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orographic effects may extend well downwind due
to the activation of mesoscale rain bands (see
Figure 9.13 ).
In tropical highland areas, there is a clearer
distinction between orographic and convective
contributions to total rainfall than in the mid-
latitude cyclonic belt. Figure 5.16 shows that in the
mountains of Costa Rica the temporal character
of convective and orographic rainfalls and their
seasonal occurrences are quite distinguishable.
Convective rain occurs mainly in the May to
November period, when 60 percent of the rain
falls in the afternoons between 12:00 and 18:00
hours; orographic rain predominates between
December and April, with a secondary maximum
in June and July coinciding with an intensification
of the Trade Winds.
Even low hills may have an orographic
effect. Research in Sweden shows that wooded
hills, rising only 30-50m above the surrounding
lowlands, increase precipitation amounts locally
by 50-80 percent during cyclonic spells. Until
Plate 5.15 What atmospheric scientists refer to as open cell cloud formation is a regular occurrence on the back
side of a low pressure system or cyclone in the mid-latitudes. In the Northern Hemisphere, a low-pressure system
will draw in surrounding air and spin it counterclockwise. That means that on the back side of the low pressure
center, cold air will be drawn in from the north, and on the front side, warm air will be drawn up from latitudes closer
to the equator. This movement of an air mass is called advection, and when cold air advection occurs over warmer
waters, open cell cloud formations often result.
This MODIS image shows open cell cloud formation over the Atlantic Ocean off the southeast coast of the United
States and the Bahamas on February 19, 2002. This formation is the result of a low-pressure system in the North
Atlantic Ocean a few hundred miles east of Massachusetts. Cold air is being drawn down from the north on the
western side of the low and the open cell cumulus clouds being to form as the cold air passes over the warmer
Caribbean waters.
Source:Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC.
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