Ler Ss OF Or 
The Three Primary Types 
Of the principal features with regard to which tides differ, type of tide is the most 
fundamental. If the tides at two places are of the same type, but differ in time or in 
range, many of the other characteristics of the tide at both places will correspond, so 
that a knowledge of the tide at the one place permits the inferring of the characteristics 
of the tide at the other. But if the type of tide at the two places differs, then the fact 
that the range or time at the two places is the same does not prevent the occurrence of 
very profound differences in the other characteristics of the tide at the two places. 
In other words, differences in time and range of tide are merely differences in degree, 
but differences in type of tide are differences in kind. 
Type of tide has reference to the characteristic form of the rise and fall of the tide 
as revealed by the tide curves. In general, it may be said that the tide curve for any 
particular place differs from.the tide curve at any other place in some one or more 
respects. There is, therefore, great variety in tide curves. They may, however, be 
grouped into three large classes or types, namely, semidaily tides, daily tides, and mixed 
tides. Instead of the terms semidaily and daily, the terms semidiurnal and diurnal 
are sometimes used. 
As the name suggests, the semidaily type of tide is one in which the full tidal 
cycle of high and low water is completed in half a day; in other words, in a day there 
are two high and two low waters in this type of tide. ‘There is, however, the further 
implication that the two tidal cycles in each day resemble each other; that is, morning 
and afternoon tides do not differ much. In this connection, it is to be noted that a 
day in the tidal sense is a tidal day of 24 hours and 50 minutes and not the ordinary 
day of 24 hours. 
The daily type of tide includes those tides in which but one high and one low 
water occur inaday. In this type of tide the rise and also the fall of the tide each occu- 
pies a period of approximately 12 hours against a period of 6 hours in the semidaily 
tide. 
The mixed type of tide is one in which two high and two low waters occur in a 
day, but with marked differences between the two high waters or between the two low 
waters of the day. As will be seen later, the mixed type of tide arises as a mixture of 
semidaily and daily tides, and hence its name. 
To exemplify the three types of tide there are shown in Figure 2 the tide curves 
for the last four days of May 1931 at Hampton Roads (Norfolk), Va., Pensacola, 
Fla., and San Francisco, Calif. The horizontal line associated with each tide curve 
represents the undisturbed or mean level of the sea, the rise and fall of the tide above 
and below mean sea level being measured by the scale to the left of each curve. 
The upper curve, that for Hampton Roads, illustrates the semidaily type of tide. 
Two high and two low waters are seen to have occurred each day, with the morning 
and afternoon tides differing but relatively little. The middle curve, for Pensacola, 
illustrates the daily type of tide, one high and one low water occurring each day. The 
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