There is an expected geography of the timing onset of the monsoon. The classic
Indian subcontinent situation is that southern India experiences the wind shift her-
alding the onset of the wet monsoon in early May and the rains progress northwest
as far as Pakistan by the middle of July. At that time, the wind shift backpedals to
the southeast. Thus, the wet monsoon lasts longer with higher rainfall totals in the
south and east than in the northwest. In the rest of monsoon Asia the topographic/
sea configurations are very complicated, and there are many nuances to the mon-
soonal flows in these realms.
The South Asian monsoon is notoriously variable. The timing of onset and fin-
ish vary by several weeks and amount of precipitation shows large interannual
variance. El Nin ̃o conditions, occurring on the eastern side of the Pacific Ocean,
have long been shown to be closely tied to the occurrence of weak Asian mon-
soons. There is keen interest in predicting monsoonal rains for upcoming seasons,
but these efforts have been only moderately successful because of limited scien-
tific understanding of monsoon causes. Increasingly good mathematical modeling
holds the promise of reasonable monsoon forecasts in the near future.
Monsoon climate is one of the important regional atmospheric characterizations
of the planet. In a classic monsoon climate there are three seasons: the relatively
mild season of the winter monsoon; theincreasingly stifling heat and humidity
leading to the onset of the wet monsoon sometime near the summer solstice; and
the warm, prodigiously wet summer monsoon rains. The monsoon climates are
located in parts of southern Asia, tropical Africa, northern Australia, and Central
America. Freezing temperatures are quite uncommon with average monthly tem-
peratures ranging from the low- to mid-20s C. Precipitation is expected to be
hugely variable by season. The cool season months commonly average less than
20 mm of precipitation, while the wettest months of the high sun season can aver-
age in excess of 300 mm. Monsoon climates tend not to be as wet as tropical rain-
forest climates for the year as a whole but tend to be the wettest places on the
planet in the high sun season.
Cherrapunji is a city in the Khasi Hills of Assam in northeastern India. Cherra-
punji sits at 1,290 m above sea level on the first significant topography encoun-
tered by the summer monsoonal flow as it leaves the Bay of Bengal. With this
mix of monsoonal rains and orographic effect, Cherrapunji has received some
truly remarkable rainfall totals. In 1861, the city recorded almost 23 m of precipi-
tation with a stunning 9.3 m falling in July of that year.
In the United States, the Southwest has a minor monsoon. In this case the term
refers to the rainfall that peaks in the high-sun season. This effect is not related to
the classic Asian monsoon by physical connection. The precipitation is enabled by
the formation of a summertime thermal low over the Southwest drawing in mari-
time tropical air from the Pacific Ocean and Gulf of Mexico. These moist air232 Monsoon