High and Low Pressure Systems

Extratropical Cyclones:
- Develop due to accelerations caused by imbalances between Pressure Gradient Force (PGF) and Coriolis force, primarily at jetstream levels.
- Friction in the boundary layer destroys these cyclones over time.

Convergence and Divergence:
- Convergence in an air column leads to increasing surface pressure due to weight gain in the column.
- Divergence results in decreasing surface pressure as the column loses weight.
- Vertical motions are linked to these fields; hydrostatic balance requires air to rise or fall in response to air column changes.
- Cyclones (right) are associated with clouds and precipitation, whereas Anticyclones (left) correlate with clear skies.

Flow Dynamics:
- As air parcels transition from A to B, the PGF remains directed toward the center. Coriolis remains constant.
- New southward PGF component reduces geostrophic balance, resulting in curved flow between isobars.
- True flow becomes subgeostrophic (slower than geostrophic).
- Conversely, in high pressure, PGF can cause a southward component leading to supergeostrophic speeds (faster than geostrophic).

Strengthening and Weakening:
- As surface pressure declines, low pressure centers get stronger, while high pressure centers weaken; the reverse holds true when surface pressure rises.
- Curvature effects play a significant role in forming these systems.

Flow and Pressure Relationships:
- Strong divergence is typically found on the NE side of a trough with surface cyclones forming below this area.
- Conversely, strongest convergence occurs on the NW side.

Impact of Friction:
- Friction consistently decreases wind speed, affecting the Coriolis force which becomes less influential compared to PGF.
- It drives airflow towards low pressures and outward from high pressures, overall weakening these systems.
- Stronger friction leads to greater turning of airflow, varying between 20-40º on land and 10-20º over water.

Temperature Impacts:
- Heating a uniformly cold air layer leads to rising pressure surfaces as temperature increases (latent heating).
- Cooling the layer decreases temperature and pressure surfaces fall (radiative cooling).
- Heating creates outward-directed PGF aloft which draws areas of low pressure at the surface and vice versa for cooling.

A low pressure system intensifies when divergence aloft surpasses surface convergence, while it weakens if surface convergence exceeds divergence.
The role of cooling and associated convergence is critical for high-pressure systems, often more influential than curvature or jetstreaks.