Sahara - A Lush Green Land

Sahara is a desert land form that covers one third of African continent. Several factors such as sudden temperature changes, rainfall and geographical features are some reasons for the formation of the desert land forms. 

Based on the geographical location desert can be divided into four types.

(i) Polar deserts:

This part of the region has inability to contain water vapor. Because air to cold or moisture to hold the water vapor.

Examples – Arctic and Antarctic regions

(ii) Subtropical desert:

This type of deserts spread out across the two hemispheres. During summer time desert change too hot and dry and during winter it falls into cold and continuous dry. Therefore, rain evaporates quickly before it reaches to the ground as a result of hot dry climate conditions.

Examples – Sahara, Thar desert

(iii) Continental or Coastal deserts:

Distance from the sea is the main factor to create this type desert. When the air pass over the continents before it reaches to these regions it lost moisture and become as a dry air.

Examples – Nambian desert (Africa), Attacama desert (Chili)

(iv) Cold winter desert:

This desert known as semi-arid desert. This type of desert contains long dry summers and cold winters.

Examples – Great Basin in America, Colorado plateau, Red desert

                    Gobi desert – China

These regions receive only very lack of rainfall. Rain shadow effect is the main reason for this little rainfall. Because when the air rises over the mountain it prevents rain carrying clouds. Therefore leeward side become as a dry region.

Examples – Himalaya Mountain range act as a big barrier to stop the rainfall to reach Gobi desert. 

Sahara desert

 

Sahara is a world’s largest hot desert. It covered one third of the African continent and characterized as subtropical desert. 

In ancient timed, this desert was a vast green grassland area. Scientists provided following evidences that several thousand years ago, Sahara desert had grassland with moisture:

Ø  In 1990’s German scientists created thousand years of Earth’s climate model based on computer simulation. According to this simulation they have mentioned, climatic transition of the Sahara took place within 300 years.

Ø  The radar image, that had taken by Columbia shuttle in 1981 using radar reveals Western Sahara desert sand topography has been characterized by wide river valleys that located aside river valleys.

Additionally this radar image reveals that beneath the desert sand topography with wide black channels cut by meandering of an ancient river.

Ø  Beneath the Sahara desert, the radio carbon dating reveals, very porous and permeable type of rock known as the Nubian sandstone, is about 900m thick and stores enormous quantities of ground water around 18,000 cubic kilometers. 

These analyses shows, the existence of 35,000 years old ground water and indicating that this water is result of precipitation fell in last ice age. 

 Desertification of Sahara

(1) Changes in the Earth’s orbit

Scientists suggest Sahara desert dried up due to the changes of Earth’s orbit. These changes affect the amount of electromagnetic energy which receives by Earth from sun. 

In a simple world, the shining of sunlight on a particular place depends on geographical location, time of day, seasons, landscape and local weather. 

Climatologist Gavin Schmidt (NASA’s Goddard Institute for space studies) suggests that 8000 years ago, Earth’s orbit slightly different from today’s position. Earth tilt changes from 24.1⁰ to 23.5⁰. Additionally Schmidt explained, Earth’s closest approach to the Sun in Northern hemisphere took place in August at summer and today’s this movement happen in January.

Earth’s orbit tilt gets changes by gravitational force that reveals by other bodies in solar system.

In every year earth’s tilt get changes from 22 to 25 degree. 

(2) Climatic changes

Green Sahara suddenly became to end in 5,500 years ago. Schmidst believed that slowest changes in Earth’s orbit disturbed that ecosystem. 

A timeline of Sahara occupation:

Ø  10,500 - 9000 years ago: Sahara turned as a habitable zone by monsoon rainfall.

Ø  9000 – 7,300 years ago: Continuous rainfall, growth of vegetation, migration of animals leads to development of human settlement. 

Ø  7,300 – 5,500 years ago: Retreating of rainfall reason for human migration. End of rainfall and desert conditions took place in 5,500 years ago.

 References:

(1) https://na.unep.net/atlas/africaWater/book.php

(2) http://www.livescence.com

(3) National Institute of Science Research/ Eagle.com

(4) http://www.oxfordpresents.com/ms/nance/water-in-the-sahara-desert/

(5) The Encyclopedia of Earth (2008); Weldon Owen pty Ltd; Australia

Optical phenomena of the Atmosphere

1. Nature of light

Light is weightless, immaterial and transverse. Light generated by incandescence and luminescence. Incandescence is the emission of light from hot and luminescence is the emission of light when excited electrons fall to lower energy level. 

The speed of light is constant in vacuum and speed of light through medium is slower than vacuum. It depends on medium which it travel or interact.

2. Properties of light

2.1 Radiation

The sun emits heat and light. These forms constitute only small portion of energy of the total energy that radiates from the sun. This energy called as radiation or electromagnetic radiation. All types of radiation travel in a speed of light (x-rays, radio waves, or heat waves). Electromagnetic radiation comes in various sizes or wavelengths. Earth gets sun radiation in the form of short wave length. 

Radiation often identified when the rays interact with an object. When it is easy to show that will call visible light. It appears “white” in color. White light is corresponding to various colors and each color represent various wave length and over 95% are between 0.1 and 2.5 micrometers and much of this energy concentrated in the visible and near-visible parts of the electromagnetic spectrum. 

 2.3 Reflection

Light travel through vacuum of space in a straight line. Rays enter the surface and bounce back from the surface is to be called reflection. When the rays bounce back from the surface it reflects the surface in same angle. This principle is called as law of reflection.

 Angle of incidence = angle of reflection

Incoming rays = Outgoing ray

This is not perfectly suited for all objects. It depends on interfere particles and wavelength of the light. When light rays reach the rough surface the image appear as multiple images and travel in various directions.

 

2.4 Refraction

When light travel through transparent material it’s not bounce back from material. It transmitted through that material to another medium called refraction. For example when the light passes through water it get bending in the opposite direction. The amount of refraction in air is small and in water is much larger.

We can explain this illusion through sun setting. When the sun near the horizon we can see the sun few minutes after it has set below the horizon. Solar radiation takes eight minutes to reach the earth. Therefore we can see only the eight minutes earlier of the suns apparent located position. This illusion is cause by the result of bending of solar radiation. 

Usually air density increases in earthwards. This gradual density changes are equal for gradual slowing and bending of light. 

3. Rainbows

This optical phenomenon can see when the observer is situated with the sun on one side and rain shower occur in opposite side. The rainbow outer part start with red color and it gradually blends to orange, yellow, green, blue and violet. In this situation observer can see the view of dimmer secondary rainbow near the primary one. The second bow will appear 8⁰ above the primary bow and it in reverse colors order that red in inner part and violet in outer part of the rainbow. 

The sunlight and water droplets are needed for the formation of rainbow. When the light passes through water it speed slowed and gets refracted in different angle and each color of light travel at different velocity in water. 

This phenomenon was demonstrated by Sir Isaac Newton through using a prism. Newton noted when the light passes through prism it refracted twice. Once it enters into the prism and after it exists. In this occasion water droplets act as a prism and disperse the sunlight. The rainbow appears in curved shape because rays reach the observer in the angle of 40⁰ and 42⁰ from the sunlight path. 

    

3. Halos, Sun dogs and Solar pillars 

Halos:

This optical phenomenon can see any observer and it often appear morning and late afternoon when the sun is near the horizon. It also generated by dispersion of sunlight. These halos are mostly associated with cirrus clouds. 

Four types of ice crystals that compose cirrus clouds are contribute to the formation if halos: plates, columns, capped columns and bullets. Sunlight strikes these crystals in all angles and light get scattered and disperse equally in all directions. 

Halos are commonly whitish in color. Ice crystals are dispersing the light as a rain droplets or prism. The color produced by the light reddish band is in inner portion of the ring and other colors tend to washout each other and leave the red surrounded by whitish ring. 

Most spectacular effect that associated with halos is sun dogs or perihelia and sun pillar. Sun dogs forms under the same condition. This phenomenon is happen when the ice crystals are allowed to descend slowly. Sun pillar are most often viewed near sunset or sunrise and appear in vertical from the sun. 

 

 References:

1. Frederick K. Lutgens, Edward J. Tarbuck, Dennis Tasa (2010); The Atmosphere : An introduction to Meterology; Pearson Education, Inc. USA. 

2. http://physics.info/light

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