Fog and Cloud

When air is cooled below its dew point, then the water vapour within air starts to condense out into water droplets. These water droplets are called either fog or cloud, depending on the heights and process of formation of water droplets. Fog forms through cooling of the air by conductive process, where as cloud normally forms by adiabatic cooling of the rising air. Again fog forms near earth surface, where as cloud forms at higher altitudes.

Fog

Fog forms when the difference between temperature and dew point is generally less than 2.5 °C, with relative humidity of about 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. So fog may form, (1) by cooling air to its dew point, or (2) by adding moisture to air, near the surface. So for fog formation, it is better to have low dew point and high humidity. At hygrometer there are two thermometers, wet bulb thermometer and dry bulb thermometer. The difference between these two thermometer’s readings is called “depression of wet bulb below the dry bulb”. From page….., it is understood, a small depression of the wet bulb below the dry indicates the higher humidity, whereas a large depression of the wet bulb below the dry indicates the lower dew point, both are favorable for fog formation. For that reason it is called, small and large depressions of the wet bulb, at hygrometer below the dry bulb indicate the formation of fog.

Initially fog forms over land with shallow depth and air turbulence brings this fog to upwards & upper air to downwards. Then this downwards air becomes fog, when it comes in touch with cold surface. This way an up & down cycle is going on and making a deeper and stronger fog layer. This air turbulence has to be smaller; otherwise it will throw the fog to more upwards, then it will be no more fog, but a cloud layer at higher altitude. This way turbulence helps to continue fog life. So fog usually lies in relatively shallow layer and turbulence is an essential condition for maintaining the fog.

Fog is a visibility hazard. While navigating at fog, we should be more active. Range of visibility is to be considered with supreme importance. When coasting, range of visibility is determined by the distances of shore objects, at the time of first sighted or last sighted, with the help of admiralty charts and radar. In the open sea, visibility can be calculated by noting the radar range of a ship, when that ship is first sighted visually from disappear position, at foggy weather. Visibility-meter is also a good instrument to measure the distance of visibility, as well as the presence of fog at sea.

Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury

Visibility at Bay of Bengal is normally good. Here fog is normally rare. The average frequency of fog in ship observations is less than 2% over the whole area. Some parts are free of fog in all months but a few patches occur near the cost of Orissa in June. Here fog is mainly confined to the dawn period and extends over inshore waters for an hour or so. Sometimes, during early winter-morning, dense fogs are observed for 3 / 4 hours, at the coast of Bay of Bengal and Karnafuly River.

There are mainly 2 types of fog, as below – 1. Radiation fog 2. Advection fog.

There are some less common types of fog also.

Radiation fog is formed by the cooling of land surface, during night time, by thermal radiation in calm conditions with clear sky. The cold ground produces condensation in the nearby air by conduction, which produces the water droplets for radiation fog. In perfect calm, this fog layer can be less than a meter deep but turbulence can promote a thicker layer of 150 meters. It does not last long after sunrise. Its maximum time is about half hour after sunrise, as air temperature goes to its lowest position at this time. It is a relatively shallow fog. It may be dense enough to hide the entire sky or may conceal only part of the sky. Favorable conditions for radiation fog are clear sky, with little or no wind.

Radiation Fog is disappeared due to below reasons,

1. The spreading of a cloud sheet over fog reduces the cooling, which disappears fog. 2. Heat from the ground lifting the fog into low cloud (Stratus). 3. Increasing of wind causes fog dissipation.

4. In mature fog, the settling of droplets to the surface contributes significantly to the fog dissipation. 5. Except in mid-winter at higher latitudes, solar radiation can penetrate to the ground in places of thinner fog, heats the ground and evaporates the Fog. Radiation fog also called “Ground Fog”, as it normally forms over land. There is a large diurnal range of air temperature over land, but over sea, it is very less. So giving off heat over land is much quicker than sea, accordingly the air over land becomes cold, which is much faster than air over sea. On the other hand, the sea surface can not be so cool to form this fog. For that reason, radiation fog forms over land but not over sea and can drag towards costal sea areas.

Radiation fog, which forms over land, can drift towards rivers or other coastal areas. The fog at Karnafuly River, at winter time is the good example of this type of fog. Due to same reason, radiation fog may be encountered in UK coastal waters also.

Advection fog forms, when moist air moves over colder land or water surfaces. When this moist air comes in contact with colder surface and becomes cool below its dew point, then the excess water vapour condense into small droplets of water, on minute particles of dust or salt, which is moving through air. This way advection fog is developed. It is most common along coastal areas but often develops deep in continental areas. But it is mainly formed at sea, so it is called “Sea Fog” also. Advection fog deepens as wind speed increases up to about 15 knots. Wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds.

Advection fog is very common on the Grand Banks of New foundland. Here the warm and moist westerlies first blow over the warm Gulf Stream and gets hotter and more moisturized, then cross over the cold Labrador Current and forms advection fog. These fog banks are occurring along the coasts of Newfoundland & Labrador, with quite deep height. It often continues for days at a time and hampering shipping activities.

This advection fog is also available at east cost of Japan (here moist westerlies first blow over warm Kuro Shio, then over cold Oya Shio), south coast of UK, west coast of the United States, central and eastern United States (with extension as far north as the Great Lakes).

Forecasting of advection fog can be done by plotting on a graph paper. Here sea temperatures and dew point temperatures of fixed intervals of time are needed to plot on this graph paper, where temperature is the vertical line and local time is the base line. Then we will get two graph lines of dew point and sea temperature, if we extent these two lines (in figure which is given as dotted line), then we will get the forecasting time of advection fog, (which is 1200 hrs local time, at below graph). So the knowledge of sea surface temperature is very important in forecasting of advection fog (sea fog).

Published by GSP Training Centre (GTC), to distributive with FREE OF COSTS. For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. Haz For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury

e is an atmospheric condition, where dust, smoke and other dry particles obscure the visibility. It often occurs, when dust and smoke particles accumulate in relatively dry air. When weather conditions block the dispersal of smoke and other pollutants, then they concentrate and form a low-hanging blanket of foggy condition, which impairs visibility. Industrial pollution also can produces dense haze.

Mist is formed, when small droplets of waters are suspended at air. It can occur as part of natural weather or volcanic activity. The only difference between mist and fog is “range of visibility”. This phenomenon is called fog, if the visibility is one kilometer or less, otherwise it is known as mist. So mist has better “range of visibility” then fog.

If we look to mist from a distance, we find it as bluish, whereas haze is more brownish. Mist makes a beam of light visible from the side, via refraction and reflection of the suspended water droplets. Mist usually occurs near the shores and is often associated with fog. Mist can also forms on mountain tops.

Sea smoke is a peculiar kind of surface fog or mist, which is formed close to the open sea surface, when very cold dry air moves over relatively warmer sea surface. Here the water vapour evaporates from the sea surface and is condensed into water droplets. These way vertical streaks of smoke rise from the sea surface, which is the sea smoke (in other words steam fog). If the wind blows across cold icy areas like the Arctic, then the icy surface steams up into the air, produces this fog, for that reason it is also called Arctic sea smoke (frost smoke). The condition for such fog formation requires not only low surface temperature but also requires surface inversion of temperature to restrict the rapid development of instability. Normally it is shallow layer of fog, with variable visibility.

Along some coastal regions, such as at the Gulf of Maine, sea smoke frequently takes place in autumn and early winter, when an arctic air mass slips off the continent onto the warmer Atlantic waters. It is most common in Arctic and Antarctic areas and also at some areas like Cape Hatteras.

This Cape Hatteras is a cape on the coast of North Carolina. Two major Atlantic currents (the southerly-flowing cold water Labrador Current and the northerly-flowing warm water Florida Current -Gulf Stream) collide just off Cape Hatteras. Sea smoke is formed off Cape Hatteras at winter, when cold winter offshore wind meets the relatively warm waters of the Gulf Stream. For that reason, when passing north of Cape Hattera, on the US east coast during the winter, we may find the visibility is severely reduced, due to sea having the appearance of smoking.

Mixing fog (Frontal fog) forms at the boundary of cold air current and warm moist air current, when they meet with each other. The warm moist air cools at this boundary and forms this type of fog.

Upslope fog (Hill fog, orographic fog) forms, when winds blow air, up a hill or mountain slopes. So the air adiabatically cools, as it rises and moisture in air, condenses into water droplets, to form this fog.

Valley fog forms in mountain valleys, at winter seasons. It is the result of a temperature inversion, caused by heavier cold air settling into a valley, with warmer air passing over the mountains above. It is basically a radiation fog, restricted by local topography. This fog can last for several days in calm conditions.

Freezing fog occurs, when liquid fog droplets freeze to surfaces and form white soft or hard rime. This is very common on mountain tops, which are exposed to low clouds. It is almost same as freezing rain. 7

Ice Fog forms when the air temperature is well below freezing and is composed entirely of tiny ice crystals that are suspended in the air. Ice fog is seen in cold Arctic and Polar areas. Generally this requires temperatures at or below −35 °C.

Artificial fog is artificially generated fog, which is usually created by vaporizing water with glycol-based or glycerine based fluid. This fluid is injected into a heated block, which evaporates quickly.

Garua fog is a nearly invisible type of fog, which mainly occurs near the coast of Chile and Peru. Here the normal fog suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, produces a transparent mist.

Hail fog sometimes occurs in the locality of significant hail accumulations, due to decreased temperature and increased moisture. It most often occurs, when there is a warm, humid layer of atmosphere with light wind. It can be extremely dense and abrupt. It may forms shortly after the hail falls, when there are times and environments for hails, to cool the air.

Cloud Clouds are visible accumulations of water droplets or solid ice crystals, which are floating in the Earth's troposphere (the lowest part of the Earth's atmosphere) and moving with the wind. Dense deep clouds exhibit a high reflectance (70% to 95%). They thus appear white, at least from the top. On the other hand, it is white, gray or even sometimes dark appearance at the base.

When air parcel is cooled below its dew point temperature, then water vapor begins to condense on condensation nuclei, such as dust, ice and salt, which remain suspended in the air and forms clouds.

The air is normally cooled by any one of below four processes, a. Contact with cold surface of land or sea. b. Adiabatic cooling during ascent. c. Contact with colder masses of air. d. Radiation of heat into space by upper layers of air.

At Bay of Bangle, during South West Monsoon seasons, cloudy skies are observed with average coverage of 6 or 7 oktas. But normally the cloud amounts in the dry season are about 3 or 4 oktas, over most of the Bay of Bengal but amounts increase over SW sector, as air becomes moist during the long sea fetch.

As per height, there are mainly four types of clouds,

Low level clouds have their bases between sea level and 2 km height. This Cloud types include stratus, nimbostratus and stratocumulus.

Mid-Level Clouds have their bases between 2 km and 6 km above sea level. This Cloud types include altocumulus, altostratus.

High-Level Clouds have their bases between 6 km and tropopause. This Cloud types include cirrus, cirrocumulus and cirrostratus

Special Clouds have their bases at low cloud level but tops may extend well into high cloud level. This Cloud types include cumulus and cumulonimbus.

There are ten clouds, which are described as below,

Cirrus (Ci) is a silvery cloud, with appearance like feathers or fibers. This is composed of ice crystals, which originate from the freezing of super-cooled water droplets. It generally occurs in fair weather, although an especially heavy layer of cirrus can indicate an incoming storm system. Many cirrus clouds produce hair like filaments, which is made of heavier ice crystals. Due to the sparse moisture at a high altitude, it tends to be very thin. At this altitude, cirrus cloud is also formed from aircraft’s hot exhaust. This happens, when hot exhaust, mostly water, freezes and leaves as a visible trail.

Cirrostratus (Cs) is sheet like thin, generally uniform cloud, composed of ice crystals, usually covers the entire sky. When thick enough to be seen, they are whitish, usually with no distinguishing features. The sun or moon can shine through cirrostratus cloud. Sometimes, the sun or moon will appear to have a halo around it. The ice crystals from the cloud refract the light from the sun or moon, creating this halo. Cirrostratus clouds may indicate a large amount of moisture in the upper atmosphere. So sometimes, this cloud signals the beginning of a warm front, with precipitation with in the next 12 to 24 hours. The outline of sun and moon are sufficiently clear for sextant altitude measurement.

Cirrocumulus (Cc) is thin, small, puffy, patchy and wavelike appearance clouds. These high clouds are also sometimes available in the form of small flake or cauliflowers. They are white in colour, with no dark shadows in between. It represents a degraded state of cirrus or cirrostratus, as both of them may change into it.

Altostratus (As) cloud is named, because they are middle height cloud ("alto") and are spread out & layered ("stratus"). Altostratus clouds are grayish or bluish veil of clouds, which frequently cover the whole sky. The sun can be seen shining through them, though the sun or moon appears very dim through this cloud and the outline of the sun and moon are hazy. Altostratus cloud can be found over wide-spread areas. Altostratus clouds can cause ice accretion on aircrafts, as they are also made from ice crystals. Altostratus cloud forms, when a front of warm, moist air meets a body of cold, dry air. If these clouds thicken, then precipitation starts. It sometimes looks like thick cirrostratus, but no halo phenomena.

Altocumulus (Ac) consists of a thin patch, sheet or layer of white or gray clouds, which are broken into bands, blobs, rolls or waves. Altocumulus cloud usually forms by convection, in an unstable layer aloft, which may result from the gradual lifting of moist air, in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms, later in the day. It also sometimes indicates the formation of rain or snow.

Altocumulus can be confused with cirrocumulus, but it’s shading confirming the presence of altocumulus cloud. It also can be confused with high-level cirrostratus. Since there is no halo (which is common feature associated with cirrostratus clouds) in altocumulus, so we can easily separate it from cirrostratus clouds.

Stratus (St) is a horizontal layering with a uniform base cloud. Stratus means “layer” or “blanket” in Latin. More particularly, the term “stratus” is used to indicate flat, featureless clouds of low altitude, varying in color from dark gray to nearly white. A "cloudy day" means the sky is filled with this stratus cloud, obscuring the disk of the sun. These clouds are basically ground fog, formed either through the lifting of morning fog or when cold air moves at low altitudes. These clouds do not usually bring heavy precipitation, although drizzle and snow may occur. It can obscure the sun completely and can greatly weaken daylight. When the sun is visible through the cloud, its outline is clearly visible. Sometimes stratus appears in the form of ragged patches or irregular slices, which are called as stratus fracture (St fra).

Nimbostratus is threatening in appearance, low, even layered, dark gray cloud. "Nimbo" comes from a Latin word "nimbus", which means “rain”. So it is a low cloud, which produces rain. In rare cases, it can be very thin and accompanied by a separate layer of altostratus, divided by a cloudless layer. It is more common at middle latitudes. Usually it produces steady, moderate to heavy precipitation. The base of a nimbostratus cloud is normally dimmed by precipitation and not clearly visible.

Nimbostratus forms when warm, moist air is lifted steadily over a large area. This can happen at a warm front or less commonly, at a cold front. Stratocumulus (Sc) cloud is a low, lumpy layer of cloud, composed of tessellations, rounded masses, rolls etc. Stratocumulus varies in color from dark gray to light gray, with some darker patches. Huge areas of subtropical and polar oceans are covered with this cloud.

When warm, moist air is mixed with drier, cooler air and the mixture is moving, beneath warmer, lighter air above, then stratocumulus cloud forms. It can also form, when air is forced to rise over hills. Sometimes, especially in summer, there are gaps, through which the sun can be seen. It may looks threatening, but unless it is very thick, usually only drizzle or light precipitation occurs from it. Generally it brings only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather, so it may indicates storms to come, in the form of thunderheads or gusty winds.

Cumulus is a bright white, thick, puffy, sharp out lined cloud and looks like pieces of floating cotton, with noticeable vertical development and clearly defined edges. It is a flat based cloud and may be only 1 km above the ground. If the top of the cumulus looks like the head of a cauliflower, then it is called “towering cumulus”. Cumulus cloud forms, when warm air rises and reaches a level of reasonably cool air, where the moisture in the air condenses. The size of the cloud depends on the temperature profile of the atmosphere and the inversion rate. If the top of the cumulus cloud reaches above the altitude, where the temperature is at or below the freezing level, then the precipitation may forms. In windy conditions, this cloud can form cloud streets, which are parallel with the wind. Over the sea, cumulus clouds may be found in regularly spaced lines or patterns. It can be also small with very ragged edges and with outlines, which is continuously undergoing rapid changes. This type of cloud is called Cumulus fractus (Cu fra)

Cumulonimbus comes from the words "cumulo" (puffy) and "nimbus" (rain). So it is grey coloured heavy cloud, with threatening appearance, associated with heavy rain, hail, lightning, and tornadoes. It is also a thunderstorm with considerable vertical growth cloud. It forms due to atmospheric instability, usually from cumulus clouds. Well-developed cumulonimbus clouds can be characterized by a flat, anvil-like top (anvil dome), caused by straight line winds, at the higher altitudes. Cumulonimbus cloud contains severe convection currents, with very high, unpredictable winds, particularly in the vertical plane (updrafts and downdrafts

Clouds are formed in four following ways,

Convective lifting occurs due to convective currents, which is a strong vertical movement of air, caused by heating of the earth’s surface. As air gets heated, then expands and becomes less dense than surrounding air and rises. Finally becomes adiabatically cool to dew point, then this cloud is formed. Cloud types depend on moisture content and stability of the air, mainly cumulus types of clouds are formed with this process.

Turbulence is caused by friction between moving air and the earth’s surface. The friction causes eddy currents in the air, which extend upwards from the surface of the earth. During this process, the air gets cooled adiabatically below its due point and cloud is formed. The vertical extent of these currents depends on the wind speed, roughness of the land and stability of the air. This type of cloud is common with tropical-maritime air, when reaches temperate latitudes.

Orographic lifting occurs, when warm, moist air is forced to raise along sloping ground, such as mountains, hills etc. During this ascent, air cools and orographic cloud is formed. This is mainly stratus type of cloud and for higher ascent; it can be nimbostratus cloud, with continuous precipitation. The ‘table cloth’ of table mountain and ‘levanter’ cloud over Gibraltar are good examples of this cloud. For extensive mountain area, this orographic cloud can be wide with heavy and prolonged precipitation.

Frontal lifting occurs, when two different air masses that vary in temperatures meet. Since the cold air is denser, so the boundary between them is inclined towards the colder air mass. Here the warmer air is forced upward by colder air. So the moist warmer air becomes cool to form this cloud. This lifting mechanism can produce different types of clouds, according to the types of fronts involved, mainly nimbostratus, altostratus, cirrostratus, cirrus; in same cases cumulonimbus and cumulus are also formed.

Thunderstorm (electrical storm or lightning storm) is a form of weather associates with lightning, thunder, heavy rain, snow, hail etc.

Thunderstorms have three stages: 1. Cumulus stage, 2. Mature stage, 3. Dissipation stage.

Cumulus stage is developing stage. Here unstable moist air is lifted vertically into the atmosphere. The process to initiate vertical lifting can be caused by: 1. unequal warming of the surface of the Earth. 2. orographic lifting due to topographic obstruction of air flow, or 2. dynamic lifting, because of the presence of a frontal zone. So when the air goes up, the moisture on it rapidly cools into liquid drops of water, which appears as cumulus clouds. As the water vapor condenses into liquid, latent heat is released, which warms the air, causing air to become less dense than the surrounding dry air. So the air tends to raise more.

Mature stage is a stage, where the warmed air continues to rise and at the same time cools, until the achievements of the surrounding air’s cool temperature, which altitude can be the same of top-tropopause. At this altitude, the air is instead of spreading, making the storm-characteristic anvil-shaped cumulonimbus cloud. Here the water droplets freeze to become ice particles. As these fall, then they melt to become rain. If the updraft is strong enough, the droplets are held aloft long enough, to fall as hail.

Dissipating stage is the stage, where the thunderstorm is dominated by the downdraft. This stage occurs quickly, which can be approximately 20-30 minutes. 13

The downdraft will push down out of the thunderstorm, hit the ground and spread out. The cool air, which is carried to the ground, by the downdraft, cuts off the inflow of the thunderstorm, the updraft disappears and finally the thunderstorm dissipates.

Lightning is an electrical discharge, which occurs in a thunderstorm. It can be seen as a bright spark from the sky. During this time, an electrical charge is built up, within a cloud, due to static electricity. This static electricity is generated by the super cooled water droplet’s collisions with ice crystals near the freezing level. When the whole cloud are filled up with electrical charges, the positive charges or protons form at the top of the cloud and the negative charges or electrons form at the bottom of the cloud. Due to opposite’s attract, a positive charge also build up, on the ground beneath the cloud. So the lightning sparks can occur between clouds, between the cloud and air or between the cloud and ground. When the difference between negative and positive charges become high enough, then a giant spark jumps, which is called lightning.

The temperature of this lightning can be five times hotter than the surface of the sun, but due to shorter duration of it, 90% of strike victims survive.

Tornado is a violent, revolving column of air, which is in contact with both the surface of the earth and a cumulonimbus cloud or in rare cases, the base of a cumulus cloud. It can be many sizes but normally they look like funnel, whose narrow end touches the earth.

Precipitation is any of all of the forms of water particles, whether liquid or solid, that falls from the clouds and reach the ground. The forms of precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail, ice pellets etc. It occurs when the atmosphere, a large gaseous mixture, becomes saturated with water vapour and the water condenses, falling out of mixture. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Air resistance naturally causes the water droplets in a cloud to remain stationary. Due to air turbulence water droplets collide with each other and produce larger droplets. So droplets become heavy enough to overcome the air resistance and fall as precipitation. The standard way of measuring Precipitation (mainly rainfall or snowfall) is the standard rain gauge, which is available in 100-mm plastic and 200-mm metal varieties.

Rain is the liquid precipitation. It is the condensation of atmospheric water vapor into water drops, which is heavy enough to come out from clouds. Rain is available from Ns, As, Sc, Ac, Cu and Cb clouds. Its size is normally more than 0.5mm in diameter.

As per rate, the rain can be classified as follows,

Very light rain — when the precipitation rate is < 0.25 mm/hour.

Light rain — when the precipitation rate is between 0.25 mm/hour to1.0 mm/hour.

Moderate rain — when the precipitation rate is between 1.0 mm/hour to 4.0 mm/hour.

Heavy rain — when the precipitation rate is between 4.0 mm/hour to 16.0 mm/hour.

Very heavy rain — when the precipitation rate is between 16.0 mm/hour to 50 mm/hour.

Extreme rain — when the precipitation rate is > 50.0 mm/hour.

Freezing rain is the name given to the rain, which falls to surface with below freezing temperatures. It becomes supercooled, while passing through a sub-freezing layer of air, which is many hundred meters above the surface and then freezes upon impact with any object they encounter.

Drizzle (also called mizzle) is a light precipitation consisting of liquid water drops, which are smaller than those of rain, and generally smaller than 0.5 mm in diameter. It is most frequent form of precipitation, over large areas of the world's oceans, mainly in the colder regions of the subtropics. Drizzle is normally produced by low stratiform and stratocumulus clouds.

Virga is an observable strip or shaft of precipitation, which falls from a cloud but evaporates before reaching the ground. It is very common in the desert and in temperate climates. It is also common in the Southern United States, during summer.

Mango showers are the pre-monsoon showers in the Indian states of Karnataka and Kerala, which helps in the ripening of mangoes. This shower prevents the mangoes from dropping prematurely from trees and is essential for the mango cultivations at South India. It is also known as April rains or summer showers and a result of thunderstorms over the Bay of Bengal. These summer rains normally come in the second half of the month of April.

Snow is a type of precipitation, in the form of crystalline water ice, consisting of a huge number of snowflakes, which fall from clouds. The process of this precipitation is called snowfall. It is composed of small ice particles and soft in structure, unless packed by external pressure.

Graupel (also called snow pellets) is a precipitation that forms when supercooled droplets of water condense on a snowflake, forming a 2–5 mm ball of rime. Here the snowflake acts as a nucleus of condensation.

Ice pellets (also known as sleet in the United States) are a form of precipitation, consisting of small, translucent ice balls. Ice pellets are usually (but not always) smaller than hailstones. They often bounce, when they hit the ground and generally do not freeze into a solid mass unless mixed with freezing rain.

Ice pellets form, when a layer of above-freezing air is located between 1500 meters and 3000 meters above the ground, with sub-freezing air both above and below it.

Hail is a form of precipitation, which is ball shaped or irregular pieces of ice (hailstones), with diameter between 5 and 150 millimeters.

It is produced by cumulonimbus, associated with thunderstorms or tornado. Hail forms in this storm clouds, when super cooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. The storm's updraft blows the hailstones to the upper part of the cloud. When the updraft disperses and the hailstones fall down, back into the updraft and are lifted up again. This way an up and down cycle is going on. The hailstone gains semi- transparent soft ice layer at upper part of cumulonimbus and also again gains coating of clear ice, while falling. So this way, hailstones consist of transparent ice or alternating layers of transparent and semi- transparent ice of at least 1 mm thick.

Hail is most common in mid-latitudes during early summer, where surface temperatures are warm enough to promote the instability associated with strong thunderstorms, but the upper atmosphere is still cool enough to support ice. Hail is also common along mountain ranges, because mountains force horizontal winds upwards (which are known as orographic lifting), thereby increasing the updrafts within thunderstorms and forming hails, which is mainly observed at northern India and Bangladesh.

Diamond dust is a ground-level cloud composed of tiny ice crystals. Diamond dust generally forms, when there are clear or nearly clear skies, so it is also called as “clear-sky precipitation”. It is most commonly observed in Antarctica and the Arctic. In Polar areas diamond dust may continues for several days without interruption.

Diamond dust is similar to ice fog, in other words it differs from ice fog. The term ‘ice fog’ usually refers to a fog, which is formed as liquid water and then freezes and where as diamond dust forms directly as ice. Also, ice fog is dense enough to reduce visibility (which is one of the main characteristics to be a fog), while diamond dust is usually very thin and may not have any effect on visibility.