Transcript ppt
isobars and wind barbs sea level pressure factors affecting wind wind is the result of horizontal differences in pressure • air flows from higher to lower pressure • wind tends to balance inequalities in air pressure • since unequal heating of Earth’s surface generates pressure differences, solar radiation ultimately drives winds factors affecting wind (cont.) if Earth did not rotate, and if there was no friction, air would flow directly from high to low pressure pressure gradient force pressure gradient force to change velocity (accelerate) requires a force when air is subjected to greater pressure on one side than on another this imbalance produces a force directed from high to low pressure pressure differences cause the wind to blow the greater these differences, the greater the wind speed isobars - lines of equal pressure change of pressure over a given distance is pressure gradient closely spaced isobars --> high pressure gradient, strong winds widely spaced isobars --> low pressure gradient, weak winds Hurricane Andrew “Storm of the Century” coriolis force coriolis force coriolis force Figure 6.6 coriolis force planetary boundary layer factors affecting wind (cont.) on a rotating Earth with friction, wind is determined by a combination of forces 1. pressure gradient force 2. coriolis force 3. friction Figure 6.8 Figure 6.12 upper air flow is geostrophic balance between pressure gradient force and coriolis force ==> winds blow parallel to the isobars cyclonic and anticyclonic flow geostrophic wind balance between pressure gradient and coriolis force Figure 6.10 Figure 6.11 force balance at upper and lower levels surface high and low pressure systems Table 6.1 Table 6.1 Newton’s second law for horizontal and vertical motions (PGF denotes the pressure gradient force) Type of Motion Newton’s Second law Horizontal motions above the boundary layer Acceleration = Horizontal PGF + Coriolis Force Horizontal motions within the boundary layer Acceleration = Horizontal PGF + Coriolis Force + Friction Vertical motions Acceleration = Vertical PGF + Gravity Table 6.2 part 1 Table 6.2: Summary of the properties of forces acting on air in the Earth’s atmosphere Force Direction in which force acts Strength depends on Effect on air Balances Vertical Pressure Gradient Force Upward, from higher to lower pressure Magnitude of the vertical pressure gradient Accelerates air vertically toward lower pressure Hydrostatic balance when equal and opposite to gravitational force Horizontal Pressure gradient force Horizontally, from higher to lower pressure Magnitude of the horizontal pressure gradient Accelerates air horizontally Geostrophic balance when equal and opposite to Coriolis force Coriolis Force To the right (left) of the wind direction in the Northern (Southern) Hemisphere Wind speed and latitude Affects wind direction, but no effect on wind speed Geostrophic balance when equal and opposite to horizontal pressure gradient force Table 6.2 part 2 Table 6.2 continued Summary of the properties of forces acting on air in the Earth’s atmosphere Force Frictional force Gravitational force Direction in which force acts Strength depends on Effect on air Opposite the direction of the flow The roughness of the underlying surface Reduces air velocity, important primarily in boundary layer Toward the center of the earth Essentially constant in the troposphere Accelerates air downward Balances _________ Hydrostatic balance when equal and opposite to vertical pressure gradient force. how winds generate vertical air motion vertical motion associated with a cyclone • winds spiral into a surface low (horizontal convergence) • compensation occurs aloft (horizontal divergence) • net upward movement vertical air motion is: • small relative to horizontal • very important weather maker • rising air associated with clouds and rain • sinking air (subsidence) -> adiabatic heating, clear skies convergence and divergence