TIDAL DATUM PLANES . 47 
Indeed, as a first approximation, any arm of the sea may be regarded as constituting 
a huge inverted water barometer. When the barometric pressure over this arm of the 
sea rises, the level of the water will be lowered, while with a decrease in barometric 
pressure the level of the water will rise. 
Daily Sea Level 
Wind and weather vary from day to day; this, together with the variation due 
to the long-period tides, brings about variations in the height of sea level from day to 
day. Figure 22 shows in diagrammatic form the changes in sea level from day to day 
for 2 months of the year 1939 at Atlantic City, N. J., one a winter month, the other a 
_ summer month. 
During January, as the upper curve of Figure 22 shows, sea level from one day to 
another varied from less than a tenth of a foot to more than 1% feet. Furthermore, 
from the 19th to the 23d sea level fell 2% feet. Such large differences are obviously 
to be ascribed to the wide variations in wind and weather which characterize the 
month of January on the North Atlantic coast. An examination of the weather record 
for January 1939 shows that on the 18th easterly winds prevailed, with velocities up 
to 38 miles per hour. The next day the wind shifted to the northwest and continued 
from that direction with velocities up to 20 miles per hour until the 21st, when the 
wind shifted to the west, with velocities increasing up to 53 miles per hour on the 22d, 
and up to 40 miles per hour on the 23d. 
1 10 20 30 
January 
FG. 22.—Daily sea level, Atlantic City, January and July 1939. 
