Air Pressure & Wind Essentials

Air Pressure Basics

  • Unequal surface heating → density changes → horizontal pressure differences.

  • Isobars: lines of equal station-level pressure (QFF); standard chart spacing 2 hPa.

  • Pressure (isobaric) surface: a level of constant pressure in the atmosphere.

Isobaric Patterns

  • Low / Cyclone: closed low-pressure centre; cyclonic flow; deepens when central pressure falls.

  • Trough: V-shaped extension from a low.

  • High / Anticyclone: closed high-pressure centre; anticyclonic flow; builds when central pressure rises.

  • Ridge (wedge): U-shaped extension from a high.

  • Col (saddle): neutral area between two highs & two lows; slack winds.

Horizontal Pressure Gradient

  • Gradient (ΔpΔn)\left(\frac{\Delta p}{\Delta n}\right) = pressure change per unit horizontal distance.

  • Closely spaced isobars → strong gradient → stronger winds; wide spacing → weak gradient.

  • Pressure-gradient force (PGF):

    • Acts perpendicular to isobars toward lower pressure.

    • Strength G=1ρΔpΔnG = \frac{1}{\rho} \frac{\Delta p}{\Delta n} (per unit mass).

Vertical Pressure Gradient (Lapse Rate)

  • Pressure decreases much faster with height than horizontally.

  • Variations arise from uneven warm/cold air distribution; warmer column ⇒ greater spacing between pressure surfaces.

Wind Generation

  • PGF accelerates air high → low pressure; wind speed proportional to isobaric slope.

  • Example: slope 1/3000 ≈ 50 KT at 60∘60^{\circ} latitude.

Local Circulations (Sea Breeze & Similar)

  • Daytime: warm land / cool sea → surface onshore flow (sea breeze); return flow aloft.

  • Same mechanism around individual convective clouds.

Large-Scale Circulation (Non-Rotating Earth)

  • Hadley-type cell: equatorial ascent, poleward flow aloft, polar subsidence, equatorward surface return.

Coriolis (Geostrophic) Force

  • Result of Earth’s rotation (anticlockwise viewed from N pole).

  • Moving air retains original eastward speed v=ωRv = \omega R; changing latitude changes underlying ground speed → apparent deflection.

    • Deflects right in Northern Hemisphere, left in Southern Hemisphere.

  • Acts perpendicular to motion; together with PGF leads to geostrophic balance aloft.