WIND HYDROSTATIC RELATIONSHIP 700 mb 800 mb 900

WIND

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

HYDROSTATIC RELATIONSHIP Despite very strong vertical pressure gradients, vertical winds (rising and sinking air) are generally weak.

Pressure Gradient Force

Pressure Gradient Force 4 mb/100 km

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

Coriolis Force

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). 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. 210– 214)

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

Geostationary Satellite Same orbital period as Earth and in same direction

Polar-orbiting Satellites 700 -800 km elevation. Passes each location at same time of day

(pp. 135– 141)

GLOBAL WINDS (Chapter 10)

Names of Latitude Bands

Non-Rotating Homogeneous Planet surface wind follows PGF

Rotating Homogeneous Planet

Average Surface Pressure and Wind in January

Average Surface Pressure and Wind in July

Climate Features on a Weather Map

Columbus’ First Voyage (1492– 93) Mid-latitude Westerlies

Average Precipitation Variation by Latitude (Chap. 10, pp. 266– 274)

Quick Summary—Large-scale Winds (Chapter 10): • Large-scale prevailing winds vary by latitude about every 30 degrees. • The Hadley Cell is a very persistent feature associated with rising air at the equator, sinking air at 30°, and easterly trade winds at the surface from 0– 30° in each hemisphere. • The latitude zone from 30°– 60° has prevailing westerly winds. • Polar latitudes have highly variable winds that tend to be easterly, with cold, high pressure systems over the poles. • This simple, zonal (west–east) wind pattern is altered by the presence of coasts, continents, and mountain ranges.

CYCLONES AND ANTICYCLONES

Geostrophic Wind PH = PGF; C = COR

Geostrophic Wind

Geostrophic Wind As PGF increases, geostrophic wind speed increases.

(or low heights) Westerlies Aloft (or high heights) (or low heights)

Gradient Wind PGF = pressure gradient force CF = Coriolis force

Gradient Wind If PGF=COR, wind is not parallel to isobars If PGF>COR, gradient wind is parallel to isobars

Gradient Wind

Gradient Wind If PGF=COR, wind is not parallel to isobars If PGF<COR, gradient wind is parallel to isobars

Gradient Wind (Aside: “Net” = inward force minus outward force = centripetal acceleration)

Summary of Winds at Surface and Aloft (no friction) Geostrophic Gradient Surface (with friction) Gradient

Buys-Ballots Law If wind is at your back, low pressure is to your “left. ”

Upper Air Chart

Cyclones and Anticyclones Surface friction produces surface convergence and divergence.

Southern Hemisphere Winds at Surface and Aloft Cor Trough PGF Ridge

S. Hemisphere Surface Map

Cyclones Northern Hemisphere Southern Hemisphere (Chap. 8, pp. 214– 224)

Quick Summary—Cyclones and Anticyclones (Chap. 8): • Geostrophic Wind (straight isobars, no friction) • balance between PGF and COR • wind is parallel to isobars • Gradient Wind (curved isobars, no friction) • inward force > outward force • wind is parallel to isobars around a curved path • cyclonic flow is counter-clockwise in N. Hemisphere and clockwise in S. Hemisphere • Surface Wind (friction is present) • friction reduces wind speed • slower wind reduces COR • flow is across the isobars toward low pressure • more friction increases cross-isobaric flow