Geoscience Reference
In-Depth Information
Figure 5.16
Orographic and con-
vective rainfall in the Cachi region of
Costa Rica for the period 1977 to
1980. (A) The Cachi region, elevation
500 to 3,000 m. (B) Typical accumu-
lated rainfall distributions for individual
convective (duration 1 to 6 hours, high
intensity) and orographic (1 to 5 days,
lower intensity except during convec-
tive bursts) rainstorms. (C) Monthly
rainfall divided into percentages of
convective and orographic, plus days
with rain, for Cachi (1018 m).
Source
: From Chacon and Fernandez
(1985), by permission of the Royal
Meteorological Society.
Figure 5.17
Classic view of the cycle of a local thunderstorm. The arrows indicate the direction and speed of air currents. (A) The
developing stage of the initial updraft. (B) The mature stage with updrafts, downdrafts and heavy rainfall. (C) The dissipating stage,
dominated by cool downdrafts.
Source
: After Byers and Braham; adapted from Petterssen (1969).
distinguishable. Convective rain occurs mainly in the
May to November period, when 60 per cent of the rain
falls in the afternoons between 12:00 and 18:00 hours;
orographic rain predominates between December and
April, with a secondary maximum in June and July
coinciding with an intensification of the trades.
Even low hills may have an orographic effect.
Research in Sweden shows that wooded hills, rising
only 30 to 50 m above the surrounding lowlands,
increase precipitation amounts locally by 50 to 80
per cent during cyclonic spells. Until Doppler radar
studies of the motion of falling raindrops became
