# Atmospheric Circulation and Weather Systems - Class 11

### Atmospheric Circulation and Weather Systems – Class 11

##### Question.What do you mean by atmospheric pressure?

Answer: (Atmospheric Circulation) Atmospheric pressure means the weight of air in a column of atmosphere from the mean sea level to the top of the atmospheric level. Atmospheric pressure is expressed with the help of the unit of millibar and pascals. The average atmospheric pressure is 1013.2 mb.

##### Question. What are the factors affecting the velocity and the direction of winds?

Answer: There are so many factors affecting the velocity and direction of winds. Which are as follows:

• Pressure gradient: Pressure gradient means the rate of change of pressure in respect to distance. That means the difference between two pressure zones (High and low) create a gradient or slope is known as the pressure gradient. If the air pressure differences between two pressure zones increase the wind velocity also increase.
• Frictional force: Frictional force includes the location of different objects on the earth surface resist the wind speed or the velocity of the wind. For example due to the presence of different objects like, buildings, plants etc. wind speed decrease in respect to the open area like ocean or desert area.
• Coriolis force: Due to the rotation of the earth Coriolis force originates. Because of this coriolis force wind direction deflects towards the right direction in the northern hemisphere and towards the left direction in the southern hemisphere. This deflection increases if the wind velocity is more.
##### Question. Explain in detail about the ‘Geostrophic wind’?

Answer: In about 2-3 km above the surface the wind is free from the different kind of frictional forces. Here wind velocity is mainly controlled by the pressure gradient and the Coriolis force. In this part the isobar become straight lines.

When the isobar becomes straight and there is no frictional force – here the pressure gradient also controlled by the Coriolis force. As a result wind flows parallel to the isobar. This type of wind is called as ‘geostrophic wind’.

##### Question. What do you mean by ITCZ or Inter Tropical Convergence Zone?

Answer: In the tropical region or near the equator forms low pressure centre. Because, this region gets more temperature throughout the year and due to this low pressure centre wind reaches from the both sub tropical region.

In this way two branch of air converge in the tropical region and forms a calm zone. Here north east and south east trade wind meets together. This calm condition zone is called as ‘doldrum’ or ITCZ (Inter Tropical Convergence Zone).

Answer: Due to the low pressure formation in the tropical region warm air moves upward. Then these air masses become cool and sink in the sub tropical area (in the tropic of cancer or tropic of Capricorn). This zone is developed in between 30 degree north and south of the globe. After that these sinking cool air masses horizontally moves towards the equatorial region from the subtropical area. In this way a circulatory wind system developed between the tropical and sub tropical zone. This zone is called as Hadley cell as this cell is discovered by George Hadley.

Ferrel cell: Like Hadley cell another circulatory wind system developed between the subtropical high pressure zones to sub-polar low pressure zone. This circulatory wind system is discovered by the William Ferrel.

In this zone comparatively cool air masses moves horizontally from the subtropical high pressure zone to sub-polar low pressure zone. Then this air masses moves upward from the sub-polar low pressure zone and sink to the sub-tropical high pressure area.

##### Question. What do you know about ‘El-Nino’ and ‘ENSO’?

Answer: Generally Peruvian current is cold and flows from south to north direction in the western part of South America. But, sometimes the mid-Pacific and warm current drift towards the South America. In this time the Peruvian current replace its direction and starts to flow from north to south as a warm current. This situation is called as ‘El-Nino’. This is very important for global atmospheric circulation.

Due to the presence of El-Nino the air pressure system between the Mid-Pacific Ocean and Australia. This phenomenon in the South-Pacific ocean area is known as ‘Southern Oscillation’.

Here the combine phenomenon of ‘El-Nino’ and ‘Southern Oscillation’ is known as ‘ENSO’. Read more..

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