Go through these RBSE Class 11 Geography Notes Chapter 14 Airmasses, Front, Cyclone and Anticyclone contain important concepts so that students can score better marks in the exam.
Rajasthan Board RBSE Class 11 Geography Notes Chapter 14 Airmasses, Front, Cyclone and Anticyclone
- The broad and dense portion of the atmosphere is called the airmass, whose physical properties, especially the temperature and humidity are almost the same in horizontal extent.
- When atmospheric conditions are stable on a wide surface, the air contains the surface humidity and temperature-related properties, from which the origin of airmasses takes place.
- Airmasses flow forward without being stable after their origin.
- The regions from where airmasses are originate, called the Origin Areas.
- An atmospheric region of wide extent and similar nature, lesser wind speed and minimal divergence, and unvarying atmospheric conditions prevailing for a long time are the conditions essential for the origin of airmasses.
- Among areas with major origins of airmasses on the Earth, the polar sea region, sub-polar continental region, monsoon region, tropical ocean region, tropical continental region and equatorial regions are included.
Classification of Airmasses
- Airmasses are divided on the basis of the nature of their region of origin and on the basis of their displacement.
- On the basis of nature of the region of their origin, the airmasses are divided into tropical and polar airmasses.
- The tropical and polar airmasses are again divided into two sub-classes sea tropical, sea-polar and continental tropical and continental polar regions.
- The Conversion of the airmasses is divided into two parts: Thermodynamic Conversion and Mechanical Conversion.
- On the basis of thermodynamic conversion, the airmasses are divided into hot and cold, while on the basis of mechanical conversion, they are divided into stable and unstable.
- The word ‘Front’ was originally used during the World War.
- When two types of wind with opposite nature meet, they try continuously to retain their identities of temperature and humidity. In this process, a sloping boundary develops between them,which is called Front.
- According to Blair, “The surface or line by which the airmasses are separated is called front”.
- The origin of the fronts takes place due to the interaction of different types of airmasses, differences in their humidity and atmospheric circulation.
Types of Front
- The fronts are classified as warm front, cold front, stationary front, occluded front.
- When warm air replaces the cold air by advancing over it, cold front is formed, when cold air advances over warm air, warm front is formed, when the wind motion on both sides of the front is parallel to the front, stationary front is formed and when a cold airmass overtakes a warm airmass and goes underneath it, occluded front is formed.
- Cyclone refers to the center of low air pressure in which the winds move from the periphery to the center.
- In the northern hemisphere, cyclones move in an anti-clockwise direction while in the southern hemisphere, the cyclones move in clockwise direction.
- The shape of the cyclones is usually elliptical, circular or in the shape of letter ‘V’.
Types of Cyclone
- Cyclones are mainly divided into temperate cyclones and tropical cyclones.
- Temperate cyclones occur between 35° – 65° latitudes in both hemispheres. They move from west to east direction due to the effect of westerlies winds.
- Clouds originate from temperate cyclones, by which rainfall or snowfall takes place in favourable conditions.
- Temperate cyclones originate mainly on polar fronts. Their origin and development is more frequent and rapid in winter.
- In the northern hemisphere, temperate cyclones originate the most in the western coastal parts of the Pacific Ocean, the western edge of the North Atlantic and 60° latitude in the southern hemisphere.
- Temperate cyclones are divided into thermal cyclones, dynamic cyclones and migrant cyclones.
- These cyclones are found on both sides of the equator between Tropic of Cancer and Capricorn.
- Their speed ranges from 32 km. to 200 km. per hour and their diameter is 80 – 300 km.
- Tropical cyclones originate in the mid oceans in 8° to 15° North latitudes.
- They are also known as Hurricane, Typhoon, Cyclone and Willy-Willy.
- Tropical cyclones are divided into weak cyclones, furious (strong) cyclones, hurricanes, or typhoons and tornadoes.
- Local warming theory, dynamic theories and polar front theory have been developed for the formation of cyclones.
- The anticyclone is the center of high air pressure, in which the winds move from the center to the periphery.
- The first use of the word anticyclone was done in 1861 by Galton.
- The origin of the anticyclone is near 30° latitudes in both the hemispheres.
- Anticyclones are divided into cold anticyclone, warm anticyclone and blocking anticyclone.
- Cold anticyclones originate in arctic regions, warm anticyclones originate in the temperate high air pressure belt and blocking anticyclones in north ‘western Europe, and in the western part of the Atlantic Ocean and the Pacific Ocean.
- Cold anticyclones are again divided into temporary and transient anticyclones and semi-permanent anticyclones.
- In the upper layers of the mid-latitudinal troposphere, the winds blowing at very high speed near the tropopause are called the Jet Stream.
- These are narrow, spiral and rapidly-blowing strips of currents. Their speed is more than 120 km. per hour.
- Jet streams slide toward the poles in the summer and towards the equator in the winter.
- The jet stream is divided into two parts: Sub-Tropical Jet Stream and mid Latitudinal or Polor Front Jet Stream.
- The Sub-Tropical Jet Stream is found between 30°-35°latitudes in both the hemispheres. Their origin is due to the rotation of the earth.
- The equatorial airmasses are deflected to the right side in the northern hemisphere due to the Coriolis force and on the left side in the southern hemisphere. This is how the sub-Tropical Jet Stream is formed.
- The mid-Latitudinal Jet stream is originate between 40° to 60° latitudes in both the hemispheres.
Airmasses, Front, Cyclone and Anticyclone Notes Important Terms
→ Air Mass: Air Mass refers to the wide region of the atmosphere, in which similarity is found in the climate conditions of temperature and humidity in the horizontal plane at different heights.
→ Front: Any land border or air border formed above the ground, with the help of two opposite-natured air masses. Generally, when the cold and warm air masses come from opposite directions, then the mid-point of their surface of contact is the front.
→ Cyclone: A low air pressure region where the winds move from the outside towards the center. Generally, their shape is circular.
→ Anticyclone: Wind flow system related to high-pressure region located in the atmosphere. Their shape is often spherical, with the difference between atmospheric pressure of the center and the perimeter of the system, usually being 10 to 30 millibars.
→ Divergence: The action of moving forward from one point or source to different directions, in which the amount of dispersion increases along with increasing distance.
→ Thermodynamic Conversion: The change caused by the warming up and cooling down of the airmasses, which changes the nature and form of the airmasses.
→ Mechanical Conversion: The conversion, that occurs in the air mass, which is free from the heating and cooling provided by the surface, is called Mechanical Conversion.
→ Stationary Front: When two winds do not move even after being opposite in nature, they form a Stationary Front.
→ Occluded Front: When a cold front overtakes a warm front and moves beneath it due to its rapid speed, Occluded Front is formed.
→ Tropical Cyclone/Revolving Storm: Cyclone arising in the zone (tropical zone) located between Tropic of Cancer and Capricorn.
→ Mid latitudes: Latitude located between the Cancer circle and the Arctic circle and the Capricorn circle and the Antarctic circle. Usually latitudes ranging from 30° C to 60° are considered to be mid latitudes.
→ Air Turbulence: Irregular and interrupted flow or motion of air or water, whereby minor whirlpools/maelstrom arise.
→ Westerlies or west winds: Permanent or regular winds flowing to sub-polar low pressure belt from subtropical high pressure belt in both the northern and southern hemispheres, which flows from the south-west to north-east direction iii the northern hemisphere and from the north-west to south-east direction in the southern hemisphere.
→ Thermal cyclone: Cyclone generated from thermal difference. Centres of low air pressure are formed over land part in the temperate zone during summer season due to excessive heat, and on the relatively warmer sea part during winter season due to which cylones originate, which are called Thermal cylones.
→ Dynamic Cyclone: The temperate zone cyclone, whose origin takes place on the collision of cold polar airmass and relatively warmer sea airmass. This cyclone affects a large region.
→ Migrant Cyclone: Cyclone entering into another cyclone area leaving its area of origin.
→ Hurricane: Normally, the average wind speed greater than 120 km/h, whose strength is 12 on the Beaufort scale, is called Hurricane.
→ Typhoon: The tropical cyclone originating in the north-west Pacific Ocean and the China Sea, in which the winds blow violently from the outside to the center.
→ Cold Anticyclone: Anticyclones arising in the polar regions in the Arctic circle.
→ Warm Anticyclone: Anticyclones arising in the sub-tropical zone. They are often caused by radiation from the surface due to it being cold and due to divergence of the winds.
→ Blocking Anticyclone: Anti cyclone which originates in the tropospheric part of the atmosphere (mainly in 140° – 170° western longitude) due to obstruction taking place in the free flow of winds.
→ Pressure Gradient: Between two points on the land, the difference between the air pressure is called Pressure/Barometric gradient. It is displayed by the distance between the isobars on the weather chart or map.
→ Tropopause: The border present between the troposphere and the stratosphere in the atmosphere. Its thickness is about 1.5 km.
→ Sub-Tropical Jet Stream: Fast winds flowing in both the hemispheres near 30° – 35° latitudes near the troposphere.