30 
PACIFIC SCIENCE, VoL XII, January, 1958 
TABLE 2 
Mean Monthly Wind Velocity in La Paz, 
Guaymas, and Mazatlan 
(Serv. Met. Mex., Boletin Anual) 
LA PAZ 
GUAYMAS 
MAZATLAN 
m/ sec 
m/sec 
m/sec 
January 
2.6 
4.8 
6.1 
February 
2.4 
4.2 
5.9 
March 
2.2 
5.3 
5.5 
April 
2.3 
5.1 
5.0 
May 
2.1 
4.6 
4.8 
June 
2.4 
4.3 
4.6 
July 
2.0 
3.5 
4.3 
August 
2.0 
3.6 
4.9 
September 
1.5 
4.4 
5.0 
October 
1.8 
4.8 
5.1 
November 
2.3 
4.6 
4.1 
December 
2.3 
4.9 
5.5 
Year 
2.2 
4.5 
5.1 
In La Paz winds are northerly from November until 
March and southerly from April until October. 
In Guaymas winds are northerly from August until 
May and southerly in June and July only. 
In Mazatlan winds are northerly from November 
until April and westerly from May until October. 
They originate in the Caribbean Sea or off the 
coast of Central America and it takes them 
between five to seven days to reach the Gulf 
(Serv. Met. Mex., Boletin Anual). 
If one compares the distribution of winds 
and the occurrence of the rainy season with 
the migrating low pressure system over west- 
ern Mexico (Fig. 7) it is seen that northerly 
winds are found with the low pressure to the 
east, and southerly winds with the low pres- 
sure to the northwest. The beginning of the 
rainy season is connected with the appearance 
of southerly winds that carry moist air into 
the Gulf and give rise to uplift rain. 
Evaporation is one of the most important 
factors in the Gulf and is responsible for the 
high salinity observed in the surface layers. 
The amount of evaporation has been deter- 
mined from pan observations at a few shore 
stations (Serv. Met. Mex., Boletin Anual). 
These measurements show a main maximum 
in summer and a main minimum in winter. 
The amount of annual evaporation varies be- 
tween 1800 mm. in La Paz and about 3000 
mm. in Guaymas (Table 3). The walues are 
consistent from year to year for each station 
and the deviations from the above values are 
generally less than 10 per cent (Serv. Met. 
Mex., Boletin Anual). 
Since evaporation from pans is not strictly 
representative for evaporation from the sea 
surface, it is of interest to see how the ob- 
served evaporation from pans compares to 
evaporation derived from indirect methods. 
The use of the energy budget method is 
restricted, because the advection term canno: 
be evaluated. 
In the meteorological approach a straight 
line relationship between the evaporation, E, 
and the product of wind velocity, W, and 
vapor pressure difference, Ae, is assumed 
(Jacobs, 1951); the coeficient of proportion- 
ality depends upon the height where the wind 
velocity and the humidity has been measured, 
and upon the roughness of the sea surface. 
In the following the coeficient of propor- 
tionality will be taken as 4.26 (Jacobs, 1951) 
so that 
E = 4.26 WAe 
where E is in mm/month, W in m/sec, and 
Ae is in mb. 
A comparison between the pan and com- 
puted evaporation for the year 1938 is shown 
in Table 3, and it is readily seen that notable 
differences exist. The computed evaporation 
is less than the observed evaporation (Fig. 8) 
for La Paz and Mazatlan. In La Paz the maxi- 
mum evaporation occurs in summer and the 
minimum in winter; in Mazatlan the observed 
evaporation shows a winter minimum and a 
summer maximum, whereas the computed 
evaporation has a summer minimum and a 
fall maximum. The summer minimum is due 
to the low wind velocities observed during 
this season. Values for Guaymas are not 
strictly comparable because the pan observa- 
tions were made on a nearby station farther 
inland;' nevertheless it is interesting to note 
that there the total annual amount of evapora- 
tion is considerably higher than in La Paz or 
Mazatlan. 
