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Summary

Geography Class 23

## Last class revision(5.10 PM).

## Development of pressure belts(5.19 PM):

- Air rising at the equator due to high temperature causes expansion and therefore the development of a low-pressure belt at the equator.
- This connecting air at the equator spreads out as it reaches the top of the troposphere and moves toward the poles.
- This air as it converges towards the polar region its density increases and begins to sink which is subsiding, leading to a high-pressure belt at 30 degrees N and S.
- Some of the high-pressure air moves toward the equator and some toward the pole.
- The air moving towards the equator replaces the air rising there completing a cell.
- Low temperatures at the poles, resulting in a contraction of air and the development of high pressure.
- Air blowing away from poles spread out to larger space and pressure falls leading to a low-pressure belt along 60% north and south.
- Some of the air from the sub-tropical high-pressure belts moving toward the pole reaches 60 degrees north and south and converges with air from the poles leading to convection along 60 degrees north and south.
- The rising air 60 degrees north and south spread out at the edge of the troposphere, a part of it moves towards the equator and the rest towards the poles.

## Distribution of pressure across the world(5.41 PM):

- The distribution of pressure across the earth can be studied using isobars.
- Isobars are the lines joining places with equal pressure.
- With the apparent movement of the sun between the tropics, the pressure belts shift north and south.
- **==Pressure in January:==**
- The equatorial low-pressure belt extends into the southern hemisphere.
- The sub-tropical high-pressure belt of the southern hemisphere is found only over the oceans.
- The low temperature in the northern hemisphere produces a continuous high-pressure system, linking it with the high-pressure cells of Siberia and North America.
- **==Pressure in July:==**
- Equatorial low-pressure belts extend too much into the northern hemisphere due to intense heating in India and Tibet.
- Sub-tropical high-pressure belt in the northern hemisphere is not continuous and exists only over oceans as weak high-pressure cells.
- In the southern hemisphere, a continuous belt of high pressure is formed.

## Winds(6.10 PM):

- The wind is air in motion.
- ==Forces affecting the motion of wind:==
- ***1. Pressure grading force:***
- The rate of change of pressure concerning distance is called a pressure gradient.
- The pressure differential in the atmosphere causes the movement of air from high pressure to low pressure.
- The force causing this movement is called the pressure gradient force.
- Pressure gradient force acts in a direction perpendicular to isobars.
- Pressure gradient forces increase with the increase in the rate of change of pressure.
- That is pressure gradient force is more when isobars are placed closely.
- ***2. Frictional force:***
- Any moving object moving near land experiences a frictional force in the direction opposite to that of its movement.
- It is maximum near the surface than at higher altitudes.
- It is maximum over the continents than oceans.
- ***3. Coriolis force:***
- It is a fictional force produced due to the rotation of the earth
- It is the combined effects of various forces and factors such as centrifugal force, angular velocity, and variation in speed of rotation at different latitudes.
- It causes any moving object to defect towards the right in the northern hemisphere and the left in the southern hemisphere.
- The degree of Coriolis force depends on the speed of the moving body, the latitude at which the body is moving, speed of rotation of the earth.

## Geostrophic winds(7.17 PM):

- At some latitude where the isobar is straight and there is no friction, the pressure gradient force is balanced by Coriolis force and the result wind flows n a parallel direction of isobars. This is called geostrophic winds.
- Geostrophic winds along low-pressure and high-pressure cells result in the formation of cyclonic and anticyclonic circulations.
- |  |  |  |  |
  | --- | --- | --- | --- |
  | Pressure system | Pressure condition at the center | The pattern of winds in the north Hampshire | The pattern of winds in the South Hampshire |
  | Cyclone | Low | Anticlockwise | clockwise |
  | Anti-cyclone | High | clockwise | Anticlockwise |

## Planetary winds(7.37 PM):

- These are the winds blowing throughout the year and are found across the entire planet.
- ***Trade winds/Easterlies:***
- They blow from subtropical high to equatorial low.
- In German trade means Track as they blow in the same direction, throughout the year.
- Trade winds are dry and stable in the area of origin.
- Moving toward the equator picks up moisture and causes precipitation along the eastern margins.
- Reaching the western margins, as they are devoid of moisture the offshore trade winds will not result in precipitation causing desert formation.
- In addition to this nearness to the sub-tropical high-pressure belt and pressure belt and presence of cold ocean current creates dry conditions, eg Sahara desert, Kalahari deserts, etc.

## The topic of the next class: Planetary winds to continue.