Winds, Clouds, Rainfall, Islands & Rocks

Winds, Clouds, Rainfall, Islands & Rocks

Winds

• Wind is the movement of air caused by the uneven heating of the Earth by the Sun.
• Sometimes wind blows gently, refreshing us. At other times, it blows strongly creating storms that cause widespread damages.
• We need measurements of two quantities: direction and speed, to give a description of the wind.

Wind measurement Instruments

Wind vane or Weather -cock measures the wind- direction. Anemometer measures the wind velocity.

Trade Winds

• They blow from the Sub-tropical High Pressure Belt to the Equatorial Low Pressure Belt in the tropics between 30° North and 30° South latitudes.
• They blow as the N.E.  Trades in the Northern Hemisphere and as the S.E. Trades in the Southern Hemisphere.
• The name 'Trade' is derived from a nautical expression to blow tread' meaning to blow along a regular path or 'tread'.

Westerlies

• They blow from the Sub-tropical high Pressure Belt to the Sub-polar low Pressure Belt in the temperate latitudes between 30° and 60°, on either side of the Equator.
• They are more constant and stronger in the Southern Hemisphere because there are no large landmasses to interrupt them.
• In places they become so strong that these winds are known as the Roaring Forties or the Brave West Winds and the Furious Fifties.
• The belts of the Westerlies move north and south following the Sun's movement. These are known as Westerlies because they blow out of the west.

Polar Winds

• They blow from the Polar High Pressure Belt to the Subpolar Low Pressure Belt between latitudes 60° and the poles on both sides of the Equator.
• These winds blow from the east to form the Polar Easterlies.
• They are more regular in the Southern Hemisphere.
• Polar winds are extremely cold and dry.

Jet Stream

• These are the winds blowing with great velocity near the Tropopause. The Jet Streams are active in 150 km wide and 2-3 km thick transition belt.
• The general velocity of these winds is about 150-200 km/hour. But sometime the velocity at the core of the Jet Stream is found to be 325km/hour.
• Jet Streams are generally found in the Northern Hemisphere only. In the Southern Hemisphere, they are found over the South Pole, though they are found in the form of light Rossy Waves over other latitudes also. Jet Streams are of four types-

(a) Polar Night Jet Streams: These are found beyond the 60° latitudes in both the hemispheres.

(b) Polar Frontal Jet Streams: These are found over the zone Between 30° to 70°  N latitudes at a height of 9-12km. These are related to polar fronts and follow wave like inconsistent paths. Since this jet stream was made known by Swedish scientist Rossby, it is called Rossby waves-

(c) Sub-Tropical Westerly Jet Streams: These are found between 20°-35°  N latitudes at a height of 10-14 km. The main reason behind their origin is the north-eastern flow of the air rising through the convection in the equatorial region in the Tropopause. These jet streams are responsible for bringing the western disturbances to India in the months from December to February.

(d) Tropical Easterly Jet Streams: All the other jets follow a westerly direction, but, these jet streams follow the north-easterly direction. These jet streams found in the zone of 25°-35° N originate in the North Hemisphere in the summers. These jets originate in a zone of 100-150mb pressure at a height of 14-16km. This is responsible for the origin of the Indian monsoon. Since these jets are warm, they lift the hot and humid surface air and cause convectional rainfall and in this way jet streams is responsible for the bursting of monsoon in India.

Climatic Winds or Periodic Winds

• These winds change their direction along with change in time or change in climate. Land and sea breezes and the Monsoon winds are typical examples of periodic winds.

Monsoon Winds

• Monsoon winds are seasonal winds characterised by a complete reversal in their direction from one season to another.
• They blow from the sea to the land in summer and from the land to the sea in winter.

Local Winds

• These winds blow due to local variation in the temperature and pressure, and influence of very small area.

Chinook: Chinook means the snow-eater- This is the hot and dry wind blowing along the eastern slope of the Rockies and covers an area from the southern part of Colorado in the south to British Columbia in Canada in the north. Due to its effect, the snow melts and green grass sprout even in the winter. This wind is helpful for the animal rearing as it makes rakes the grasslands snow free.

Foehn: This is similar to Chinook and blows along the northern slope of Alps. It affects the snow, makes the weather pleasant and helps in early ripening of the grapes.

Sirocco: This is a warm and dry and dusty wind which blows in northerly direction from the Sahara Desert and after crossing over the Mediterranean Sea reaches Italy, Spain etc., where it is also known as blood rain because of its reddish sand brought along with it from the Sahara desert. It is very much destructive to agricultural and fruit crops.

Black Roller: These are the warm and dry dusty winds, blowing in the great plains of North America.

Yoma: This is the warm and dry wind like Santa Ana, blowing in Japan.

Temporal: This is the monsoon wind blowing in the Central America.

Simoom: This is the warm and dry wind blowing in the Arabian Desert. It causes dust storms and obstructs visibility.

Karaburan: these are the dust laden fast blowing winds in the Tarim Basin in the central Asia. These winds blow towards the North-East.

Harmattan: This is the warm and dry wind blowing from north-east and east to west in the Sahara desert. The weather becomes suddenly dry and pleasant in the western coast of Africa, at the arrival of Harmattan. Therefore, it is called Doctor in the Guinea coastal.

Loo: This is a hot and dry wind blowing in the northern India from the north west and west to east. It is sometimes called heat wave.

Santa Ana this is the warm and dry wind blowing in California (USA).

Levanter: It is a strong easterly cold wind in southern Spain.

Clouds

• Clouds are defined as aggregates of innumerable tiny water droplets, ice particles or mixture of both in the air generally much above the ground surface. They play a vital role in the heat budget of the earth. Based on the average height, clouds are classified into:

1. High Clouds

(Height: 42,500 ft. to 16,500 ft or 13 to 5 km, according to Encyclopaedia Britannica)

Cirrus Clouds the high altitude detached clouds having fibrous (Chain like) or silky appearance are called cirrus clouds. They are composed of tiny crystals. Therefore, they do not cause rain fall. These clouds are indicator of cyclones.

Cirrocumulus these are white coloured clouds having patches of small white flakes or small globules which are arranged in distinct groups or wave like form. They generally appear as ripples similar to sand particles in the desert.

Cirrostratus Clouds These clouds are generally white in colour and spread in the sky like milky thin sheets. In fact, cirrostratus is a thin veil of cirrus clouds. These are transparent clouds and create halo around the sun and the moon. These clouds are seen just after the cirrus cloud at the onset of cyclone. In this way, they also indicate the arrival of a cyclone in the near future.

2. Middle Clouds

(Height: 23,000 ft. to 6,500 ft. or 7 km to 2 km.)

Altostratus Clouds: These are thin sheets of grey or blue clouds having fibrous or uniform appearance. When theybecome thick sheets, the Sun and Moon are obscured and they appear as bright spots behind the clouds. They yield widespread and continuous precipitation.

Altocumulus Clouds: These clouds are characterized by white and grey wavy layers or globular forms. They form fairly regular patterns of lines, group or waves. High globular groups of altocumulus are sometimes called as Sheep clouds or Wool pach clouds.

3. Low clouds

(Height: 6,500 ft to zero (0) feet or 2 km to 0 km)

Stratus Clouds: These are dense, low lying fog-like clouds of dark grey colour, close to the ground surface. They are composed of several uniform layers. These clouds are formed in the subtropical region in the winter season belt due to convergence of two air masses of opposite nature. When these clouds are associated. With rains or snow, they are called nimbostratus clouds.

Cumulostratus Clouds:

These are light grey or whitish colour clouds. They are generally composed of globular masses or rolls which are arranged in lines, waves or groups. Generally, they cover the entire sky in winter season. They are generally associated with fair or clean weather.

Cumulus Clouds these are dense widespread and dome-shaped with flat bases. They are associated with fair weather.

Cumulonimbus Clouds:

These are thunderstorm clouds. They are very dark and dense clouds. They show great vertical development. They are spread over large areas. They are associated with heavy rainfall, hailstorm, thunder, etc. The rainfall in both tropical and temperature cyclone is generally caused by these clouds.

Nimbostratus Clouds: These are low clouds of dark colour, very close to the ground surface. They create dark-ness because the sunlight is obscured due to presence of its high thickness. These clouds increase the humidity in the atmosphere. Consequently, the precipitation starts soon. The rainfall in the subtropical cyclones is generally caused by these clouds.

Rain fall

• Rainfall occurs only when cloud droplets become so large due to coalescence that the air becomes unable to hold them. Based on the mode of origin, rainfall is classified into these type-

1. Convectional Rainfall

When the land surface is heated, the air in its contact rises up and in this way convectional currents are produced. When this warm and moist air reaches up to a certain height, it becomes saturated and forms dark Cumulo-Nimbus clouds, which cause heavy rainfall. This type of rainfall is termed as convectional rainfall and it occurs in the equatorial belt, especially in the zone of Doldrums.

2. Orographic or Relief Rainfall:

This type of rainfall occurs more frequently where hills or mountains are situated near and parallel to the coast. When warm and moist air is obstructed by any hill or plateau, it starts ascending along the slope of the hill or plateau and gets cooled. As a result it starts to get saturated and the process of condensation starts. The rainfall caused by this process is called orographic rainfall.

3. Cyclonic or Frontal Rainfall

Cyclonic rainfall is caused due to ascending of moist air and adiabatic cooling caused by convergence of two extensive air masses of entirely different physical properties (warm and cold air mass). In the temperate region, warm air is lifted upward along the front where the westerlies and polar winds meet, whereas cold air being heavier settles down. The warm air lying over cold air is cooled and gets saturated and condensation begins resulting into heavy rainfall.

Islands

Land masses surrounded by water from all the sides are called Islands. It may occur in oceans, seas, lakes or rivers, a group of islands is called archipelago. On the basis of their origin, Islands are classified into-

1. Tectonic Islands:

The islands are formed due to the subsidence of land, upliftment of land in sea areas formation of rift valleys, or segmentation of continental parts, all due to tectonic activities. Eg. West Indies.

2. Volcanic Islands:

Volcanic eruption gets deposited in the water body to such a height that it emerges out of the water surface and looks like Islands are called Volcanic Islands. E.g. Hawaiian Islands.

3. Erosion Islands:

Soft rocks are subjected to easy erosion, but certain hard portions remain found here and there. When these scattered positions of hard rocks are surrounded by water from all sides, Erosional islands are formed. E.g. Greenland.

4. Depostional Islands:

These islands are formed by the deposition of sediments brought by rivers, glaciers and sea waves on the surface.

5. Coral Islands:

The islands formed by the accumulation of coral polyps on the sub-marine platforms of the tropical oceans are called Coral islands. Example: Lakshadweep, Maldives etc.

Internal Structure of the Earth

Crust

The Earth's Crust the outermost solid cover or shell of the earth is known as the earth's crust.

The thickness of the crust is about 30-100 km.

• It is thicker in the region of the continents and thinner in the region of the ocean floors.
• The density of the rocks in the earth's crust ranges from 2.7 to 3 g/c.c (grams per cubic centimeter).
• The upper part of the crust consists of silica and aluminium in greater proportions. That is why, it is called 'SIAL'.

Mantle:

This layer lies below the crust. The lower part of the crust is called SIMA' because the proportion of sillica and magnesium is higher in this part.

• Its thickness is about 2900 km and the density of substances in the mantle ranges from 3.0 to 4.7.

Core

The earth's core lies below the mantle. Its thickness may be about 3,471 km.

• Its radius is 6,371-78 km., according to IUGG.
• It is divided into two parts-the outer core and the inner core. The outer core is probably in a liquid state and the inner core in a solid state.
• The core mainly consists of Nickel and Ferrous i.e. Iron Hence, it is called' NIFE'.
• After the mantle, the earth's density goes on increasing rapidly towards its centre and finally is more than 13.
• The temperature of the central part of the earth may be about 5000°C.
• The study of the earth's interior helps us to understand the original rocks in the earth's crust and their later transformation.

Rocks

• The solid parts of the earth's crust are called rocks. Most of the rocks are made up of two or more minerals.
• In the same type of rocks, the proportions of minerals may be different in different areas.
• Rocks may not always necessarily be hard.
• Minerals are obtained from rocks.
• Rocks are classified in three main types depending on the process of their formation:
  (a) Igneous, (b) Sedimentary, (c) Metamorphic.

Igneous rocks

• Hot lava pours out at the time of volcanic eruptions and cools down later on, forming rocks.
• The molten materials known as magma sometimes cool down beneath the earth's crust, again forming rocks.
• Both these types of rocks are known as Igneous rocks.
• When the earth's surface first became solid after it cooled down from its hot liquid state, the original rocks of the earth's crust were formed. They are the Primary Igneous rocks, which are usually not found today.
• Igneous rocks are generally harder and granular.
• There are no layers in Igneous rocks.
• Fossils are not found in Igneous rocks.
• The formation of Igneous rocks takes place beneath and above the surface of the earth.
• Rocks formed by the cooling of molten matter beneath the earth's surface are called intrusive Igneous rocks. 'Granite' and 'Gabbro' are the main examples of these rocks.
• The intrusive rocks are thus crystalline rocks.
• Sometimes, the molten matter oozes out through cracks in the earth's crust and spreads on the surface, forming extrusive Igneous rocks.
• Gabbro, Obsidian, Basalt etc. are examples of extrusive igneous rocks.
• A very large area of the Deccan Plateau consists of basalt rocks.
• These rocks contain silica from 40 to 80%, others are felspar, magnesium and iron etc.
• Other examples of Igneous rocks are- Granite, Pumic stone, Basalt and Gabbro.

Batholith:

Large body of igneous rock formed beneath the Earth's surface by the intrusion and solidification of magma. It is commonly composed of coarse-grained rocks (e.g., granite or granodiorite) with a surface exposure of 100 square km (40 square miles) or larger. A well-known batholith is located in the Sierra Nevada range of California, U.S. The" Idaho batholith" is a geologic formation that covers about 90% of the Boise National Forest. Massive granitic rocks of the Coast Range Batholith, North America is bigger than Idaho batholith.

Stocks:

Igneous intrusion shaped plutons are called either stocks or batholiths, depending on their sizes. Plutons larger than 100 square kilometres in area are termed batholiths, while those of lesser size are called stocks.

Laccolith

Laccolith, in geology, any of a type of igneous intrusion that has split apart two strata, resulting in a domelike structure; the floor of the structure is usually horizontal. A laccolith is often smaller than a stock usually is less than 16 km (10 miles) in diameter; the thickness of laccoliths ranges from hundreds of metres to a few thousand metres.

Lopolith:

Igneous intrusion associated with a structural basin, with contacts that are parallel to the bedding of the enclosing rocks. In an ideal example, the enclosing sediments above and below the lopolith dip inward from all sides toward the centre, so that the lopolith is concave upward.

Phacolith

A concordant lens-shaped pluton that typically occupies the crest of an anticline or through of a syncline.

Dike:

Also called dyke or geological dike, in geology, tabular or sheet-like igneous body that is often oriented vertically or steeply inclined to the bedding of pre-existing intruded rocks; similar bodies oriented parallel to the bedding of the enclosing rocks are called sills.

Sill: A relatively thin tabular concordant body intruded along bedding planes.

Sedimentary rocks

• They are formed by the deposition, sedimentation and lithification of sediments over a long period of time.
• As layers over layers get deposited, over a period of time, unified sedimentary rocks are formed on account of the tremendous pressure exerted by the layers above.
• Sometimes the remains of plants, dead animals etc are found in the deposited material. Such fossil containing sedimentary rocks are useful for studying life on earth. Sandstone, limestone, shale are some examples of sedimentary rocks.
• Limestone is white as well as black.
• Sandstone is dull white, pink, bright red or sometimes black.

Metamorphic rocks

• The nature of igneous and sedimentary rocks changes due to the effects of tremendous heat or pressure, and new transformed rocks, called metamorphic rocks, are formed.
• Minerals in the rocks get restructured on account of heat and pressure. This brings about a change in the original formation of the rocks.

Some examples of metamorphic rocks formed from igneous and sedimentary rocks: