Rainfall varies seasonally and can be described as follows: In January
it amounts to 25 to 50 mm. in the coastal region and 10 to 25 mm. in
the rest of the state. In April there is more than 25 mm. to the north
of about 23 deg. north latitude, 10 to 25 mm. in the Sierra de Tamaulipas
and Sierra Madre Oriental, and less than 10 mm. in the extreme
southwestern part of the state.
In July rainfall amounts to less than 25 mm. in Nuevo Laredo and San
Fernando, is from 25 to 50 mm. in the northeastern and central parts of
the state, 50 to 100 mm. in the Sierra San Carlos and Sierra Madre
Oriental, and 100 to 200 mm. in the area south of Soto la Marina and
east of the Sierra Madre Oriental. In October rainfall is less than 50
mm. in the northern half of the state, including the Sierra de
Tamaulipas, and 50 to 100 mm. in the rest of the state, except on the
east side of the Sierra Madre Oriental and in the area near Tampico,
which receive between 100 and 200 mm.
The number of rainy days per year varies from 60 to 90 at Sierra San
Carlos, Sierra Madre Oriental, and in the lowlands south of 23 deg. north
latitude; the rest of the state has about 60 rainy days, excepting the
Mexican Plateau, which has fewer than 60.
Although Tamayo (1949) followed the Koeppen System in classifying types
of climate and thereby recognized 10 different kinds of climate in
Tamaulipas, these can be grouped into three major categories as
follows: