, although sprinkles or occasionally light showers may occur from a thick alto-stratus deck.
Altocumulus clouds exhibit "cumulo" type characteristics in mid levels, i.e., heap-like clouds with convective elements. Like cirrocumulus, altocumulus may align in rows or streets of clouds, with cloud axes indicating localized areas of ascending, moist air, and clear zones between rows suggesting locally descending, drier air. Altocumulus clouds with some vertical extent may denote the presence of elevated instability, especially in the morning, which could become boundary-layer based and be released into deep convection during the afternoon or evening.
Low-level clouds:
Low-level clouds are not given a prefix, although their names are derived from "strato-" or "cumulo-", depending on their characteristics. Low clouds occur below 6500 feet (2000 mts), and normally consist of liquid water droplets or even supercooled droplets, except during cold winter storms when ice crystals (and snow) comprise much of the clouds.
The two main types of low clouds include stratus, which develop horizontally, and cumulus, which develop vertically.
Stratus clouds are uniform and flat, producing a gray layer of cloud cover which may be precipitation-free or may cause periods of light precipitation or drizzle. Low stratus decks are common in winter in the Ohio Valley, especially behind a storm system when cold, dismal, gray weather can linger for several hours or even a day or two.
Stratocumulus clouds are hybrids of layered stratus and cellular cumulus, i.e., individual cloud elements, characteristic of cumulo type clouds, clumped together in a continuous distribution, characteristic of strato type clouds. Stratocumulus also can be thought of as a layer of cloud clumps with thick and thin areas. These clouds appear frequently in the atmosphere, either ahead of or behind a frontal system.
Nimbostratus clouds are generally thick, dense stratus or stratocumulus clouds producing steady rain or snow .
In contrast to layered, horizontal stratus, cumulus clouds are more cellular (individual) in nature, have flat bottoms and rounded tops, and grow vertically. In fact, their name depends on the degree of vertical development. For instance, scattered cumulus clouds showing little vertical growth on an otherwise sunny day used to be termed "cumulus humilis" or "fair weather cumulus," although normally they simply are referred to just as cumulus or flat cumulus.
A cumulus cloud that exhibits significant vertical development (but is not yet a thunderstorm) is called cumulus congestus or towering cumulus. If enough atmospheric instability, moisture, and lift are present, then strong updrafts can develop in the cumulus cloud leading to a mature, deep cumulonimbus cloud, i.e., a thunderstorm producing heavy rain. In addition, cloud electrification occurs within cumulonimbus clouds due to many collisions between charged water droplet, graupel (ice-water mix), and ice crystal particles, resulting in lightning and thunder.
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Mammatus: Drooping underside (pouch-like appearance) of a cumulonimbus cloud in its latter stage of development. Mammatus most often are seen hanging from the anvil of a severe thunderstorm, but do not produce severe weather. They can accompany non-severe storms as well.
Contrail: Narrow, elongated cloud formed as jet aircraft exhaust condenses in cold air at high altitudes, indicative of upper level humidity and wind drift.
Fog: Layer of stratus clouds on or near the ground. Different types include radiation fog (forms overnight and burns off in the morning) and advection fog.
Wall Cloud: A localized lowering from the rain-free base of a strong thunderstorm. The lowering denotes a storm's updraft where rapidly rising air causes lower pressure just below the main updraft, which enhances condensation and cloud formation just under the primary cloud base. Wall clouds take on many shapes and sizes. Some exhibit strong upward motion and cyclonic rotation, leading to tornado formation, while others do not rotate and essentially are harmless.
Shelf Cloud: A low, horizontal, sometimes wedge-shaped cloud associated with the leading edge of a thunderstorm?s outflow or gust front and potentially strong winds. Although often appearing ominous, shelf clouds normally do not produce tornadoes.
Hole-Punch Clouds: Also known as a fallstreak hole, this type of cloud is usually formed when the water temperature in the cloud is below freezing but the water has not frozen. When sections of the water starts to freeze, the surrounding water vapor will also freeze and begin to descend. This leaves a rounded hole in the cloud.The theory on its creation is that a disruption of the cloud layer stability, which can be caused by a passing jet aircraft, creates a descending motion that can lead to the stimulation of evaporation, producing a hole.
Mackerel sky is a popular term for a sky covered with extensive cirrocumulus or altocumulus clouds arranged in somewhat regular waves and showing blue sky in the gaps. The pattern resembles the scales on a mackerel fish, thus, the name.
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| Mackerel sky |
Clouds above the troposphere
Clouds are found almost exclusively in the troposphere. The stratosphere is
very dry, because vertical transfer is limited by the high stability and
because any transfer would have to occur through the tropopause, which is so
cold that the saturation vapour pressure is negligibly small. Yet on occasion
thin veils of clouds are observed above the tropopause.
Nacreous clouds, also known as mother-of-pearl clouds, are
stratospheric; they occur between 15 and 30 km. Large
volcanic eruptions emit dust particles in the lower stratosphere. These may
combine with ice to produce nacreous clouds. In fact, in the year following Mt
Pinatubo eruption in 1991, many nacreous clouds where spotted by airline pilots
flying in the twilight. Polar stratospheric clouds occur at about 20 km, where
the air is at -80°C during the
Antarctic winter. Their presence is a factor in the formation of the Antarctic
ozone hole.
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| Nacreous Clouds |
Noctilucent clouds occur in the
upper mesosphere, at about 80 km. Their name derives from the fact that they
can be seen from the ground when the Sun is 7-10 degrees below the horizon
and only reflects off these very high clouds . It arises from the water
vapour released upon oxidation of methane. The recent observed increase of
such clouds is related to increased atmospheric concentrations of methane, a
greenhouse gas. Noctilucent clouds are most common in the summer in polar
regions. At this time the mesospheric lapse rate is close to neutral, and
this makes uplift easier.
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| Noctilucent Clouds |
Clouds and Weather
Certain types of clouds produce precipitation. Clouds also produce the bolt of electricity called lightning and the sound of thunder that accompanies it. Lightning is formed in a cloud when positively charged particles and negatively charged particles are separated, forming an electrical field. When the electrical field is strong enough, it discharges a superheated bolt of lightning to the Earth. Most of what we consider to be single lightning strikes are in fact three or four separate strokes of lightning.
The sound of thunder is actually the sonic shock wave that comes when the air, heated by the lightning bolt, expands very rapidly. Thunder sometimes sounds like it comes in waves because of the time it takes the sound to travel. Because the speed of light is faster than the speed of sound, lightning will always appear before its thunder is heard.
Meteorologists measure cloud cover, or the amount of the visible sky covered by clouds, in units called oktas. An okta estimates how many eighths of the sky (octo-) is covered in clouds. A clear sky is 0 oktas, while a totally overcast or gray sky is 8 oktas.
Scientists have experimented with a process called cloud seeding for many years. Cloud seeding aims to influence weather patterns. Seeds, or microscopic particles, are placed in clouds. These seeds are artificial cloud condensation nuclei (CCN), which are tiny particles of dust, salt, or pollution that collect in all clouds. Every raindrop and snowflake contains a CCN. Water or ice droplets accumulate around CCN. Scientists hope that cloud seeding will allow people to control precipitation.
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