Schematic diagram showing the main synoptic scale circulation systems that control water vapour and cirrus cloud cover advection patterns over the Southeastern Pacific Ocean. The satellite image is taken at 11µm in the atmospheric IR window. Warmer surfaces appear dark and cold surfaces light. Clouds behave essentially as blackbodies at this wavelength, hence low clouds (being warmer) are medium grey and higher cloud (being cold) appear light grey or even white for the highest (about 15km), coldest cirrus clouds.

In summer the pressure systems move southwards and the Subtropical high pressure (H) strengthens and expands its area of influence. Subsidence in the high creates a strong temperature inversion at about 1000m above the surface which traps low level stratocumulus clouds below it (notice the area of medium grey speckelled cloud in and around the high). The subtropical high is a semi-permanent circulation feature that only experiences minor position changes as the high develops east or west of its mean position. The high produces clear dry conditions over Northern Chile. Occasional incursions of moisture and cloud may occur over Northern Chile from the north in summer if the high weakens and the anticyclonic flow over the Amazon basin intensifies.

In winter the pressure systems move northwards and the subtropics comes under the influence of migratory wave-like systems that propagate from west to east in the prevailing flow. The high pressure area of the wave (the ridge) is warm and dry while the low pressure area (the trough) is cool and moist. Middle and upper tropospheric clouds can typically be found along the leading edge of the ridge and trough in association with surface warm and cold fronts. Trough and ridge development, which is usually slight over the Southeastern Pacific, may add a meridional component to the cloud cover and water vapour advection patterns.