# WIND: PART III - RESULTANT ATMOSPHERIC MOTIONS

## Summer 2004

### Today's Lecture Objectives:

• To explain how the various forces are joined in hydrostatic equilibrium, geostrophic wind, gradient wind and surface winds.
• To describe the hydrostatic equilibrium concept and its implications.
• To describe the formation of a sea/land breeze regime and discuss the principle it illustrates.
• To list the assumptions and properties of the geostrophic wind approximation.
• To compare and contrast geostrophic winds with the observed surface winds.
• To describe the effect of friction upon the speed and direction of the horizontal surface winds.
• To sketch a diagram (with isobars and wind arrows) showing the direction of the winds associated with cyclones and anticyclones in both the Northern and Southern Hemispheres.
• To explain the relationship between horizontal and vertical motions.
• To demonstrate Dines' Compensation Principle and to present examples of how the vertical motion of the air is linked to the horizontal motion of air.
• To explain why cyclones are typically cloudy and stormy, while anticyclones are commonly fair weather systems.

### Outline:

Series continues from THE THEORY OF WINDS: PART II - FUNDAMENTAL FORCES

#### A. INTRODUCTION

• Overview
• Fundamental assumptions

#### B. HORIZONTAL EQUATION OF ATMOSPHERIC MOTION

• Reviewing the Word Equation
• The Hydrostatic Balance Concept
• As a Result

#### C. FLOW RESPONDING TO PRESSURE GRADIENT FORCE - LOCAL WINDS

• Assumptions
• Examples
• Sea-Land Breeze Circulation
• Mountain-Valley Breeze Circulation
• City-Country Circulation

#### D. STRAIGHT-LINE, BALANCED, FRICTIONLESS MOTION - "GEOSTROPHIC FLOW"

• Definition
• Assumptions
• Concept of Geostrophic Balance
• Resultant Diagrams
• Implications of the Geostrophic Balance

#### E. BALANCED FLOW IN THE FRICTION LAYER

• The Nature of Friction
• The Friction Layer
• The Effect of Friction upon the Geostrophic Wind
• Assumptions
• Resultant Diagrams
• Variations of Near-Surface Winds with Height

#### F. CURVED, HORIZONTAL BALANCED MOTION - "GRADIENT FLOW"

• Assumptions
• Forces (accelerations) Associated with Curved Flow

#### G. GRADIENT FLOW WITH FRICTION

• Applicability to the Atmosphere
• Situation
• Resultant Diagrams

#### H. RELATIONSHIPS BETWEEN HORIZONTAL & VERTICAL MOTIONS

• Dilemma
• Convergence / Divergence
• Principle of Mass Continuity
• Dines' Compensation
• Resultant Vertical Motions
• Implications of Dines' Compensation

#### I. VORTICES AND VORTICITY

• Definitions
• Characteristic Vortex Features

The Forces & Wind module from Weather World 2010 (Univ. of Illinois-Urbana-Champaign).
See the mpg viewer of the geostrophic wind as well as the sea and land breeze sections.

Latest revision: 26 July 2004 (0330 UTC)

Produced by Edward J. Hopkins, Ph.D.
Department of Atmospheric and Oceanic Sciences