Category: Geography Notes PPT

According to the terminology of geography and geology, a vertical or nearly vertical exposure of a surface of a rock or a hill or a mountain is known as a Cliff. They are generally formed because of the erosion of landforms by the weathering forces of nature. For example, a cliff near the water body such as a sea or an ocean is caused by the erosion of the rocks by the water waves over a period of years. Aside from the sea-shores or coasts, a cliff edge is most commonly found in mountainous areas, escarpments, and along the rivers. If a rock is resistant to weathering and erosion, it generally results in the emergence of a cliff. 

Formation of a Cliff

The formation of a cliff happens mostly with sedimentary rocks. Sedimentary rocks such as sandstone, limestone, chalk, and dolomite are most likely to form a cliff. Sometimes, if there is a landslide formed because of a geologic activity then it can also lead to the emergence of a cliff edge. In old mountains, a mountain cliff can appear because of landslides caused due to the differential erosion of rock layers of differing hardness.  

In arid areas, a cliff edge can be determined by the jumbles of fallen rock. It means that in such areas, a rock that has fallen because of erosion, can give rise to the emergence of a cliff side. Opposite to that in areas of high moisture, a soil slope alongside an elevated portion of the earth can give rise to a cliff side. Certain mountain cliffs are part of the tributary waterfalls or rock shelters as well. Sometimes the edge of a cliff is created at the end of a ridge with different types of rock columns such as mushroom rocks. And as already stated before, coastal erosion by the waves can lead to the creation of the edge of a cliff which is the sea-cliffs on the receding coastline. 

The Ordnance Survey – a national mapping agency of Great Britain differentiates between cliffs and outcrops. According to them, a vertical exposure can be called a cliff only when it forms a continuous line along the top edge of the exposure (top edge of a cliff) with downward projections towards the land surface. They call the continuous lines along the lower edges as outcrops.

Large and Famous Cliffs

As per the above definition of a cliff, there is confusion in the extent of vertical exposure that is to be considered. There are various questions such as how much of the vertical slope be counted as a cliff. One can totally consider the combination of a vertical rock wall with a very steep slope as the cliff in terms of vertical exposure or only consider the rock wall as the cliff. Setting apart these doubts, one can find ascertaining qualities in some of the cliffs that are found around the world. 

Some of the largest cliffs on Earth are actually found underwater. A glistening example of such a cliffy feature is the ridge found sitting inside the Kermadec Trench. It has an 8000 m drop with a 4250 m width.

Another example of the highest cliffs is the steep non-vertical cliffs of Nanga Parbat: The Rupal Face and Gyala Peri. Both of them rise approximately 4600 meters or 15000 feet high above their base. 

The east face of the Great Tango in the Karakoram range of mountains of northern Pakistan is said to be one of the highest cliffs with a height of 1340 meters steep face. This height of this mountain cliff is considered by referring to the nearly vertical headwall of two stacked pillars which adds a very steep approach bringing the total drop from this cliff top of the east face to the nearby Dunge Glacier making it to 2000 meters.

One of the highest sea-cliffs stated by the Guinness World Records is Kalupapa in Hawaii. The height of this cliff from the cliff top is measured to be 1010 meters. The north face of Mitre Peak in New Zealand is also one of the competitors for the title of the world’s highest sea-cliff in the world. The north face of the Mitre peak has a height of 1683 meters. But these cliffs are taken into consideration by referring to the less stringent definition of the cliff as they also include it even though they have average slopes of 1.7 corresponding to 60° angle. A more stringent view of the definition is accepted for the vertical cliff found at Maujit Qaqarssuasia situated in the Torssukátak fjord area located at the tip of South Greenland having a vertical height of 1560 meters.

Mount Thor on Baffin Island in Arctic Canada is often referred to as the highest cliff with a height of 1370 meters. This height is inclusive of the cliff hanging part which does not form part of the 1250 meter which is said to be the longest vertical drop on Earth. But there is a possibility that the Polar Sun Spire in Sam Ford Fjord, in the Baffin Island may be higher than this. 

One interesting fact is that the highest cliff in the Solar System is said to be Verona Rupes which is approximately 20 kilometers of height found on the fault scarp on Miranda, a moon of Uranus.

Life Forms and Folklore

Clifftops and surrounding areas provide a unique ecosystem that is part of diverse habitat niches of a variety of plants and animals. An interesting fact is that many birds have decided preferences for choosing cliff locations for nesting and are often chosen because of their defensibility and absence of certain predators. 

The word ‘cliffhanger’ or cliff hanging became popular because the character in the novel To Kill a Mockingbird is hanging on the edge of a cliff. Thus, apart from being a source of scenic beauty cliffs form an integral part of the ecosystem and habitats with cultural values as well. 

Coral islands are kinds of islands, and they are formed from coral detritus as well as connected organic materials. A coral island happens in subtropical and tropical areas, commonly in the form of a portion of a coral reef. Coral islands begin in the form of the volcanic island on a hot spot. When the volcano does emerge from the sea, then a fringing reef enlarges on the volcano’s outskirts. Eventually, the volcano shifts to the hot spot via a process that is called plate tectonics. When this happens, the volcano finds it incapable of keeping up with the erosion of the wave, and so, it undergoes subsidence.

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What is a Coral Island in India?

A coral island in India is considered one of the most dynamic and earlier ecosystems of India. A coral reef does not only propose a sanctuary to countless marine life but turns important for shielding the coastline from wearing away. In India, there is nearly 7517km of coastline, and it includes islands though the mainland coast happens to be 6100 km.

Some popular Indian coral reefs remain confined to the Gulf of Manner, Andaman & Nicobar Islands, Palk Strait, the Lakshadweep Islands, and the Gulf of Kutch. All these reefs happen to be Fringing reefs, excluding Lakshadweep as they are Atolls. You will also come across some Patchy corals that remain present all along the inter-tidal zones. 

Some Features of Coral Islands

Though many people know what is coral island, they aren’t aware of its features. A coral island is acknowledged as a tropical island that is built of some organic materials that are derived from the corals’ skeletons and various other plants and animals connected with corals. The coral islands comprise low land but only some metres above sea level, commonly with coconut palms. They remain surrounded by many white coral sand beaches.

The coral islands extend to many kilometres, and they comprise nearly all the tropical limestone islands whose structures are a portion of the living coral reefs. The building of reef takes place below the level of high tide, and a common coral island surmounts the comparatively flat peak of the entire reef system. 

The coral island examples are The American territories of Baker, Jarvis, and Howland Islands etc. Some Indian examples are-

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Gulf of Mannar

The Gulf of Mannar happens to be a huge shallow bay that forms a portion of the Laccadive Sea and it has an average depth of 19 feet or 5.8 meters. This part lies between the Sri Lankan west coast and India’s southeastern tips. The chain of some low islands, as well as reefs, is called Adam’s Bridge or Ram Sethu and it comprises Mannar Island.

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Palk Bay

This is a fifteen thousand square kilometer biodiversity conglomeration that is nestled between India’s South East Peninsula and Sri Lankan. It has a coastal length of two hundred and fifty kilometers. 

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Gulf of Kutch

The Gulf of Kutch happens to be the bay of the Arabian Sea besides the Indian west coast located in Gujarat. It is well-known for its huge daily tides.

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How are Coral Islands Formed?

The formation of coral islands happens to be a dynamic process, and it takes many thousands of years to form a coral island. A coral island is formed with a freshly-formed volcanic island, and it rises above the ocean’s surface. When the island stabilises and magma no longer flows into the ocean, a coral begins to grow. This process happens in the shallow water all across the volcanic island.

Coral reef islands comprise rocks, and they emerge from coral skeletons. They are biologically formed calcium carbonate compounds that are the derivatives of the adjacent coral reefs. The sizes of coral reef islands range from some sq. metres to many sq. km. Again, you will find them in all proportions and shapes. The soils of a coral reef island comprise calcareous algae, coral fragments, various other limestone detritus, and guano that is found from sea birds, various amounts of humus, drifted pumice, and volcanic ash.

Coral reef islands become capable of developing only when ideal conditions maintain coral growth with passing the time. These conditions comprise some favourable physical factors, like high salinity, high temperature, low nutrients, and excellent penetration of light, besides some biological factors in a tropical region. 

Types of Coral Islands 

Coral Atoll

An atoll is acknowledged as a coral reef that is ring-shaped. At times, it emerges as a series of islets and sometimes islands too. The atolls surround bodies of water known as lagoons. Most of the time, lagoons and atolls shield a core island. A channel between islets connects a lagoon to a sea or an open ocean. Atolls form with underwater volcanoes, known as seamounts. At first, a volcano erupts, and it piles up lava. When the volcanoes continue to erupt, then the elevation of the seamount becomes higher, and it finally breaks the water surface. This way, the topmost part of the volcano turns into an oceanic island.

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Fringing Reef

A fringing reef is considered one of the different kinds of coral reefs. These reefs grow closer to the shore on a high island or mainland coasts. Commonly, a fringing reef is shore-attached. Some examples of fringing reefs are Timor-Leste, Indonesia, Philippines, Red Sea, East Africa, The Caribbean and the Australian western coast.

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Barrier Reef

A barrier reef gets featured by a reef and it is separated from the chief coast through a lagoon or deep channel. The initial formation happens on the edge of an open edge. Afterwards, it grows towards the coast or subsequent growth in parallel. The largest and renowned barrier reef example is the Great Barrier Reef of Australia. Some other examples are the New Caledonian Barrier Reef and the Belize Barrier Reef.

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Patch Reefs

These are the isolated little reefs which grow at the bottom of the platform island or even in continental shelf. It is present in between fringing and barrier reefs. They almost never reach the water surface. Patch reefs vary in size.

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Conclusion

A coral island is a low island that is produced in warm waters by very small sea animals known as corals. The corals build up the hard exterior skeletons of calcium carbonate. It is called limestone, and it has huge similarities to the shells of some sea creatures such as mussels and clams.

Earthquake magnitude is the measure of the “size” or amplitude of the seismic waves generated by the sources of the earthquake and recorded by the seismograph. As the magnitude of the earthquake changes tremendously, it is necessary to compare in order to compress the range of amplitude of waves measured on seismograph using the Mathematical device. 

In 1933, the American seismologist Charles. F. Righter introduced the earthquake magnitude scale (known as the Richter magnitude scale) as the logarithm to the base 10 of the maximum seismic waves amplitude reported on a standard seismograph (in thousandths of ml) at a distance of 60 miles or 100 km from the earthquake epicentre. 

Reductions in amplitude are observed at various distances to the expected amplitude at a standard distance of 100 km is formulated based on the empirical tables. Righter earthquake magnitude scales are computed based on the assumption that the ratio of the amplitude of the maximum wave at two given distances is equal for all earthquakes and is different from the azimuth.

Due to the various shortcomings of the Richter scale, most seismologist authorities now use another scale known as the moment magnitude scale to record the magnitude of an earthquake.

What is the Magnitude of Earthquakes?

The earthquake magnitude is a measure of the amount of seismic energy released by it, so it is a quantitative scale. Magnitude is the most commonly used measure to describe the overall strength or size of an earthquake. The magnitude of an earthquake is expressed in decimal fractions and whole numbers. For example, a magnitude of 5.3 is considered a moderate earthquake whereas a magnitude of 6.3 is a strong earthquake. Due to the logarithm basis of scale, each whole number increases in magnitude represents a 10-fold increase in measured amplitude as measured on a seismograph.

In modern times, several different magnitude scales for measuring the relative size of an earthquake are used by scientists and engineers. An earthquake scale for measuring magnitude has no lower or upper bounds. Sensitive seismographs can even record earthquake magnitudes of negative values and have reported magnitude up to about 9.0 (For example, the 1906 San Francisco earthquake had a Richter magnitude of 8.5).

Earthquake Frequency

An earthquake frequently defines how often a given earthquake with a certain magnitude comes about. On average, an earthquake with a magnitude of 2 or less comes about several hundred times a day throughout the world.

However, a major earthquake range with a magnitude of 7.0 – 7.9 occurs more than once per month throughout the world. Whereas, an earthquake ranges to a great extent with a magnitude of 8.0 or greater or comes about only once a year.

Knowing the earthquake frequency is important for engineers as they not only strengthen a building against earthquake shock but also minimize the force a building is subjected to. To minimize the loss, they install a base isolator that helps to isolate the base of the buildings from the earth’s movement. 

Earthquake Range in Magnitude Scale

How Strong is a 3.5 Magnitude Earthquake?

An earthquake of magnitude 3.5 on the scale is considered to be minor. A 3.5 magnitude earthquake is often felt by people, but rarely causes any damage to the buildings. You can even observe the shaking of an object inside the buildings.  

Did You Know?

• The largest-ever earthquake with a magnitude of 9.6 occurred in China in 1916.

• The largest earthquake in the US with a magnitude of 9.2 struck the Prince Willian studio, Alaska on March 28, 1964, UTC.

• An earthquake can occur in any type of weather.

• It is estimated that approximately 500,000 earthquakes are detected in the world each year. 100,000 among those can be felt, and 100 of the earthquakes can cause damage.

• The most deadly earthquake occurred in Shaanxi, China in 1556. It is estimated that 850,000 people were killed in that period.

• Florida and North Dàkota are the places with the least earthquakes.

Ferrel cell refers to a model that belongs to the mid-latitude region of the Earth’s wind flow. The term was first proposed by William Ferrel in 1856. The air inside the Ferrel cell flows eastward and poleward near the equator and in higher altitude areas westward. It was the first model ever to account for the westerly winds, and the Ferrel cell latitude is between 35o Celsius and 60o Celsius in both the north and south hemispheres. However, the Ferrel cell is not yet considered a perfect representation of reality because it needs the wind to flow westward in the upper level of the mid-latitude. 

Hadley Cell and Ferrel Cell 

Hadley and Ferrel’s cells are characterized by the current atmospheric energies. There are various weather systems that weave around the globe at the same time. However, if an average is calculated, then the global order of air movement can be estimated.

Differential Heating

The reason behind so many weather patterns, deserts, jet streams as well as prevailing winds is only because of the circulation of the global atmosphere that is caused due to the Earth’s rotation and the heat that the different parts of the globe receive individually. 

Sun is the primary source of heat on the Earth, and as the Earth is in a tilted position, the different regions of the Earth receive heat in different ways. The circulation of wind is caused due to the circulation of Hadley polar and Ferrel cells. 

A huge difference in the temperature is experienced due to the difference between the equator and the poles. The global circulation helps in providing inherent air conditioning to stop the poles from becoming too cold and the equator from becoming too hot. 

The Global Circulation and its Relation with Polar Cell, Ferrel Cell and Hadley Cell 

Vigorous wind circulation covers a significant part of the Earth’s surface. The global circulation is often regarded as a world-wide system of winds necessary to transport the heat from the tropical latitudes to the polar latitudes. 

There are three cells in each of the hemispheres, which are named Hadley Ferrel polar cells. The air circulates by the depth of the entire troposphere. The vertical extent of the atmosphere that starts from the surface and then goes directly to the top between 10km to 15km high is known as the troposphere. This is the place in the atmosphere where almost all the weather changes take place. 

Hadley cell

Hadley cell refers to the most prominent cell that extends from the equator and goes up to 30 to 40 degrees south and north. It is named after the famous meteorologist named George Hadley. In the Hadley cell, the winds blow towards the equator and then climb near the same place in the form of a broken thunderstorm as a line. This line then forms the Inter-Tropical-Convergence Zone (ITCZ). From the top of the storms, air flows towards the direction of high latitudes. It then sinks into there to produce high-pressure in the Earth’s hottest deserts region and the subtropical oceans, like the Sahara Desert of North Africa. 

Ferrel Cell

The middle cell is known as the Ferrel cell, and the air here converges at very low latitude to ascend through the boundaries between the warm tropical air and the cool polar air that appears between 60 to 70 degrees south and north. The Ferrel cell circulation is a mid-latitude circulation that was named by a famous person named Ferrel in the 19th century. 

It often takes place across the latitude of the UK that gives unsettled weather to the area. There is a connection between the Hadley cell Ferrel cell. Within the circulation process, air from the Ferrel cell flows at a high latitude and joins the Hadley cell through a sinking air.

Did You Know?

• The weakest and the smallest cells are known as the Polar cells. 

• These extend from 60 to 70 degrees south and north to the poles. 

• Air in these particular cells sink through the highest latitudes and then flow out to the lower latitude on the surface. 

• The polar front is the junction that connects the polar cell and the Ferrel cell. This is a low-pressure zone where the relatively warm moist air runs into a relatively dry and cold air of the Polar cell.

So, this is all about the global circulation of winds and their relationship with the polar cell, Hadley Cell and Ferrel Cell. A minute study of the diagrams along with the theoretical part will further clarify your concept of these topics. Practice the diagrams as you read! 

The time interval occupied by the geological history of the earth is known as the Geologic time. Or a system of chronological dating which classifies geological strata in time is known as the geological time scale. The geologic time is estimated to have started at the Archean Eon which was approximately 4.0 to 2.5 billion years ago. This geological time scale still continues to this day. Sometimes modern geological time scales often in addition include the Hadean Eon which is an interval in geologic time that ranges from 4.6 billion years to 4.0 billion years. The geological periods can be observed by looking at the rock strata which serves the recorded geologic history of Earth. 

Calendar of Earth’s Geology

As can be observed from the geologic time scale definition, the time scale of geologic time is huge in millions of years. Geological periods in order of their decreasing duration divide the geologic time into certain units of time scale which are – Eons, Eras, Periods, Epochs, and Ages. Eons are divided into Eras which are further subdivided into geological Periods, Epochs, and Ages.

The calendar of Earth’s geologic history is currently divided into four eons which are the primary and largest divisions of time scales. They are as given below:

1. Hadean eon: Started with the formation of the earth and lasted for 600 million years.

2. Archean eon: Earliest known life forms emerge as the Earth has cooled down enough to form the continents.

3. Proterozoic eon: Oxygen generating photosynthesizing single-celled organisms appear in the beginning of this geologic time period.

4. Phanerozoic eon: Encompasses development of diverse forms of living organisms and continues till present day encompassing 541 million years.

The naming of these geologic time scale units is based upon the stratigraphy which correlates and classifies with the rock strata. Although the rock strata provide significant information about the changes taking place in the geological landscape, the fossil remains provide additional information and help in the demarcation of the time scale. The fossil records provide primary evidence and means of establishing a continuous geological record when correlated with the time of the emergence and disappearance of universally common species. This fossil record sets an outline with appropriate boundaries for the beginnings and endings of geological periods, epochs, ages, and other intervals. 

The International Commission on Stratigraphy (ICS) provides the nomenclature, dates, and colour codes for preparing charts that provide information on the relationships between different geologic time periods. The chart that agrees with this information is given below:

Establishment of Principles 

Rock strata, fossil remains, and living things play important roles in establishing and developing the geological periods in order as they have undergone geological and evolutionary changes respectively over geologic time. Many organisms are found to have existed only during particular geological periods which enhances specificity in dating and establishing the time scale. The reconstruction of the geological timeline of the geologic history of various regions and of Earth as a whole is done by correlating the data from the rock strata and the specific types of fossils that are found in that region. This timeline has been numerically quantified by means of absolute dates obtained from the radiometric dating methods. Radiometric dating evidence estimates that the age of the earth is 4.6 billion years.

The difference between the geological periods is also marked by extreme events such as mass extinctions. There have been major mass extinction events and they divide the geologic time scale into five different periods. An example of this is the demarcation between the Cretaceous geological period and the Paleogene geological period marked by the Cretaceous-Paleogene extinction event. Many groups of life forms, especially the non-avian dinosaurs, went extinct during this event. 

Although, the time interval mentioned by the geologic time scale definition, can be estimated from the fossil remains and living organisms, many times it leads to different nomenclature of geological periods around different locales. An important aspect of the work of ICS is to solve this issue of variance in nomenclature and bring a universal terminology to the work. Not only that but also the main work of ICS is to establish the principles for making the geological time scale more structured.

These geologic time scales can be identified for other planets as well. Planets and moons having rigid structures preserve the various geological events at least in the form of stratification of rocks. This is not the case with planets which are made up of gases. But on a cautionary note, the geological events taking place on one planet have very little or no effect on the geological changes taking place on other heavy objects such as planets in the universe. 

Naming and Dating of Geological Time Scale

The earliest geological activities were done based widely upon the stratification of the rocks. These activities were mostly carried out by the Britishers and Scottish geologists. The effect of this is seen in the names of various geologic time periods. For example, the name “Carboniferous” of the carboniferous period was named after the adaptation of “the Coal Measures” an old biologists’ term. The carboniferous period is named for the set of strata of coals from the peat deposits because of the swampy forests around 360 to 300 million years ago. British geologists also contributed to the grouping and eras and division of the time scale units greatly. 

Earlier geologists and paleontologists studied various rock strata and fossil remains and estimated different time scales by understanding and involving the different rates of weathering, erosion, sedimentation, and lithification. After the discovery of radioactivity, the 20th century brought all the previous methods and time scales to the table for questioning. After that radiometric data was relied upon more and more for determining the age of the earth and subsequently determining the geologic time periods.

Conclusion: Geologic Time

Geologic Time Scale definition and particulars are briefly explained in this article. The huge time scales provide a large storehouse of the knowledge in understanding the different geologic changes taking place in the geology of the Earth which help us understand what and how of these changes and may possibly aid in predicting what can be. Although that will be a rare chance of occurrence. But nonetheless, this does not hamper the appreciation that the geologists and their work deserve to understand many aspects of geology and note it in geologic time scales helping us to know about the presently changing seismic activity landscape.

Harmattan is a season that is observed in the Western African nations of the African continent. It occurs in-between the end of November and the middle of March month. It is a dry season with typical characteristics of a dry and dusty season. The season is brought over by the northeasterly trade wind which is also named the Harmattan and it blows from the Sahara desert over West Africa and moves into the Gulf of Guinea. Depending on the local circumstances the temperature during harmattan can range from cold in most places to hot in certain regions. 

Characteristics of the Harmattan

The harmattan season brought in by the dry and dusty northeasterly trade wind occurs during the months of the lowest sun i.e. from the months of November to January with the highest during the period of December. During this season the subtropical ridge stays over the central region of the Saharan desert and there the low-pressure Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. While passing over the Sahara the Harmattan wind picks up the fine dust and sand particles which becomes the typical characteristic of the Harmattan season. The wind flow during the winter season of the Harmattan is given in the below picture:

The language from which the harmattan meaning is derived is the Twi language, a dialect of Akan language spoken in southern and central Ghana. The name has become so popular that the harmattan meaning has become synonymous with the particular season as in the case of monsoon and winter. Due to its strong dryness, as compared to the humid tropical air, the trade wind is also known as “doctor wind”. 

The season is particularly different from the winter season as it is prominently associated with cold, dry, dust-laden wind and wide fluctuations in the room temperatures of the day and the night. For example, during the entire day, the temperature may remain as low as 9℃ but during the afternoon it can rise up to 30℃ with relative humidity dropping below 5%. Also, it is not cold in all regions but in some regions of the Sahara, the weather will be hot for the Harmattan season. It is well established that the air of Harmattan is dry and desiccating. 

It is known for the desert-like conditions because of the following characteristics: lowering of the humidity, dissipation of cloud cover, prevention of rainfall and sometimes the creation of big clouds of dust that lead to the occurrence of desert storms or sandstorms. The fire risk increases because of the dry wind of the Harmattan season anc can also cause severe damage to the crops in all the regions where the season is prevalent. The interaction of the Harmattan with the monsoon winds can cause tornadoes. 

Effects of the Harmattan Haze

An effect known as the Harmattan Haze is quite prevalent in some countries of West Africa, with a heavy amount of dust present in the air. It can severely lead to the limitation of visibility and block the sun for several days, comparable to heavy fog. Because of this, there is a loss of millions of dollars to airlines because of the cancellations and diversions of the flights each year. When the haze gets weak and the sky gets clearer because of the extreme dryness of the air the trees will begin to die. Two distinct images of the Harmattan haze at two different places is shown below: 

The drop in humidity below 15% can/might result in spontaneous nosebleeds for some of the people. There are also other health effects on the people caused by the Harmattan. They include conditions like dryness of the skin, dried or chapped lips, eyes, and respiratory problems. This season is also adverse for people suffering from asthma because it can cause aggravation of asthma.