ENV201: Air Pressure Systems

Pressure Gradient Force (PGF)

• Definition: Differences in atmospheric pressure between two distinct regions create a pressure gradient force between those regions (Aguado and Burt, 2007).
• Conceptual Basis: The PGF exists specifically because of a horizontal pressure gradient (HPG) between two points (van Heerden and Hurry, 1985).
• Calculation: The horizontal pressure gradient is calculated using the following equation (adapted from van Heerden and Hurry, 1985):
  • $\text{Horizontal Pressure Gradient } (Pa/m) = \frac{\text{Pressure}_1 - \text{Pressure}_2\,(Pa)}{\text{Distance between two points}\,(m)}$

Characteristics of Wind

• Mechanism of Movement: The PGF is the force that causes the movement of air, which is commonly referred to as wind (Aguado and Burt, 2007).
• Direction of Flow: Air moves from regions of high pressure to regions of low pressure. This is often characterized as moving "down the gradient" or "downhill" (Aguado and Burt, 2007).
• Pressure Distribution: The unequal distribution of pressure on the Earth's surface establishes these gradients, which initiate air movement.
• Wind Speed Dynamics:
  • The pressure gradient is defined as the change in pressure per change in distance (Vasquez, 2003).
  • Wind speed generally increases as the pressure gradient increases.
  • The greater the PGF, the stronger the resulting wind (Aguado and Burt, 2007).
• Isobars and Wind Strength:
  • Isobar Definition: A line on a map connecting points having the same atmospheric pressure at a given time or on average over a given period.
  • Close Isobars: Indicate a larger HPG, a larger PGF, and stronger winds.
  • Widely Spaced Isobars: Indicate a smaller HPG, a smaller PGF, and weaker winds.

Five Forces Affecting Atmospheric Motion

While air moves from high to low pressure, winds do not always blow in straight lines because five distinct forces affect motion in the atmosphere:

1. Pressure Gradient Force
2. Coriolis Force
3. Centrifugal Force
4. Friction
5. Gravity

Coriolis Force

• Definition: The Coriolis force causes a deflection of the wind due to the Earth's rotation (Aguado and Burt, 2007).
• Operating Direction: The force acts at right angles ($90^\circ$) to the direction of air movement.
• Hemispheric Deflection:
  • In the Southern Hemisphere (SH), the Coriolis force causes a leftward deflection of a moving air parcel (Vasquez, 2003; Aguado and Burt, 2007).
• Factors Determining Magnitude:
  1. Latitude: The Coriolis force is $0$ at the equator and increases as latitude increases toward the poles (Vasquez, 2003).
  2. Velocity (Wind Speed): The magnitude of deflection is directly proportional to the velocity of the air parcel. As wind speed increases, the deflection increases.
• Coriolis Force Scenario Comparison: In a comparison between air moving at $5\,m/s$ at $10^\circ S$ versus $25\,m/s$ at $90^\circ S$, the smallest deflection would occur at the lowest speed and lowest latitude ($5\,m/s$ at $10^\circ S$).

Centrifugal Force

• Direction: This force always points away from the center of rotation (van Heerden and Hurry, 1985; Vasquez, 2003).
• Magnitude Decay: The force weakens as distance from the center of rotation increases.
• Orientation: It acts perpendicular (at right angles/$90^\circ$) to the direction of movement (Vasquez, 2003).
• Role in Air Flow: It specifically plays a role in atmospheric motion when isobars are curved (van Heerden and Hurry, 1985).

Geostrophic Wind

• Equilibrium: The geostrophic wind results from a balance between the Pressure Gradient Force (PGF) and the Coriolis Force (Vasquez, 2003).
• Assumptions: For the wind to follow geostrophic flow, the isobars must be assumed to be straight (Buckle, 1996).
• Buys Ballot's Law (Southern Hemisphere): The direction of the wind is parallel to the straight isobars, with the lower pressure located to the wind’s right (the opposite is true in the Northern Hemisphere).
• Life Cycle of Geostrophic Development (SH):
  1. Particle at Rest: Velocity is zero; therefore, Coriolis Force (CF) is $0$. Moving air is initially moved by PGF from high to low pressure.
  2. Initiation: As the air parcel begins moving, it is influenced by CF, which acts at $90^\circ$ to the left in the SH.
  3. Acceleration: As the parcel accelerates, CF increases, causing further leftward deflection.
  4. Balance: CF increases until its magnitude is equal and opposite to the PGF. The air parcel then moves parallel to the isobars.

Gradient Wind

• Curved Isobars: Gradient wind develops where isobars are curved (Buckle, 1996).
• Force Balance: The gradient wind is a result of a three-way balance between the PGF, the Coriolis force, and the Centrifugal force (CE) (Vasquez, 2003).
• Curvature Mechanism: Curvature in air flow arises because the air is moving along a curved path, necessitating the centrifugal force to maintain balance.
• Gradient Wind around High Pressure (SH):
  • Initial state: $CE + PGF > CF$, pulling the air parcel outward.
  • Adjustment: As CE reduces away from the center of rotation, CF becomes more dominant, pulling the parcel back to the original radius/path.
  • Final state: Wind blows parallel to curved isobars. According to Buys Ballot's Law (Low pressure on the right in SH), this results in anti-clockwise flow.
• Gradient Wind around Low Pressure (SH):
  • Force balance: $PGF$ is balanced by the sum of $\sum(CF + CE)$.
  • Final state: Results in clockwise wind flow parallel to curved isobars.

High Pressure Systems (Anticyclones)

• Structure: Enclosed regions of high pressure where the highest pressure is in the center (Aguado and Burt, 2007; van Heerden and Hurry, 1985).
• Circulation in SH: Anticyclonic circulation is anti-clockwise.
• Vertical Motion: Involves the downward sinking of the air mass.
• Mass Movement: Associated with upper-air convergence (air masses coming together) and surface divergence (air masses moving apart).

Low Pressure Systems (Cyclones)

• Structure: Enclosed regions of low pressure where the lowest pressure is in the center (Aguado and Burt, 2007; van Heerden and Hurry, 1985).
• Circulation in SH: Cyclonic circulation is clockwise.
• Vertical Motion: Consists of an upward-moving parcel of air.
• Mass Movement: Associated with surface convergence and upper-atmosphere divergence.

Troughs and Ridges

• Shape: Pressure cells are not always circular and often manifest as elongated areas (Aguado and Burt, 2007).
• Trough: An elongated area associated with a low pressure system.
• Ridge: An elongated area associated with a high pressure system.

Geopotential Height

• Definition: An approximation of the actual height of a pressure surface above mean sea-level. In weather mapping, lines of geopotential height connect points of equal height (in meters) for a constant pressure surface (contour lines).
• Utility: It is often more convenient than standard pressure maps in regions with large variations in altitude (van Heerden and Hurry, 1985).
• Relationship: Geopotential height identifies the altitude where a specific atmospheric pressure surface (e.g., $500\,hPa$) is found. The higher the altitude of a constant pressure surface above a specific point, the higher the surface pressure at that point.

Questions & Discussion

• Calculating Coriolis Magnitude: If asked which scenario provides the smallest deflection (Options: $5\,m/s$ at $10^\circ S$, $25\,m/s$ at $10^\circ S$, $5\,m/s$ at $90^\circ S$, or $25\,m/s$ at $90^\circ S$), the answer is $5\,m/s$ at $10^\circ S$ because Coriolis force is directly proportional to both velocity and latitude.
• Synoptic Map Analysis:
  • Question 4: Identifying atmospheric pressure on an isobar labeled 'X'.
  • Question 5: Determining the vector (arrow) describing the resulting direction of PGF (movement from high to low pressure).
  • Question 6: Determination of the resultant wind direction at point Y in the Southern Hemisphere given gradient wind development. The answer must account for Buys Ballot's Law (low pressure to the right) and the clockwise/anti-clockwise circulation rules.
  • Question 7: Identifying whether a feature is a ridge or trough based on the elongation and pressure values.
• Essay/Comparison Questions:
  • Question 8a: Explain the difference between geostrophic and gradient wind in the SH, focusing on the forces (PGF and CF for geostrophic; PGF, CF, and CE for gradient).
  • Question 8b: Discuss how high versus low pressure systems impact the development of these winds.