PERPENDICULAR (90°)
ZERO AT EQUATOR
PARALLEL TO ISOBARS
Winds & Forces
Pressure Gradient · Coriolis Deflection · Frictional Drag · Geostrophic Balance · Cyclonic Circulation (28 Micro-Realms)
The Atmospheric Engine: Velocity and Direction of Wind (NCERT Chapter 10)
Air in horizontal motion is called Wind. The velocity and direction of wind are the net result of three interacting forces: 1) Pressure Gradient Force (PGF) which drives air from High to Low pressure perpendicular to isobars; 2) Coriolis Force (due to Earth's rotation) which deflects wind to the right in the Northern Hemisphere and left in the Southern Hemisphere, and is zero at the equator; and 3) Frictional Force, which drags surface winds and reduces Coriolis deflection, causing surface winds to cross isobars at an angle. In the upper atmosphere (free of friction), PGF perfectly balances Coriolis, creating Geostrophic Winds that flow parallel to straight isobars.
I. Definition of Wind & The Pressure Gradient Force (PGF)
What is Wind? (NCERT)
- Air in horizontal motion is called wind
- Transfers heat and moisture horizontally
- Vertical motion is called a "Current"
The Triad of Forces
- Velocity and direction are the net result
- Of these three interacting forces (NCERT)
- Plus gravity acting downward
1. Pressure Gradient Force
- The driving engine of all winds
- Differences in atmospheric pressure produce a force
- Directed from High to Low pressure (NCERT)
PGF Direction ($90^\circ$)
- PGF operates at right angles ($90^\circ$)
- To the isobars (NCERT)
- Always points directly from High to Low
Isobar Spacing & Velocity
- Rate of change of pressure with distance
- Close isobars = Steep gradient = Strong wind
- Wide isobars = Weak gradient = Gentle breeze
2. Coriolis Force Definition
ROTATION
- Rotation of Earth on its axis affects wind direction
- Named after French physicist G.G. Coriolis (1844)
- It is a deflective apparent force (NCERT)
Ferrel's Law of Deflection
- Deflects wind to the RIGHT in Northern Hemisphere
- Deflects wind to the LEFT in Southern Hemisphere
- Regardless of the wind's initial direction!
II. Coriolis Physics, Frictional Drag & Net Surface Winds (NCERT)
Coriolis Latitude Dependence
- Directly proportional to angle of latitude ($\sin \theta$)
- It is MAXIMUM at the poles
- And it is ZERO at the equator (NCERT verbatim)
Why No Equator Cyclones?
- At equator, Coriolis force is zero
- Wind blows perpendicular to isobars; fills low pressure immediately
- Hence tropical cyclones DO NOT form near equator!
Coriolis Velocity Dependence
- Coriolis force is directly proportional
- To the velocity of the wind
- Faster winds experience greater deflection
Direction of Coriolis Pull
- Coriolis force acts PERPENDICULAR ($90^\circ$)
- To the direction of the wind
- It only changes direction, never speed!
3. Frictional Force (Drag)
- Surface irregularities drag on moving air
- Greatest at the surface; extends to 1–3 km
- Called the Planetary Boundary Layer (PBL)
Oceans vs. Land Friction
SMOOTH / FAST
ROUGH / SLOW
- Over the oceans, friction is MINIMAL
- Winds blow much faster over water
- Rugged mountains exert maximum friction
Net Surface Wind Vector
- Friction slows wind $\to$ Coriolis weakens
- PGF overpowers weakened Coriolis
- Result: Surface winds CROSS isobars at an angle (10°–45°)
III. Geostrophic Balance, Gradient Winds & Cyclonic Circulation (NCERT)
The Upper Atmosphere
- Above 2–3 km, friction is negligible
- Wind is controlled ONLY by PGF and Coriolis
- Free Atmosphere (Geostrophic regime)
Geostrophic Wind (NCERT)
- When isobars are straight and no friction exists
- PGF is balanced perfectly by Coriolis force
- Resultant wind blows parallel to isobars!
The Perfect Balance
- PGF pulls wind towards Low Pressure
- Coriolis deflects it exactly 90° away
- They lock into a balanced opposing tug-of-war
Geostrophic Flow Profile
- The wind flows PARALLEL to the straight isobars
- This is known as the Geostrophic Wind (NCERT)
- Forms high-altitude Jet Streams
Gradient Wind (Curved Flow)
- When isobars are curved (Low/High cells)
- A third force enters: Centripetal Force
- Wind flows parallel to curved isobars
Cyclonic Circulation (Lows)
- Wind circulation around a Low Pressure centre
- Northern Hemisphere: Anti-clockwise
- Southern Hemisphere: Clockwise (NCERT Table 10.2)
Anticyclonic Circulation (Highs)
- Wind circulation around a High Pressure centre
- Northern Hemisphere: Clockwise
- Southern Hemisphere: Anti-clockwise (NCERT Table 10.2)
IV. Buys Ballot's Law, Convergence, Ascent & Wind Measurement
NH vs SH Rule-of-Thumb
ANTI-CLOCKCYCLONE SH:
CLOCKWISE
- Due to Coriolis deflection direction
- Rightward in NH creates counter-clockwise Low
- Leftward in SH creates clockwise Low
Buys Ballot's Law (1857)
- If you stand with your back to the wind in NH
- Low pressure will be on your LEFT
- High pressure will be on your RIGHT
Surface Wind Spirals
SPIRAL INHIGH:
SPIRAL OUT
- Because surface friction makes wind cross isobars:
- Lows feature converging inward spirals
- Highs feature diverging outward spirals
Convergence & Ascent (Lows)
- Surface convergence around a Low
- Forces air to rise vertically (Ascent)
- Rising air cools $\to$ condensation & rain storms!
Divergence & Subsidence (Highs)
- Surface divergence around a High
- Pulls air down from above (Subsidence)
- Sinking air warms $\to$ clear skies & dry weather!
Measuring Wind
- Velocity measured by Anemometer (Knots / kmph)
- Direction measured by Wind Vane
- Winds are named by direction they blow FROM
Master Wind Forces Matrix
- Completes Forces of Wind (NCERT Ch 10)
- Locks Geostrophic & Cyclonic flow rules
- Transitions to Realm 69 (General Circulation)
Core Distinctions
• Geostrophic Wind (Upper Air): When isobars are straight and there is no friction, the Pressure Gradient Force is perfectly balanced by the Coriolis force. The resultant wind blows parallel to the isobars.
• Surface Wind (0–3 km): Surface friction slows the wind down. Since Coriolis force is proportional to wind speed, a slower wind means less Coriolis deflection. The Pressure Gradient Force wins, causing the wind to cross the isobars at an angle towards the low pressure.
• Cyclonic (Low Pressure Center): Winds converge inward. Because Coriolis deflects to the right in the NH, the inward spiral turns Anti-clockwise. In the SH, it turns Clockwise.
• Anticyclonic (High Pressure Center): Winds diverge outward. Deflected right in the NH, the outward spiral turns Clockwise. In the SH, it turns Anti-clockwise.
The Coriolis force is directly proportional to the angle of latitude ($\sin \theta$). It is maximum at the poles and is zero at the equator. Without Coriolis force to deflect winds into a spinning vortex, air rushes straight into low-pressure centers and fills them immediately.
Wind Dynamics Matrix
NCERT Chapter 10 Reference Guide