The Weather Notebook: Monsoon

Monsoon
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Dave Thurlow, Host

Each spring, as the temperature climbs ever higher and the earth cracks for lack of moisture, a single question oppresses the Indian subcontinent: when will the monsoon begin? Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook.

Flooding is a annual occurence in India
Monsoon, from the Arabic word 'mausim' meaning season, technically refers to the winds that alternate by season in Southeast Asia -- southwest in summer, northeast in winter -- but it is the rain-bearing summertime monsoon that gets most of the attention. As the vast Rajasthan Desert heats rapidly under the spring sun, a thermal low develops that draws moist air swirling in from the Indian Ocean. When the oceanic air rises over the subcontinent, it dumps out its moisture in the form of heavy, daily rain.

Here in the US, we have our own monsoon -- the so-called Arizona monsoon that operates in much the same way. On satellite loops from late June to mid-September, the Arizona monsoon looks like a river of clouds that runs straight up along the spine of Mexico until it relaxes in a broad delta over New Mexico and Arizona. On the ground, you can expect a monsoonal storm when temperatures top 100 F and huge cumulonimbus clouds boil up from the south.

We know that the arrival of the Indian monsoon can vary by as much as 40 days and that its intensity can swing from flooding downpours to severe drought. And recently a link has been established between El Nino and the failure of the monsoon. But science has yet to find a way to pinpoint the onset or strength of the monsoon in any given year -- critical questions on a planet in which 65% of the population lives in monsoon regions.

One final word: Never make monsoon plural, as in when the monsoons arrive. There are monsoon storms and monsoon rains, but there is only one monsoon. With a meteorological event this massive and mysterious, one is enough.

Thanks to contributing writer David Laskin, who experiences monsoonal type rains in his hometown of Seattle, WA. Thanks also to Subaru and the National Science Foundation.