Aviation Meteorology Revision Guide

In other contexts, a katabatic wind (or adiabatic fall wind) refers to cold, dense air draining downhill under the force of gravity, often occurring at night or off ice plateaus.

Global Circulation Models

To understand large-scale weather patterns, pilots must grasp the three-cell model, which describes how air circulates between the equator and the poles in each hemisphere.

The Three Cells

1. The Hadley Cell: At the equator, rising air creates the Intertropical Convergence Zone (ITCZ), a belt of convective rain. This rising air eventually subsides around $25^\circ\text{--}35^\circ$ latitude, creating sub-tropical anticyclones (high-pressure zones). The surface flow from these highs toward the equator is deflected by the Coriolis force to become the trade winds.
2. The Ferrel Cell: Located in the mid-latitudes, this cell is driven by the interaction of the other two cells. Surface flow from the subtropical highs moves poleward and is deflected into the mid-latitude westerlies.
3. The Polar Cell: At high latitudes, cold air sinks at the poles and flows toward the mid-latitudes as polar easterlies.

Exam Tip: When sketching the three-cell model, remember that rising air corresponds to low pressure (convergence) and sinking air corresponds to high pressure (divergence).

Jet Streams and Upper-Level Dynamics

The boundaries between these circulation cells are home to the jet streams—narrow bands of extremely fast-moving air in the upper troposphere.

Polar vs. Sub-tropical Jet Streams

Both jet streams are explained by the thermal wind relationship, which links vertical wind shear to horizontal temperature gradients.

• The Polar Jet Stream: This sits near the polar front at approximately $250\text{--}300\text{ hPa}$ (~$9\text{--}10\text{ km}$). It is driven by the intense temperature contrast between polar and mid-latitude air masses. The polar jet is not a straight line; it meanders in large, slow planetary waves known as Rossby waves. These waves are vital for aviation meteorology because shortwave troughs within them can create upper-level divergence, leading to surface pressure falls and cyclogenesis (the formation of low-pressure systems).
• The Sub-tropical Jet Stream: Located near $30^\circ$ latitude at the boundary of the Hadley and Ferrel cells, this jet sits slightly higher (around $200\text{ hPa}$). It arises from the conservation of angular momentum in the upper poleward Hadley flow and is generally more persistent than the polar jet, as it is not directly tied to surface frontal zones.

Summary Table for Revision