- How are clouds formed?
- Why do clouds form at different heights in the atmosphere?
- Different Types of Clouds and How to Distinguish Them
- High-level clouds
- Mid-level clouds
- Low-level clouds:
- How is fog formed?
- Why are clouds white?
- Why do clouds turn gray?
- Why are clouds red during sunset?
- How do clouds move?
- Why do clouds float?
How are Clouds Formed?
Clouds are made of tiny water drops or small ice crystals sticking on dust particles in the atmosphere. The droplets have a diameter of about a hundredth of a millimeter. Depending on the height and the temperature of the atmosphere, the clouds are composed of ice or water droplets, which can remain liquid at temperatures as low as -30 °C due to their size. At temperatures below -30 °C the clouds are composed only of ice crystals.
The first stage of cloud formation is the evaporation of water in the form of tiny gas particles,surrounding us on the earth. The air can carry a certain amount of evaporated water, and the warmer the air, the higher that amount. The vapor then starts rising, and concentrates in the atmosphere, clinging to numerous particles (aerosols) floating in the air (from automobiles, fires, volcanoes, etc.).
Together with vapor rising, a cooling process begins. After reaching a level, known as the saturation point of the air, when the humidity reaches 100%, and the air is “full” of water vapor, no more can evaporate. Along with the decrease in temperature with the onward movement, some of the water vapor sticks to aerosols when they collide (condensation).
When this, so called, “dew point” is reached, the rising water vapor begins to form tiny bits of water – droplets or ice crystals, initiating the primary structure of the cloud.
Later the cloud is created, as bigger water droplets are formed around the particles, sticking together with other droplets, forming massive collections of tiny water droplets and crystals. After the formation of the cloud, the droplets or ice particles, can either continue growing by joining together,or fall to the ground as rain or snow, or change back into water vapor. The abundance of ways in which clouds are formed result in the existence of an amazing variety of picturesque shapes, sizes and textures of clouds.
Why Do Clouds Form at Different Heights in the Atmosphere?
The various cloud types have different specific features because of the interaction among numerous factors, such as the water vapor amount, the temperature, the character and direction of wind, and the availability and interplay of other air masses. At higher altitudes colder clouds are formed, while the warmer ones are created near the earth’s surface. The warmer, lower clouds are formed when humidity is higher, and they can contain more water vapor than the higher and colder ones. This causes the greater size and thickness of the clouds, closer to the earth. These features are related to temperature and pressure of the air, as well as the amount of the water evaporated. After the sun has hidden in the evening, the air near the earth’s surface begins cooling down, diminishing its vapor holding capacity. This leads to a rise of humidity, although the water vapor amount in air has not changed. The cooling continues until100% humidity is reached, and water in the air condenses into little droplets.
When the air moves up, higher into the atmosphere, it becomes cooler. Physically, this is the result of its expansion, since gases that expand get cooler.The air close to the surface is a subject to a lot of pressure, caused by the large quantities of air above it. Although air is light, it does have a mass. The miles of air above the ground cause significant pressure on the air below, compressing it. Moving up in the atmosphere, the air is gradually released by the pressure and expands. Since the atmosphere is evenly distributed across the globe, and the amount of pressure is based on altitude,air cools at a constant rate per the amount of height it rises. On the other hand, water vapor is also spread quite evenly through the air in a given geographic place. Therefore, the two factors determining the height of cloud formation,are temperature, and humidity.
Different Types of Clouds and How to Distinguish Them
Clouds can have various shapes, textures, and sizes.There are multiple factors that influence the conditions for cloud formation, such as temperature, humidity and altitude. Depending on their properties, height and appearance, clouds are divided into several main types.According to their composition, they are grouped into water, ice and mixed types. Their height determines them as high, medium, low, and clouds with vertical development. In terms of appearance,they could be buy (balls), layered,and wavy. Convective clouds are produced from warm air pockets rising directly from the underlying surface. They are smaller, a hundred yards to several miles across. Stratiform clouds, cover larger areas and are caused by broader layers of more slowly rising air, and have a more uniform, featureless appearance,covering the whole sky. The different cloud types have two parts in their name – the first part of the name refers to their height, while the second describes the appearance. When clouds form at the highest altitudes, they obtain the prefix “cirro”. Middle level clouds have the prefix “alto”, while low clouds do not get a prefix.
High clouds are located at a height between 6 km and 13 km in Earth’s atmosphere. Regardless of the season, they always consist of ice crystals. Their thickness is low and they do not cause raining. They usually move at a speed of 70 – 80 km/h, but can reach up to 350 km/h. Because of the great height at which they are located, their movement is typically imperceptible, but if it becomes visible to the naked eye, it means that their deterioration is imminent. According to the international classification, three main types of high clouds are observed – cirrus, cirrostratus, and cirrocumulus.
Cirrus (Ci) Clouds
Cirrus (Ci) clouds – high over 6 km,single, thin, made entirely of ice crystals usually white clouds, sometimes with a silky sheen. They are almost transparent with a fibrous structure, and the sun and the moon are clearly visible through them. These clouds move slowly, take many different forms: of feathers, fibers with branching white lines, or hooks screwed up. Cirrus clouds are a sign of warm, moist air rising up over the cold air.“Mare’s Tail”(or Uncinus)is a term,often used to describe wispy cirrus resembling a horse’s tail in the wind.
Cirrostratus (Cs) – soft white blanket-like, silky. When the light passes through the hexagonal ice crystals of these clouds, it is dispersed or refracted as if passing through a prism, forming a ring or halo. With the approaching of a warm front, cirrus clouds thicken into cirrostratus, which may, in turn, compress the lower cloud layers. Resembling patches, often arranged in a regular pattern, they can give the appearance of the mackerel sky.These clouds are usually located above 5.5 – 6 km, indicating a large amount of moisture in the upper atmosphere.
Cirrocumulus (Сс) – a high-altitude tropospheric, thin transparent layer of clouds, composed of small white flakes or tiny pellets, arranged in rows, or groups or rows of small waves across the sky. Theyusually appear before a warm atmospheric front, and occur at an altitude of 5 km to 12 km. Cirrocumulus that looks like fish scales, is popularly called“Mackerel Sky”.
The clouds of this group are located at an altitude between 2 km and 6 km. Depending on the height, the season, and the vertical temperature difference, these clouds can consist of water droplets, ice crystals, or a combination of the two, including supercooled droplets, whose temperatures are below the freezing point. The two main mid-level cloud types are altostratus and altocumulus.
Altostratus (As) – translucent,thin, gray or bluish clouds,with a flat and uniform texture, diapered with fibrous structures. Typical mixed clouds,occurring most often after the Perez -layer (Cs) clouds, at 6 km height, subsequently descending to 2 km, when they increase their thickness. Their formation indicates the approach of a warm front, and may thicken and lower, resulting in rain or snow. These clouds do not produce significant precipitation, apart from sprinkles or light showers.(Image Source:National Weather Service)
Altocumulus (Ас) – thin,cumulus cloud layers,or networks of separate lines, arranged in regular patterns; evenly blurry shapes, or balls with shaded areas, sometimes organized into floors. They are heap-like clouds with convective characteristics. Altocumulus clouds may align in rows, with axes indicating localized areas of ascending moist air, and clear zones with descending, drier air. This type of clouds may indicate instability, especially in the morning, which could later transform into deep convection in the afternoon or evening.
Low-level clouds lack a prefix in their names.They are derived from “strato-” or “cumulo-”, in accordance with their characteristics. Low clouds occur below 2 km, and usually consist of liquid water droplets, or supercooled droplets, with the exception of cold winter storms, when ice crystals and snow can comprise a part of the clouds.
Stratus (St) clouds form a stable, horizontal,gray,uniform, and flat layer, that can cover the entire visible sky, similar to a fog, but not reaching the ground. Sometimes they are located 50-100 m up the ground, preceding the onset of a warm atmospheric front. This cloud type is developed due to forced uplift, and may be precipitation-free, or can cause periodic light precipitation or drizzle.
Cumulus (Cu) clouds are low, individual billowy globs, with flat bottoms and rounded tops, resembling cauliflower. Commonly, they grow vertically, and are at least as tall as they are wide. This cloud typeis formed from pockets of rising air on sunny days, and is often a precursor of other cloud types, when influenced by instability. One of the specific features of this type is the constant change of the outlines, taking the shapes of almost anything (animals, faces, etc.). These are usually a sign of fair weather. When they develop in the middle or high part of the atmosphere, due to the rapid rising and sinking of air currents, they are called cumulonimbus – tall, deep and dark able to bring lightning, heavy rain and severe weather conditions (hail, stormy winds, or tornadoes).
A cumulus cloud, developed within an unstable layer, with a significant vertical growth, is called cumulus congestus. In the presence of atmospheric instability enough moisture, a strong updraft can develop, leading to the formation of adeep cumulonimbus cloud. By its vertical length, it encompasses the lower, middle and upper levels of the troposphere, contributing to thunderstorms with heavy rain.
Stratocumulus (Sc) are hybrids between the layered stratus and cellular cumulus. They form a layer of cloud clumps with thick and thin areas, appearing in the atmosphere, ahead of, or behind a frontal system. They resemble the Altocumulus (Ас) clouds, but are distinguished by a denser structure and a lower location. Most commonly lens-shaped stationary clouds, parallel bands, or spherical masses, arranged in groups, or waves,covering the entire sky.
Nimbostratus (Ns) are formless, thick, stratus or stratocumulus clouds, producing steady heavy rains or snow. They form an inverted layer at a height of 3000 m;with a dark-gray, relatively uniform composition. Developed due to a forced broad scale lifting of saturated air, these clouds can cover the entire horizon,with a high vertical thickness, which may reach 6 km. Their base is often a low-lying cloudy blanket. They consist of water drops, often supercooled, and sometimes – ice crystals. The shapes vary greatly – they are seen as pinnacles, domes, or towers, showing the atmosphere instability,or a preceding rainfall.
Nacreous – ice clouds in thе stratosphere over the polar countries, developed at a height of 15-25 km in, implicated in the formation of ozone holes, and associated with strong tropospheric winds and storms. They are overflowing in all colors of the rainbow, and are observed in the northern latitudes a few hours after sunset. These colorful clouds have the nickname “mother of pearl clouds”, and are a type of wave clouds, like the lenticular clouds.The ice crystals, forming these type of clouds, are pushed up into the stratosphere by strong wave winds, that they start oscillating.
Noctilucent – night, bluish-silver, bright and pervasive polar cloud layers,often located in the polar mesosphere, at a height between 60 and90 kilometers. Made of ice crystals, and having the form of lines and wrinkles, they can cover the sky. Extremely rare, faint to be seen,they are visible in summer,only after the sun is below the horizon in a deep twilight. These clouds form under restrictive conditions – at a high altitude, requiring the availability of water vapor, dust, and very cold temperatures to form.
Shelf Clouds – they are low (2 km), horizontal, wedge-shaped cloud, attached to the leading edge base of a parent cloud, usually a thunderstorm, or any type of convective clouds. Upward cloud motion can be seen in the front outer part, while the underside often seems turbulent and wind-torn. Formed as the cool, descending air from a storm cloud’s downdraft extends across the earth’s surface, with a leading edge (gust front). This outflow enters under the warmer air, drawn into the storm’s updraft. With the lifting of the warm, moist air by the lower and cooler air, the water condenses, developing a cloud rolling together with the different winds above and below.
Accessory clouds – (wall clouds,anvil clouds,and funnel clouds). They are connected to and dependent on a larger cloud system for their existence.
Wall Cloud - (pedestal cloud). It is a large, up to 8 km wide, a localized cloud formation that develops in the Northern Hemisphere, beneath a cumulonimbus cloud, often leading to tornadoes. It is usually under the rain-free base of a thunderstorm, indicating the area of the strongest updraft. Some of these are harmless, but rotating wall clouds are a bad sign of the available “mesocyclone” in a thunderstorm, increasing the risk of a tornado. This type of clouds is formed when, coinciding with the direction of the steering winds, an inflow region of warm, moist air rises and converges, overpowering wet, rain-cooled air from the normally downwind downdraft. The air temperature drops and the dew point increases,as the warm air continues to entrain the cooler air. With rising this air gets more moisture-saturated, forming additional wall clouds.
Funnel clouds - funnel-shaped clouds of water droplets,formed by a rotating column of air and condensation associated with a rotating wind, extending from the base of a cloud (a cumulonimbus or towering cumulus) but not reaching the ground.Most tornadoes begin as funnel clouds.It is usually the visible portion of a tornado,but it is not a tornado, itself, if the circulation does not contact the ground.
Anvils - Mostly ice crystal particle clouds, blown downwards from the top of a cumulonimbus thunderstorm clouds, initially formed in the upper parts of thunderstorms. Their shape is an effect of the rising air, which and spreads out. This happens because the stratosphere air is warmer than the rising air in the cloud, preventing the relatively cooler anvil air from further rising.
Fractus - (scud and cloud tags). They are small, ragged,and isolated;with irregular patterns, resembling torn pieces of cotton candy. Changing constantly, fractus clouds form and dissipate rapidly, not having clearly defined bases. Often unorganized,ominous,noticed around the base of thunderstorms, they develop from a larger cloud, and are generally sheared by strong winds.
Mammatus - They develop in a pouch-like shape,from cumulonimbus clouds in its latter stages. Protruding downwards, mammatus clouds can be seen hanging from the anvil of a severe thunderstorm, but usually they are not dangerous, only accompanying the storms. These clouds develop as the moisture laden air sinks, and remain visible until the air sinks to a degree, where the relative humidity drops below 100%.
Hole-punch Clouds - (fallstreak hole). Strange holes in the cloud layer,usually circular, but can also be oblong, or pentagram shaped.Their development starts when the water temperature in the cloud is below freezing, but the water has not frozen. They begin as a hole in the clouds,later filling up with cloud masses. When sections of the water begin freezing, the surrounding water vapor freezes as well, commencing to descend -a rounded hole is made.It is thought that the introduction of large numbers of tiny ice crystals into the cloud layer starts this domino effect of evaporation which creates the hole.
Lenticular - (altocumulus standing lenticularis). These clouds are stationary, formed above the mountains, with shapes resembling lens or flying saucers. They are aligned at right-angles to the wind direction, and form as air is lifted to saturation over the top of mountains.
Comma cloud - comma-like, when seen on satellite imagery, this is a low pressure cyclone,a cloud type developing and existing in cold air. The head of the comma is formed by the low pressure center and warm front, while the cold front creates the tail. There are several similar phenomena observed within cold air. It is a small- to middle-sized cloud spiral consisting of white cloud cells.
Rope cloud - these clouds are narrow funnel-shaped, like skinny and elongated ropes, often in snakelike form, along cold fronts or other boundaries. Created at about 2 km above the earth,they are usually associated with severe thunderstorms, or marking the dissipating stage of tornadoes. A narrow line of cumuliform clouds that develops along a cold front, well-developed over the sea. The line may grow when a front weakens, decreasing its slope, and a cold air outflow is available at the surface, causing increased convection ahead of the front. Often a clear band could be seen behind the rope cloud, where the air is subsiding between it and the residual cloud, along the cold front.
Contrail - These are narrow, elongated cloud formed in the cold air at high altitudes by the jet exhaust condensation.
How is Fog Formed?
Fog forms when water evaporates from a surface (the ocean, another water body, or moist ground) or is added to the air. Water vapor can also be added to the air via winds, precipitation, daytime heating, and the evaporation of water from a surface, or air rising over mountains (orographic uplift). Along with the beginning of water evaporation, the obtained vapor starts to rise, bonding with aerosol particles to form water droplets. These droplets then condense, forming fog. Fog is considered a dense, low cloud type, consisting of small water droplets, close to ground level or in contact with it. This happens when the relative air humidity slightly exceeds the saturation point. In highly polluted air the nuclei may grow sufficiently to cause fog at humidities less than 95%. “Fog”can also refer to clouds of smoke or ice particles, or their mixture. There are several different ways of fog formation. The upslope fogs that are formed by a process, in which the humid air is forced to ascend up the sides of hills and mountains.The most stable fogs develop when the surface is colder than the air above – in the presence of a temperature inversion. It is also observed when cold air moves over a warm, wet surface and becomes saturated by its the evaporation of moisture.
Inversion fogs are formed from a downward extension of a layer of stratus cloud, under the base of a low-level temperature inversion. Radiation fog develops in the evenings and nights when the heat, absorbed by the surface during the day, radiates into the air. Often the term “ground fog” is used to refer to radiation fog. Advection fog is formed after the condensation, occurring after warm, moist air masses pass over and contact a colder wet surface (advection). Valley fog starts in mountain valleys, when mountains prevent the dense air from escaping in the winter.Freezing fog is a type, developing when the liquid fog droplets freeze to form solid surfaces. The tops of the mountains are often covered in freezing fog. Frontal fog is another form, observed when raindrops, falling from a relatively warm air above a frontal surface, evaporate into cooler air close to the surface.
Why are Clouds White?
When the light beams interact with particles in the air, some of their energy is scattered, meaning that the light beam changes its direction and color. Physically explained, the amount of light scattered is expressed as a function of the particle size, relative to the wavelength of the light falling on it. Cloud particles, the billions of tiny water droplets and ice crystal particles, are large enough, and in the right amount to scatter any color of light that falls on them, a process called “Mie scattering”. This multiple,combined scattering of light at all wavelengths(red, orange, yellow, green, blue, indigo, and violet) in the clouds, is percepted by the human eye as white color. Smaller droplets allow the scattering of more sunlight, whereas larger drops let more sunlight pass through them. For this reason, the heavily raining part of a cloud is brighter than the other, only cloudy part.
Why Do Clouds Turn Gray?
The main factors causing clouds to turn gray are their thickness and height. When clouds are thin, they allow a large part of the light to penetrate and pass through them, appearing white. However, as their thickness increases, they block sunlight – too many particles prevent parts of the light from escaping, less light is able to pass through. That makes the bottoms of clouds look darker, perceived by people as gray. With increasing their vertical size, the bottoms of the clouds look grayer. It is generally assumed that a cloud around 1 km thick, hardly allows any sunlight to make its way through. That is why some vertical clouds are white on top, but and dark at the bottom.The bottom of the cloud could also be located in the shadow of other clouds when the top of a cloud casts a shadow upon its own base, or the sun could be in the process of setting and shining only on the tops of the clouds. Larger drops of water (raindrops), found near the bottom of a cloud, absorb much more light than the smaller drops. The darkness or grayness of a cloud depends also on the sky in the background – surrounded by a bright sky, clouds look darker,while they seem lighter in front of darker sky.
Why are Clouds Red During Sunset?
Clouds can beautifully catch the last red and orange rays of the setting sun, and the first light of the dawn, reflecting them to the ground. Sunset and sunrise are often colored in red due to the very low, or just below the horizon, the position of the sun, causing the light to travel horizontally from the sun, not vertically down,as during the day. The light passes a longer way through the atmosphere, crossing a greater thickness of air.The interaction with air molecules and participles leads to the scattering of a high amount of violet and blue light out of the beam, leaving the reddish undisturbed.The tremendously high number of scattering “events”, happening along the way,is a process known as “multiple scattering” (or “Rayleigh scattering”). So, just before the sun has hidden from view, its light reaches our eyes afterbeing refracted, with most of the short wavelengths efficiently scattered out of the beam of sunlight. Clouds can act like a canvas for the reddish sunset sky, taking a part of the color. A similar phenomenon, twilight “afterglows”, follows volcanic eruptions. Certain cloud types are more closely related to red sunsets than others -they must be high to intercept the light that has not suffered color loss by passing through the atmospheric boundary layer, containing dust participles. This explains why the shades of orange and red are most often with cirrus and altocumulus layers, while being rare with low clouds.
How Do Clouds Move?
Movement of the air, caused by temperature or pressure differences, is experienced as wind. Generally, clouds move at a speed and direction determined by the prevailing winds, at the altitudes where the clouds are occurring. Wind direction and speed depend on the magnitude of the pressure differences between the areas of high and low pressure. Usually, the wind speed increase with altitude up to a certain point. While the clouds, taking part in a thunderstorm travel at about 50 km per hour, rivers of fast-moving air in the atmosphere, called “jet streams,” occur at approximate altitudes from 7 to 12 km above the ground. The speed of the jet stream can reach over 280km per hour, causing the clouds at that level to move very quickly. At a lower level, the wind speed and direction may be different from those above, that is why you can often see clouds at several different levels, moving in diverse directions. Air temperature is higher at the ground,which is heated by the sun, and decreases with increasing altitude. This vertical temperature difference leads to a considerable uplift of air, since the warmer surface air is lighter than the colder air above it. Frontal wedging, a type of movement, occurs when a cloud,that is a part of a warmer air mass, encounters and collide at its surface with a colder air mass. This convergence forces the warmer cloud up, over the cold one, bringing rain along the front meeting edge.
Why Do Clouds Float?
Although water in the clouds is denser and heavier than air,it seems that gravity does not act to bring them down to earth, but leaves them to float in the air. By definition, floating describes an entity kept up within the medium by buoyancy forces, due to density differences. Actually, the clouds do not “float” in the air, because their water droplets are heavier than the surrounding air, and gravity does pull the cloud down. As known, rising air current is one of the steps in the cloud formation. Cloud droplets within this rising air are continually pushed upward at a rate greater than that at which gravity pulls them downward. The result is that the cloud droplets rise within the updraft, which can be seen by growing and changing bumps of white at the top of the cloud, an indication of updrafts and continued growth.
When the cloud droplets begin falling,the air below them cause some resistance, (aerodynamic drag), with values, depending on the object’s mass, shape and size. When an object falls through the air, or water, the aerodynamic drag force acts against the gravity, causing the achievement of the terminal velocity of the falling object after a certain time/distance, determining the falling rate toward the surface thereafter. This small fall velocity is countered by the rising air, so that the cloud droplets actually rise. Only when the droplets are caught in a downdraft,or congregate to form rain drops (300 times larger), is the falling time limited to a range of minutes.This combination of droplets slow descent, and their fast evaporation below the cloud, gives us the perception that the cloud is floating on air.