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.