Atmospheric Pressure, Wind Generation, and Deflection Forces

Introduction to Atmospheric Pressure and Density

• Conceptual Overview: Wind is primarily driven by principles of atmospheric pressure and density. Understanding these relationships is essential to comprehending why wind occurs and how it is diverted from its original trajectory.

• The Relationship Between Pressure and Density:

  • Vertical Distribution: Pressure and density in the atmosphere share a direct, proportional relationship. They generally decrease as altitude increases.
  • Graphical Representation: On a standard meteorological chart:
    • Y-Axis: Represents altitude or elevation.
    • X-Axis: Represents the magnitude of air pressure or density (increasing from left to right).
    • Pressure Line: Often colored red, showing high pressure at the surface and decreasing aloft.
    • Density Line: Often colored blue, mimicking the pressure line precisely.

• Molecular Concentration:

  • At sea level, there is the highest density of molecules (gases). As one moves upward, the air thins out rapidly.
  • Mass and Weight: Atmospheric pressure is defined by the weight of the "stuff" (air molecules) above a specific point. At sea level, the entire column of the atmosphere is overhead, resulting in the highest possible atmospheric pressure regardless of geographic location.
  • Mid-Atmosphere Analysis: At high altitudes (e.g., halfway through the atmosphere), the majority of atmospheric gases are below the observer. Consequently, there is significantly less mass above, resulting in lower atmospheric pressure.

Mapping Atmospheric Pressure

• Units of Measurement: Atmospheric pressure is documented in millibars ($mb$).

• Surface Pressure Variations: While the highest pressures are found at the Earth's surface, these values vary across the surface due to differences in atmospheric temperature.

  • Temperature Effects: Temperature instigates the rising and falling of air.
    • Cold Air: Sinks and "lands" on the surface, exerting more pressure (analogous to dropping a heavy stack of books on the floor).
    • Warm Air: Rises, exerting less pressure on the surface.

• Isopleths and Isobars:

  • Isopleth: A general term for lines on a map connecting points of equal value for a given phenomenon.
  • Isobar: A specific type of isopleth used for air pressure.
  • Verbatim Definition: Isobars are "lines of equal pressure."

• Constructing Isobar Maps:

  • Intervals: Maps are typically drawn with a standard interval of every $4\,mb$ of pressure (e.g., $1,016\,mb$, $1,020\,mb$, $1,024\,mb$).
  • Function: These lines simplify complex data, allowing scientists to easily identify pressure gradients. Perpendicular movement away from an isobar represents the greatest change in pressure over a specific distance.

The Pressure Gradient Force (PGF)

• The Generator of Wind: The Pressure Gradient Force (PGF) is the fundamental force that causes wind to blow. Wind cannot be generated by any other factor.

• Directional Rule: The PGF is always directed from high pressure to low pressure.

• Relationship to Isobars: The PGF is always perpendicular ($90^\circ$) to the isobars. This direction represents the path of the greatest change in pressure.

Wind Direction and Representation

• Wind Symbols: Meteorological maps use specific symbols to indicate wind behavior.

  • Shaft and Dot: The wind blows down the shaft toward the dot/circle at the end.
  • Flags/Markings: Attached to the end of the shaft to indicate wind speed (though detailed symbol reading is often reserved for advanced science courses).

• Observed Deviation: In reality, wind does not blow perfectly from high to low pressure along the PGF. It is redirected by the Earth's rotation and friction.

The Coriolis Force

• Definition: The Coriolis force is a force caused by the rotation of the Earth on its axis.

• Nature of the Force: It is not a physical push or pull (like a hand moving a chair). It is classified as a force because it has both magnitude and direction. It is similar to gravity in that it requires no physical contact to act on molecules.

• Magnitude and Latitude:

  • The deflection is dependent on latitude.
  • Equator ($0^\circ$): The Coriolis force is zero; no deflection occurs.
  • Poles: The Coriolis force is at its maximum.
  • Rule: The further an object moves from the equator toward the poles, the larger the deflection (the larger the Coriolis force).

• Magnitude and Speed: The faster an object (or wind) moves, the larger the Coriolis force and the resulting deflection.

• Directional Rule:

  • The Coriolis force acts perpendicular ($90^\circ$) to the direction of the wind's motion.
  • Northern Hemisphere: The wind is deflected to the right.
  • Southern Hemisphere: The wind is deflected to the left.

The Role of Friction

• Definition: Friction is a force that acts to slow objects down. It occurs when wind blows along the Earth's surface and interacts with topography such as mountains, trees, and buildings.

• Vector Direction: On a vector diagram, the friction arrow is always drawn directly opposite to the direction of the wind arrow to indicate slowing.

• Interaction with Coriolis: Because the Coriolis force is dependent on speed, friction reduces the Coriolis effect.

  • Slower Wind (Near Surface): Experiences more friction, resulting in less Coriolis deflection. Consequently, surface winds stay closer to their intended PGF path (the deflection is small).
  • Faster Wind (Aloft): Experiences negligible friction. In the upper troposphere, the wind can reach maximum deflection.

Vertical Differences in Wind: Geostrophic Flow

• Surface Level Winds: Near the Earth's surface, the interplay between PGF, Coriolis (to the right in the Northern Hemisphere), and Friction results in wind that is only slightly deflected from the PGF path.

• Aloft (Upper Atmosphere) Winds: In the upper parts of the troposphere, friction is considered negligible and is excluded from calculations.

  • Without friction to slow it down, the wind is turned to its maximum possible deflection by the Coriolis force.
  • This deflection reaches $90^\circ$ from the original PGF path.
  • Geostrophic Wind: The name given to these upper-level winds.
  • Verbatim Etymology: "Geostrophic" means "turned by Earth."

• Oceanographic Application: The principles of geostrophic flow also apply to liquid water. Oceans are fluids that respond to PGF and Coriolis force similarly to the atmosphere, leading to geostrophic flow in oceanic circulation.