THE TEXAN MONSOONS. 
305 
The other test to apply is that of the differences in atmos¬ 
pheric pressure between the plains and Gulf in summer and 
winter. Here, again, it is safe to employ charts with the 
isobars reduced to sea-level. Turning to Dr. Hanri’s isobaric 
chart for January, it appears that the mean pressure in¬ 
creases from the Gulf northward, culminating in the Dako¬ 
tas and Montana. The mean difference between the Gulf 
and this center in January is five millimeters, or a fifth 
of an inch—sufficient to give the air a tendency to move 
southward over the plains. In July the isobar of 760 milli¬ 
meters runs nearly along the hundredth meridian from the 
foot of Yucatan to Manitoba, while there is lower pressure 
to the northward and southward, with, on the whole, a more 
rapid decrease to the northward. On the more detailed 
forenoon and afternoon normal-pressure charts for the 
months, now in manuscript in the Weather Bureau, in¬ 
crease in pressure from north to south in July is decidedly 
greater, being about a fifth of an inch. These charts are 
probably based on a larger number of synchronous obser¬ 
vations taken with carefully compared instruments than are 
Dr. Hann’s, being from fifteen years of the records of the 
Signal Service. 
It appears, therefore, that, on the whole, the requirements 
for monsoons are present, but that the resulting monsoons 
are very much disturbed (sometimes promoted, sometimes 
suppressed) by the cyclones and anti-cyclones that are almost 
constantly crossing their- area. There remains to be made 
the explanation of the fact that the summer monsoons last 
longer and are more persistent than the winter ones. This 
is the more difficult because all the conditions already men¬ 
tioned are more favorable for the winter north winds than 
for the summer south winds. For this there seems to be no 
better way than to fall back on the mean wind direction 
prevailing in summer over the Gulf. The trade-winds in 
summer flow into the Gulf from the east and southeast, and 
reach the Texan coast nearly perpendicular to it. On the 
land, the surface winds are forced northward by the con- 
