Atmosphere
Download as PPTX, PDF1 like201 views
The document discusses pressure gradient force, Coriolis force, and geostrophic wind. It provides the following key points: 1) Pressure gradient force moves air from high to low pressure areas and its magnitude depends on the spacing of isobars. 2) Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere due to Earth's rotation. Its strength depends on latitude and wind speed. 3) Geostrophic wind occurs when pressure gradient force exactly balances Coriolis force, causing winds to blow parallel to isobars at constant speed with no acceleration.
Atmosphere
- 1. PRESSURE GRADIENT FORCE, CORIOLIS FORCE AND GEOSTROPHIC WIND
- 3. PRESSURE • Pressure is defined as the force per unit area. It is proportional to the the rate of collision between the molecules and the wall. • Increased pressure can result from a) Increased density ,b) Increased temperature. • Air constantly moves from high to low pressure.
- 4. IN THE DIAGRAM BELOW, THE PRESSURE AT POINT "X" INCREASES AS THE WEIGHT OF THE AIR ABOVE IT INCREASES. THE SAME CAN BE SAID ABOUT DECREASING PRESSURE, WHERE THE PRESSURE AT POINT "X" DECREASES IF THE WEIGHT OF THE AIR ABOVE IT ALSO DECREASES. Thinking in terms of air molecules, if the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure is also referred to as barometric pressure.
- 5. PRESSURE GRADIENT • The variation of heating from one locality to another is the initial factor that produces movement of air or wind. The most direct path from high to low pressure is the path along which the pressure is changing rapidly. The rate of change is called pressure gradient. • Variation of air pressure over earth’s surface are determined from barometric readings at hundred of weather stations. Which is in the surface weather map is called “isobars”. • The spacing of isobars indicate the amount of pressure change occurring at a given distance and is expressed as “pressure gradient”.
- 6. PRESSURE GRADIENT • So the pressure gradient can be defined ass a change in pressure over a given distance i.e. PRESSUREGRADIENT=∆P/distance = Phigh-Plow/distance The magnitude of pressure gradient can be addressed by nothing but the spacing of the isobars- -if the isobars are close together the pressure gradient is large. -if the isobars are far apart the reassure gradient is small.
- 7. HORIZONTAL PRESSURE GRADIENT • when surface under one air column is heated: o air column expands, following P = ρ Rd T , example: height at which 500 mb is reached: 5500 m, say (before heating)
- 8. HORIZONTAL PRESSURE GRADIENT after heating and expansion: height at which 500 mb pressure is reached is now higher up, at 5600 m, say (this is like a hill of air above the heated spot) at 5500 m the pressure is now less than 500 mb.
- 9. HORIZONTAL PRESSURE GRADIENT • Gradual poleward decrease in mean temperature . • Denser air at higher latitude. • More rapid decrease of pressure with height .
- 10. HORIZONTAL PRESSURE GRADIENT • The horizontal pressure gradient is the driving force of wind. • with a horizontal pressure gradient created in this way, air can start to flow from higher pressure to lower pressure. • It has both magnitude and direction. • Its magnitude is determined from the spacing of the isobars. And the direction of force is always from higher to lower pressure. • At the right angels to the isobars. Give rise to pressure gradient force.
- 11. VERTICAL PRESSURE GRADIENT • The vertical pressure gradient is usually in, or near , balance with gravity. • For the vertical pressure gradient the upward and downward flow in the Atmosphere is comparatively slow.(with the exception of localized updrafts and downdrafts)
- 12. WHY THE AIR IS NOT ESCAPE INTO SPACE? • The airflow is from areas of higher pressure to lower pressure. Air pressure is highest near earth surface and get progressively lower as we move upward. but the air not accelerate and escape to space because due to gravity. Which acts the opposite direction to the vertical pressure gradient. The important balance that is usually maintained between these two opposite forces is called HYDROSTATIC EQULIBRIUM.
- 14. PRESSURE GRADIENT FORCE • Pressure gradient force is the force that moves air from an area of high pressure to an area of low pressure. The velocity of the wind depends upon the pressure gradient. If the pressure gradient is strong , the wind speed is high. If the pressure gradient is weak, the wind speed is light.
- 16. CORIOLIS FORCE
- 17. CORIOLIS FORCE • The Coriolis force is an inertial force (also called a fictitious force) that acts on objects that are in motion relative to a rotating reference frame. • In a reference frame with clockwise rotation, the force acts to the left of the motion of the object. In one with anticlockwise rotation, the force acts to the right. • Though recognized previously by others, the mathematical expression for the Coriolis force appeared in an 1835 paper by French scientist Gaspard- Gustave de Coriolis, in connection with the theory of water wheels. • Early in the 19th century, the term Coriolis force began to be used in connection with meteorology. Deflection of an object due to the Coriolis force is called the 'Coriolis effect'.
- 18. • The weather map consists of high and low pressure system.as expected, the air moves from high to low pressure but not cross the isobars at right angles as the pressure gradient force directs. This deviation is the result of earth’s rotation which is called as Coriolis force. • It is essentially an extension of the centrifugal force in case of an object or air. All the air on rotating earth feels a centrifugal force that is perpendicular to earth rotation axis. But if the air is also moving so that it changes either the total speed or the distance from axis or both, then the centrifugal force is change as it is equal to V2/R . This change of the total centrifugal force and appears as a new force to the observer that deflect motion horizontally , perpendicular to motion . This deflecting force is called Coriolis force •
- 19. WHICH WAY DOES THE CORIOLIS FORCE DEFLECT MOTION? • The Coriolis force deflects movements to the right angle of the direction of the movement. The Coriolis Force affects only wind direction, not wind speed • In northern hemisphere it deflect movement to right and in southern hemisphere this rule is reverse because the earth’s rotation is clockwise as viewed at south pole. • The magnitude of Coriolis force per unit mass is proportional to the latitude and the speed of movement of air parcel. This can be written mathematically as follows: CF= - +fV • where V is the wind speed ,f is the Coriolis parameter which is written as follows • f=2*Earth’s rotation rate(ω)*the sin of the latitude(sinф)
- 21. THE CORIOLIS FORCE IS ZERO IN EQUATOR AND HIGHEST AT POLE -WHY? • As we moves towards pole, angular momentum must be conserved due to radius on axis of rotation is decreasing. So that we have to increase our tangential velocity to keep angular momentum constant. As we know faster the speed , the larger the Coriolis force. • The Coriolis parameter is given by f=2*Earth’s rotation rate(ω)*the sin of the latitude(sinф) • Where ω=rotation of earth & ф= latitude. By applying this we see f is greatest when sinф=1 which means sin 90°=1 or ф=90°.
- 22. GEOSTROPHIC WIND
- 23. GEOSTROPHIC WIND • A nonmoving parcel of air has no starting point to move. The Coriolis force is not work on it. Underneath the influence the pressure gradient force which is always directed perpendicularly to the isobars, the air parcel begun to accelerate directly toward the area of low pressure. • As some as the flow begins the Coriolis force comes into play and causes a deflection to the right for winds in the NH. • As parcel continuous to accelerate the Coriolis force intensified. Thus the increased speed results further deflection. • The wind turn and following parallel to the isobars.
- 24. • When this occur the pressure gradient force is balanced by opposing Coriolis force. • As long as the force remain balance the resulting wind continue to flow parallel to the isobars at a constant speed.
- 25. • •Geostrophic motion occurs when there is an exact balance between the PGF and the Co, and the air is moving under the the action of these two forces only. • It implies 1) No acceleration •e.g., Straight, parallel isobar 2) No other forces •e.g., friction 3)No vertical motion •e.g., no convergence
- 26. GEOSTROPHIC WIND
- 27. • Mathematically, let's see how Vg is calculated. If we equate the PGF and Coriolis force, then we can solve for Vg, which in this case replaces the actual wind (V), previously used in the Coriolis equation. • Vg is simply the pressure gradient force divided by the Coriolis parameter. • So,
- 28. Thank you… PARTHA PRATIM KANDAR SWAGATA CHAKRABORTY