The seasons redecorate this pattern. In January, the Siberian High, an intense cold anticyclone over the radiatively frozen heart of Asia, dominates the continent, while the Icelandic and Aleutian lows are at their deepest and the Azores High sits displaced toward the equator. In July, the Siberian High is gone and its place is taken by the Asian monsoon low, a vast thermal low over the heated continent; the Azores High strengthens and spreads poleward, thermal lows form over the other hot land masses, and the Icelandic Low fades to a shadow of its winter self.
Two mechanisms build these areas, and telling them apart explains nearly everything about their behaviour:
Thermal pressure areas — made by surface temperature: intense cold makes dense air and a surface high (the polar highs, the Siberian High), intense heat makes buoyant air and a surface low (the monsoon low, the summer thermal lows).
Dynamic pressure areas — made by the circulation aloft: the subtropical highs are fed by air subsiding in the descending branch of the tropical circulation of the wind module, and the subpolar lows are kept low by the endless traffic of polar-front depressions.
And the whole ladder follows the sun with a lag of a month or two: poleward in that hemisphere's summer, equatorward in its winter — the shift the two columns of the figure show.
Making highs and lows
The wind module's convergence-and-divergence columns are the machine behind the dynamic systems, and they are worth replaying here:
Convergence at high level — loads the air column with mass: surface pressure rises, and the accumulating air is forced to sink. The result is a high with subsidence inside it.
Divergence at high level — unloads the column: surface pressure falls, and air rises to replace what the upper flow removes. The result is a low with ascent inside it.
Highs are therefore built from above as much as from below, and the same is true of the deep lows.
Subsidence: the anticyclone's engine
Follow the sinking air of an anticyclone and its weather writes itself. Subsiding air warms adiabatically as it descends, so the layer becomes more stable (the environmental lapse rate is flattened, in the language of the thermodynamics module), and the descent is capped by a subsidence inversion: a lid of warm, dry air lying a few thousand feet above the ground, above which the sky is brilliantly clear. The result is the anticyclone's double character: fine, settled, quiet weather — and everything the lower atmosphere produces trapped beneath the inversion. Haze, smoke and dust accumulate for days; in winter, moisture joins them, and the settled high delivers widespread radiation fog and low stratus, the grey lid of anticyclonic gloom, with visibility often worse inside the fine-weather high than under a passing front.
Warm and cold anticyclones
The anticyclones themselves come in a short list of types (warm, cold, plus two shapes built from them, the ridge and the blocking anticyclone), and the warm-versus-cold split is the structural one:
Warm anticyclone — a deep, dynamic high: the column is warm, and high pressure persists to great altitude. Built by high-level convergence and subsidence in the subtropical belt, it is persistent and slow-moving: the Azores High is the standing example.
Cold anticyclone — a shallow, thermal high: a lens of cold, dense air near the surface, intense on the surface chart but replaced by low pressure aloft. Radiative cooling of a winter continent builds the seasonal ones (the Siberian High, the polar highs), and the cold air that floods in behind a depression family builds brief, travelling ones.