m satellite image, the pixels which
will pass over the observatory during the period predicted, and
the corresponding water vapour content.
The large scale circulation patterns affecting the Chilean coast are presented
on Fig. 2. The particular situation of the Atacama desert with respect
to cloudiness and water vapour is clearly illustrated.
Here follows a summary of the presentation by the author:
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.