WIND Pressure Gradient Force PGF Causes air to

WIND

Pressure Gradient Force (PGF) Causes air to move from high pressure toward low pressure Three characteristics of the PGF: 1) directed from H to L 2) perpendicular to isobars 3) magnitude is proportional to isobar spacing (i. e. , tight spacing = stronger PGF)

Pressure Gradient Force (PGF) 08 12 04 16 00 H L 12 08 1) directed from H to L 2) perpendicular to isobars 3) magnitude is proportional to isobar spacing (i. e. , tight spacing = stronger pgf) density horizontal distance 04 In this example, isobars are evenly spaced so PGF has the same strength everywhere.

Pressure Gradient Force

Pressure Gradient Force 4 mb/100 km

PGF = Pressure Gradient Force close isobar spacing, strong PGF farther spacing, weak PGF

Change of Air Pressure and Density with Height

HYDROSTATIC RELATIONSHIP 700 mb 800 mb 900 mb Upward pressure gradient force (PGF) is balanced by gravity (g).

HYDROSTATIC RELATIONSHIP gravity vertical distance Despite very strong vertical pressure gradients, vertical winds (rising and sinking air) are generally weak. The vertical PGF is NOT responsible for vertical winds.

Coriolis Force (COR) (viewed from above) turntable target dart gun Time = 1 (now) Dart misses target as target has rotated out of position Time = 2 (15 sec. later)

Coriolis Force (COR) (as viewed while riding on turntable) turntable A mysterious force caused the dart to veer to the right of the target dart gun Definition: force applied to a body in motion that appears to cause it to turn to its right in the Northern Hemisphere (or to its left in the Southern Hemisphere).

Coriolis Force Simple Coriolis demonstration: https: //www. youtube. com/watch? v=dt_XJp 77 -mk&app=desktop

Coriolis Force (COR) • Not a real force—arises from our frame of reference as we travel on a planet that is constantly rotating • Deflection by COR (right or left) is greater for a) faster rotation, and b) greater distance travelled (higher speed) constants wind speed latitude @ equator, sin 0°= 0, so COR = 0 @ poles, sin 90°= 1, so COR = maximum

Coriolis Force: Equatorward-moving Air

Coriolis Force: Eastward-moving Air

Coriolis Force In the Northern Hemisphere, the Coriolis force causes an air parcel to deflect TO ITS RIGHT (to the right of its direction of motion), regardless of the direction of motion. This deflection increases toward the North Pole.

Variation of Coriolis Force with Speed and Latitude Stronger wind speed = stronger Coriolis force

Coriolis Force: Poleward-Moving Air

REVIEW OF KEY FORCES Pressure gradient force (PGF) • High to Low • Perpendicular to the isobars • Proportional to isobar spacing Coriolis force (COR) • Acts to right of parcel direction in N. Hemisphere (and to left in S. Hemisphere) • Maximum at poles, zero at equator • Proportional to wind speed (Chap. 8, pp. 209– 213)

EVSC 3300 Atmosphere and Weather (3 credits) MWF, 10– 10: 50 a. m. EVSC 3301 (1 credit lab. optional for nonmajors) Prerequisite: one semester of calculus