Geography 131 Notes – Wind, Friction, and Coriolis: Surface and Aloft Winds

Pressure Gradients, Isobars, and Vertical Features

  • Geography 131: Weather, Climate, and Climate Change – Coriolis and Wind

  • Focus: how friction, Coriolis effect, and pressure systems shape surface and aloft winds

  • Key ideas: PGF (pressure gradient force) drives motion; friction slows surface winds; Coriolis redirects flow; flow around highs and lows varies with height; geostrophic winds occur aloft when friction is small; jet stream is a geostrophic feature around ~300 mb; vertical motion is linked to convergence/divergence near highs/lows

  • Isobars are lines of equal pressure, expressed in millibars (mb)

  • Pressure gradient force (PGF) accelerates air from high to low pressure; stronger PGF when isobars are packed tightly

  • Vertical features of pressure systems include: tropopause, highs (H), lows (L), and L' (a secondary low), with convergence at lows and divergence at highs

  • Tropopause marks the boundary between the troposphere and stratosphere; vertical structure of pressure systems interacts with this boundary

  • Key equation (conceptual):
    \mathbf{PGF} = -\frac{1}{\rho}\nabla P

  • The horizontal wind tendency is governed by the balance (in different layers) of PGF, friction, and Coriolis; direction and strength depend on height and latitude

Surface Winds: Friction, PGF, and Coriolis

  • Surface wind results from the sum of three forces: PGF + friction + Coriolis
  • Friction acts in the boundary layer and slows surface winds; example: a friction layer about ~1 km thick reduces wind speed near the surface
  • There is a distinct boundary layer (<2 km) where friction is important; aloft, friction is absent or negligible
  • Friction also alters the direction by reducing the Coriolis deflection near the surface; winds cross isobars within the surface layer due to friction
  • Above the friction layer (roughly >1–2 km), Coriolis is stronger, and winds tend to blow parallel to isobars (geostrophic winds)
  • Strength of Coriolis varies with height: deflection increases with height; stronger for faster winds
  • Geostrophic winds begin to dominate around the 500 mb level; jet stream is a geostrophic feature around ~300 mb
  • Summary of layer behavior:
    • Surface layer (<2 km): PGF + friction + Coriolis; winds cross isobars; deflection to the right in the Northern Hemisphere (NH) and to the left in the Southern Hemisphere (SH)
    • Above the friction layer (≈1–2 km and higher): friction weakens, Coriolis strengthens; winds align parallel to isobars (geostrophic balance)
  • Practical note: wind speed is stronger where isobars are packed tightly (steeper PGF)

The Coriolis Effect: Mechanism, Magnitude, and Height Dependence

  • Coriolis is an apparent force arising from Earth’s rotation; it deflects moving air but does not initiate motion on its own
  • It acts on existing motion and deflects it to the right in the NH and to the left in the SH
  • It is not a “real” force in the Newtonian sense, but it is essential for understanding wind directions at large scales
  • Mechanism summary (as described in lecture):
    • Earth rotates once per day; the equator travels about 25,000 miles per day, while the poles have far less linear speed
    • This variation in linear velocity with latitude causes moving air to turn as it moves (Coriolis deflection)
  • Magnitude and variability:
    • The strength of Coriolis varies with latitude (stronger toward the poles, zero at the equator)
    • Larger-scale motions feel stronger Coriolis effects; faster winds feel a stronger deflection
    • Effect is greater for larger systems and faster winds
  • Height dependence:
    • Coriolis is weaker near the surface due to friction; stronger aloft where friction is minimal
    • Deflection increases with height; higher up, winds bend more toward geostrophic balance
  • Key relationships:
    • Coriolis acts on motion, not on the initiation of motion
    • In NH: moving air turns to the right; in SH: moving air turns to the left
    • At equator, Coriolis effect is zero; at poles, it is strongest

Flow Around Highs and Lows: Convergence, Divergence, and Vertical Motion

  • PGF around high pressure (H) and low pressure (L):
    • High pressure: divergence (air sinks)
    • Low pressure: convergence (air rises)
    • Vertical motion is driven by the need to fill voids (around highs) and by air colliding or rising (around lows)
  • Northern Hemisphere (NH) circulation around H and L (aloft and near surface):
    • Around H: winds turn to the right as they leave the high; clockwise rotation; anticyclonic; divergence; sinking air
    • Approaching L: winds turn to the right as they approach the low; counterclockwise rotation; cyclonic; convergence; rising air
  • Southern Hemisphere (SH) exhibits opposite rotations around H and L
  • Surface wind pattern (NH) from high to low pressure:
    • Surface winds generally blow from H to L
    • They are slowed by friction and turn to the right in the NH
    • These patterns are not identical throughout the entire atmosphere due to friction and vertical layering
  • Aloft vs surface flow:
    • Near surface: friction causes cross-isobar flow and turns wind to the right in NH (left in SH)
    • Aloft: friction is reduced or absent; Coriolis is stronger; flow tends to be parallel to isobars (geostrophic flow)

Geostrophic Winds: Aloft, Isohypses, and the Jet Stream

  • Geostrophic winds arise when PGF is balanced by the Coriolis force (no friction) and flow becomes parallel to isobars
  • Geostrophic wind behavior:
    • Above the friction layer, winds blow parallel to isobars (isoheights on pressure maps)
    • The jet stream is considered geostrophic and occurs around ~300 mb (a higher, fast-moving air stream)
  • Height markers:
    • Geostrophic winds begin at roughly 500 mb
    • Boundary layer is typically <2 km where friction is significant
    • The jet stream is located around ~300 mb
  • Isobaric/isohypsic map features:
    • Isohypses and wind barbs illustrate how winds align with pressure patterns aloft
    • In the map, winds near the jet stream show strong, fast geostrophic flow
  • Summary of flow aloft vs at the surface:
    • Aloft: PGF + Coriolis only (no friction) → flow parallel to isobars; geostrophic
    • Surface: PGF + Coriolis + friction → flow crosses isobars; deflection by Coriolis; speed influenced by isobar spacing

Quick Synthesis: Surface vs Aloft Wind Patterns

  • Surface winds:
    • Deflected by friction and Coriolis; cross isobars due to friction
    • In NH, turn right relative to the pressure gradient and pressure contours
    • In SH, turn left relative to the pressure gradient and pressure contours
  • Aloft (above surface friction layer):
    • Friction negligible; Coriolis strong; wind follows isobars (geostrophic)
    • Winds parallel to isobars; flow shows a more organized, parallel-to-isobar pattern
  • Strength and tightness of isobars:
    • Tighter isobars -> steeper PGF -> stronger winds
  • Height and latitude considerations:
    • Coriolis strength increases with latitude and wind speed; strongest at high latitudes
    • Equator: little-to-no Coriolis effect; poles: strongest

Key Questions and Learning Targets

  • Why do winds curve instead of blowing directly from high to low pressure?
    • Because near-surface winds are shaped by the balance of PGF, friction, and Coriolis; friction causes cross-isobar flow and deflection, while Coriolis turns the flow to the right (NH) or left (SH);
    • Aloft, friction is reduced; Coriolis dominates, causing geostrophic flow parallel to isobars
  • What causes geostrophic wind, and why does it occur aloft?
    • Geostrophic wind results from a balance between the horizontal pressure gradient force and the Coriolis force; it occurs aloft where friction is negligible and the balance can be maintained over longer time scales; near the surface friction disrupts this balance, leading to cross-isobar flow

What to Know from This Lecture (Concise Points)

  • How friction influences wind at the surface and aloft
  • How the Coriolis effect influences wind at the surface and aloft
  • Hemisphere differences: NH vs SH, equator vs poles, and vertical variation with height
  • Wind patterns around high- and low-pressure systems at the surface vs aloft
  • Key terms: Coriolis effect, geostrophic wind
  • Important height references:
    • Boundary layer: <2 km (friction important here)
    • Friction layer: ~1 km (explicitly slows surface winds)
    • Geostrophic winds: begin around ~500 mb; notable at ~300 mb jet stream
  • Important map terms:
    • Isobars (pressure lines in mb)
    • Isohypses and wind barbs (aloft)
  • Practical implications: understanding wind patterns supports weather forecasting, aviation, and climate studies

References to the Transcript Content (Conceptual Anchors)

  • PGF drives motion from high to low pressure; stronger PGF with tightly packed isobars
  • Friction slows surface winds and reduces the Coriolis deflection, causing cross-isobar flow near the surface
  • The Coriolis effect deflects moving air to the right in NH and to the left in SH; it is zero at the equator and strongest at the poles; it is an apparent force arising from Earth’s rotation
  • The balance among PGF, friction, and Coriolis changes with height: surface (friction present) vs aloft (friction negligible)
  • Lows are regions of convergence with rising air; highs are regions of divergence with sinking air
  • Aloft, winds tend to become geostrophic and flow parallel to isobars; at the surface, winds cross isobars due to friction
  • The jet stream is a prominent geostrophic feature around ~300 mb; geostrophic winds can be seen in maps with isoheights and wind barbs
  • The vertical structure of wind and pressure systems links to weather patterns and their evolution across scales

Quick Reference Formulas (LaTeX)

  • Pressure gradient force (per unit mass):
    \mathbf{PGF} = -\frac{1}{\rho}\nabla P
  • Geostrophic wind (balance between PGF and Coriolis):
    \mathbf{u}_g = -\frac{1}{f\rho}\hat{\mathbf{z}}\times\nabla P = \frac{1}{f\rho}\left(\frac{\partial P}{\partial y}, -\frac{\partial P}{\partial x}\right)
  • Coriolis parameter (depends on latitude):
    f = 2\Omega\sin\phi
  • Typical height references:
    • Boundary layer: < 2 km
    • Friction layer: ~1 km
    • Geostrophic onset: ~500 mb
    • Jet stream region: ~300 mb
  • Directional deflection rule (hemispheric):
    • NH: deflection to the right of motion
    • SH: deflection to the left of motion