Page 188 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

152 part I The energy–atmosphere System
(a) Pressure gradient
Top view and side view of air movement in an idealized high-pressure area and low-pressure area on a nonrotating Earth.
Isobar Isobar
Pressure gradient force
HL
High Low pressure pressure
(b) Pressure gradient + Coriolis forces (upper-level winds)
Earth’s rotation adds the Coriolis force, giving a “twist” to air movements. High-pressure and low-pressure areas develop a rotary motion, and wind flowing between highs and lows flows parallel to isobars.
Pressure gradient force alone
High Low
High Low
Descending, Ascending, diverging converging
Arrows for all parts:
Wind
Pressure gradient Coriolis force Friction force
Isobar
Friction
force LHL
Northern Hemisphere
High pressure
Isobar
Coriolis force
Isobar
Pressure gradient
force L Low
pressure
L
Northern Hemisphere
H
High pressure
Geostrophic wind
(c) Pressure gradient + Coriolis + friction forces (surface winds)
Surface friction adds a countering force to Coriolis, producing winds that spiral out of a high-pressure area and into a low-pressure area. Surface winds cross isobars at an angle. Air flows into low-pressure cyclones and turns to the left, because of deflection to the right.
Isobar
Low pressure
Surface Coriolis wind
force
▲Figure 6.8 Three physical forces that produce winds. Three physical forces interact to produce wind patterns at the surface and in the upper atmosphere: (a) pressure gradient force; (b) Coriolis force, which counters the pressure gradient force, producing a geostrophic wind flow in the upper atmosphere; and (c) friction force, which, combined with the other two forces, produces characteristic surface winds.
Animation
Wind Pattern Development
Pressure gradient + Coriolis forces
HL Anticyclone Cyclone
Northern Hemisphere
HL Anticyclone Cyclone
Southern Hemisphere
Pressure gradient + Coriolis + friction forces
High
Low
Pressure gradient force
Northern Hemisphere
High
Low
Southern Hemisphere
SIDE VIEW TOP VIEW