20
Gilberta Rodriguez
The highest temperatures of the system are found in the Strait, reaching sometimes
during the day 2° C above the mean temperature in the Gulf and 1-2° C above the temperature of the air (Rodriguez 1973). Along the navigation channel that traverses Tablazo
Bay and the Strait (Fig. 2.3a) the temperature of the column of water decreases regularly
with depth, and follows, although does not precisely parallel, the increment in salinity.
In the lake itself the extreme minimum mean temperature at 1 m depth is 29.0° C
and the maximum 32.5° C, according to data by Escam (1991), which represents the
mean of 6 stations covering all sectors of the lake for 2 years. Redfield and Doe (1964)
found in the northern portion of the lake a mean temperature at 7 m depth of 30 ±1° C.
The yearly temperatures follow the same regular seasonal pattern of other regions of
the estuary, with a minimum in February and a maximum in September. The day-time
temperature of the water is 1.5° C above the mean temperature of the air in Maracaibo.
There is a diurnal increase in temperature up to 1° C at sunset which can reach 3 m
depth, but it is not observable at 6 m (Redfield and Doe 1964). The structure and evolution of the thermal stratification in the lake were first described by Redfield et al.
(1955) and Redfield and Doe (1964). The seasonal changes in the mixing of the hypolimnium are related to the general hydrography revealed by the chloride distributions
and will be discussed under this topic.
2.4.2
Wind Regime
Wind regime within the basin changes throughout the year. From November to April
trade winds from the north-east predominate and are replaced during the hottest
period of the year - May to October - by local winds. These local winds are the result
of the differential heating of the Lake and surrounding lands masses: During the day
a greater amount of solar radiation is absorbed by the land; the adjacent air increases
its temperature and ascends to the upper atmosphere while the colder and denser air
above the lake is stationary resulting in a south-westerly wind. During the night a reverse process takes place and the wind changes direction. The cumulus frequently
observed at sunset condense moisture at high altitudes.
2.4.3
Precipitation
The alternation in the predominance of trade and local winds originate a sharp division between rainy and dry seasons, which is typical of the arid, semiarid and central
zones to the north where 90% of the rainfall occurs during May-October when a decrease in the trade winds take place (Cockroft 1961). The Gulf of Venezuela has annual means of 100 mm in the dry season and 200 mm during the rainy season. As we
proceed southward the amount of rainfall increases and the distinction between dry
and rainy season almost disappear. Large annual means of rainfall are found in two
areas in the south: The Escalante River basin with 1400 mm for both seasons, and the
Santa Ana River with means of 1000 and 1800 mm for the dry and rainy seasons, respectively. The large precipitation over this last area is attributed to a quasi permanent low depression over the mouth of the Catatumbo River. The largest rainfall is
found in a locality in the upper Catatumbo, with a peak annual mean of 3 500 mm.
Gilberta Rodriguez
The highest temperatures of the system are found in the Strait, reaching sometimes
during the day 2° C above the mean temperature in the Gulf and 1-2° C above the temperature of the air (Rodriguez 1973). Along the navigation channel that traverses Tablazo
Bay and the Strait (Fig. 2.3a) the temperature of the column of water decreases regularly
with depth, and follows, although does not precisely parallel, the increment in salinity.
In the lake itself the extreme minimum mean temperature at 1 m depth is 29.0° C
and the maximum 32.5° C, according to data by Escam (1991), which represents the
mean of 6 stations covering all sectors of the lake for 2 years. Redfield and Doe (1964)
found in the northern portion of the lake a mean temperature at 7 m depth of 30 ±1° C.
The yearly temperatures follow the same regular seasonal pattern of other regions of
the estuary, with a minimum in February and a maximum in September. The day-time
temperature of the water is 1.5° C above the mean temperature of the air in Maracaibo.
There is a diurnal increase in temperature up to 1° C at sunset which can reach 3 m
depth, but it is not observable at 6 m (Redfield and Doe 1964). The structure and evolution of the thermal stratification in the lake were first described by Redfield et al.
(1955) and Redfield and Doe (1964). The seasonal changes in the mixing of the hypolimnium are related to the general hydrography revealed by the chloride distributions
and will be discussed under this topic.
2.4.2
Wind Regime
Wind regime within the basin changes throughout the year. From November to April
trade winds from the north-east predominate and are replaced during the hottest
period of the year - May to October - by local winds. These local winds are the result
of the differential heating of the Lake and surrounding lands masses: During the day
a greater amount of solar radiation is absorbed by the land; the adjacent air increases
its temperature and ascends to the upper atmosphere while the colder and denser air
above the lake is stationary resulting in a south-westerly wind. During the night a reverse process takes place and the wind changes direction. The cumulus frequently
observed at sunset condense moisture at high altitudes.
2.4.3
Precipitation
The alternation in the predominance of trade and local winds originate a sharp division between rainy and dry seasons, which is typical of the arid, semiarid and central
zones to the north where 90% of the rainfall occurs during May-October when a decrease in the trade winds take place (Cockroft 1961). The Gulf of Venezuela has annual means of 100 mm in the dry season and 200 mm during the rainy season. As we
proceed southward the amount of rainfall increases and the distinction between dry
and rainy season almost disappear. Large annual means of rainfall are found in two
areas in the south: The Escalante River basin with 1400 mm for both seasons, and the
Santa Ana River with means of 1000 and 1800 mm for the dry and rainy seasons, respectively. The large precipitation over this last area is attributed to a quasi permanent low depression over the mouth of the Catatumbo River. The largest rainfall is
found in a locality in the upper Catatumbo, with a peak annual mean of 3 500 mm.