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Chapter 6: Air Pressure and Winds

Chapter 6: Air Pressure and Winds. Atmospheric pressure Measuring air pressure Surface and upper-air charts Why the wind blows Surface winds Measuring and determining winds. Atmospheric Pressure. We already know that air pressure is the weight of the air above you

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Chapter 6: Air Pressure and Winds

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  1. Chapter 6: Air Pressure and Winds • Atmospheric pressure • Measuring air pressure • Surface and upper-air charts • Why the wind blows • Surface winds • Measuring and determining winds

  2. Atmospheric Pressure • We already know that air pressure is the weight of the air above you • We also know that if you move up in the atmosphere, pressure ALWAYS decreases • But how does it change in the horizontal? For that, we need a model

  3. Atmospheric Pressure • Here, we have a model that comes with several assumptions • Air molecules are NOT crowded close to the surface • Width of air column does NOT change with height • The air is unable to move in and out of the column • What happens when we force air into the column? Force air out of the column? How do we do this?

  4. Atmospheric Pressure • Now suppose we have two columns of air with the same pressure, same temperature, and same elevation • What happens when we cool one column and warm the other? What happens to the heights of the columns?

  5. Atmospheric Pressure • The cold column becomes more dense and the warm column becomes less dense • The air pressure remains the same/changes?

  6. Atmospheric Pressure • This means that a short column of air can exert the same pressure as a tall column of warm air • Warm air aloft/high pressure. Cold air aloft/low pressure

  7. Atmospheric Pressure • This pressure gradient causes a force (the pressure gradient force) to move air from high pressure to low pressure. • The pressure gradient force IS ALWAYS FROM HIGH PRESSURE TO LOW PRESSURE

  8. Atmospheric Pressure • So, if we allow air movement, air will leak out of column 2 and into column 1 • This creates what kind of pressure at the surface in each column?

  9. Atmospheric Pressure • So, we get a circulation where the air above a high pressure is sinking and air above a low pressure is rising • So, heating and cooling can cause differences in air pressure

  10. Stepped Art Fig. 6-2, p. 143

  11. Barometers • Barometer – an instrument that measures pressure changes • Mercury barometer

  12. Barometers • Aneroid barometer • Metal box called aneroid cell • Small changes in pressure cause cell to expand or contract • Different words on readings display weather types

  13. Pressure Readings • Station pressure – the reading from a barometer not corrected to sea-level • Sea-level pressure – the reading from a barometer corrected to sea-level • Adjustments are usually 10 mb / 100 m

  14. Pressure Readings

  15. Pressure Readings • Once all the readings are taken, you can create isobars – lines of constant pressure • Specifically, this is a surface map

  16. Surface and Upper Air Charts • Isobaric maps – shows height variations along constant pressure

  17. Surface and Upper Air Charts • Contour lines - lines that connect equal elevation above sea level • Isotherms – dashed red lines that show lines of equal temperature (cold air to the north/lower heights)

  18. Surface and Upper Air Charts • Ridges – elongated highs (warm air) • Troughs elongated lows (cold air) • Wind arrows blow parallel to lines in 500 mb map

  19. Why The Wind Blows • Newton’s first law? • An object at rest will remain at rest and an object in motion will remain in motion (constant velocity) as long as no force is exerted on the object • Newton’s second law? • Force exerted on an object equals its mass times the acceleration. Acceleration is change of speed OR DIRECTION • F = MA

  20. Forces that Influence the Wind • Wind is the net result of several forces. With many forces acting, remember it is the net we are looking for. • Pressure gradient force (PGF) • Coriolis force • Centripetal force • Friction

  21. Pressure Gradient Force • Pressure gradient • Difference in pressure / distance • Pressure gradient force – the force acting on air that is directed from high pressure to low pressure. The main reason for wind. • Strong PGF – sharp change in pressure with distance • Weak PGF – gentle change in pressure with distance

  22. Fig. 6-11, p. 151

  23. Coriolis Force • Coriolis force – an apparent force due to the rotation of the earth Coriolis Force Movie

  24. Coriolis Force • While the ball moves in a straight path, the merry-go-round rotates beneath it. By the time gets to the other side, the receiver has moved • It seems as though a force has acted on it, but it is an apparent force because an outside observer sees the ball go in a straight path • This occurs for all free-moving objects

  25. Coriolis Force • Cause wind to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

  26. Coriolis Force • Factors that affect the Coriolis Force (CF) • The higher the speed, the stronger the CF • The higher the latitude, the stronger the CF • CF is 0 at the equator and maximum at the poles

  27. Straight-line Flow Aloft • Example of PGF and CF in action • PGF always acts from High to Low. CF acts to the right of motion • Geostrophic wind • Wind that blows parallel to isobars in a straight path

  28. Curved Winds Around Lows and Highs Aloft • Cyclonic flow – wind that blows around a low • Anticyclonic flow – wind that blows around a high

  29. Curved Winds Around Lows and Highs Aloft • Centripetal force – inward directed force in a circular path • Gradient wind – wind that blows at a constant speed, parallel to curved isobars

  30. Curved Winds Around Lows and Highs Aloft • Thus, wind flows counterclockwise around a low and clockwise around a high in the Northern Hemisphere. Reverse for the Southern Hemisphere

  31. Winds on Upper-level Charts • Where temperatures change quickly, large height changes are noticed • Meridional – winds that blow north to south • Zonal – winds that blow west to east

  32. Stepped Art Fig. 6-19, p. 158

  33. Surface Winds • Planetary boundary layer (friction layer) • Upwards to 1000 m. • Friction – interaction with surface that decreases wind speed near the surface

  34. Surface Winds • Friction slows wind speeds, PGF becomes greater than CF, and wind flows inward to low pressure systems and outward from high pressure systems

  35. Winds and Vertical Motions • Convergence into surface lows causes rising air • Divergence from surface highs causes sinking air

  36. Winds and Vertical Motions • Hydrostatic equilibrium – the balance between the upward directed PGF and gravity

  37. The Influence of Prevailing Winds • How do you determine wind direction? • Wind is given by the direction it is coming from. Northerly wind blows from the north • Prevailing winds – wind most often blowing during a given time

  38. Wind Instruments • Wind vane – points into the wind • Anemometer – measures wind speed • Aerovane – gives wind speed and direction

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