Monsoon rain across India: a seasonal cycle of disaster

Some of the worst monsoon rains in the last 50 years have been inundating portions of India, killing as many as 100 people and leaving millions homeless. Hardest hit has been the eastern state of Orissa, in the lowlands bordering the Bay of Bengal, where 24 of its 30 districts have been flooded.

The monsoon typically begins in late May or early June in southwestern India, then progresses northward over the subcontinent from mid-July to September. Similar winds bring heavy rains to much of Southeast Asia.

Derived from Arabic, the word monsoon means ``season'' and refers to the seasonal winds, not to the rain.

Every summer as India heats up, the hot air rises and is replaced by moisture-laden air over the Indian Ocean. This moist air also warms up, rises and keeps flowing inland until it hits the Himalayas in the north. As the moisture rises over the Himalayas, it cools and condenses into rain or snow. The heavy rains that are produced often cause flooding, especially in the flat alluvial plains. Flooding becomes a human crisis because these areas are extensively farmed and hold some of the highest population densities in the world.

Compounding the problem are the tropical cyclones that come onshore from the Indian Ocean. These storms, hurricanes, are not triggered by the monsoon but are brought inland by its wind.

This combination of torrential rains and strong winds has caused hundreds of thousands of fatalities during human history. In 1970, a cyclone devastated parts of Bangladesh with an estimated death toll of 200,000 to 500,000 people.

The town of Cherrapunjee in northeast India holds the world record for the most rain in a single month with 366 inches, and the most rain in a year with 1,041 inches. This is the equivalent of 75 years of rain in San Jose.

A regular reader of the Weather Corner recently visited Cherrapunjee to experience the rain. Here's his account of his trip.

Q. Most people's idea of paradise is relaxing on a sunny beach in Hawaii; mine is just the opposite. Here is a short report on my recent trip to Cherrapunjee, one of the world's wettest spots.

Cherra, as it is locally known, is situated in eastern India about 15 miles north of the India-Bangladesh border. The terrain rises steeply from the border to Cherra, which sits at an elevation of 4,500 feet. After passing over the plains of Bangladesh, the monsoon clouds hit Cherra with a vengeance.

On June 16, 1995, an astounding 62 inches of rain fell in just 24 hours. The year 1974 saw a total of 967 inches, with 323 inches just in July. That's 10 inches a day for an entire month. Amazingly, since Cherra sits atop a very porous limestone layer, there is no flooding and hardly any standing water.

The average yearly rainfall of 460 inches puts Mount Waialeale on the island of Kauai in Hawaii at the top of the list of the world's wettest places. Close behind, at 450 inches per year, is Cherra. But whereas the rainfall on Mount Waialeale is spread over 12 months, Cherra gets almost all of its rain in the six monsoon months of April through September. And nobody lives on Mount Waialeale, while 70,000 people call Cherra their home.

For purists, I might mention that Mawsynram, just 10 miles west of Cherra, now holds the record as the world's wettest spot, with an average annual rainfall of 467 inches.

I reached Cherra on June 20. June is the month with the most rain, about 108 inches on average. My plan was to walk several hours every day and experience the heavy rain.

Unfortunately, the rain gods didn't cooperate with me. In the two days prior to my arrival, it rained 19 inches. During my seven-day stay, it rained only 10 inches, and all of it at night.

Exasperated, I took up walking at night. The locals told me that it does rain mostly at night but could offer no explanation for this unusual phenomenon. The temperature at night was around 70 degrees, just seven or eight degrees lower than the daytime highs. Do you have any explanation? Dinesh Desai - Los Altos

A. Thanks for sharing your adventure. I am not an expert on the Indian monsoon, so I can offer only an educated guess.

As for the rainfall occurring mostly at night, here's a likely scenario:

The summer monsoon in eastern India blows northward from the Bay of Bengal. As it passes over the warm land and is heated by the sun, massive thunderstorms develop and are carried northward to the base of the Himalayas, where they dump copious amounts of rain.

I think it may be very late in the day by the time these storms reach as far north as Cherra, with the bulk of the rain arriving after dark. The overnight rain and cloud cover would also account for the relatively warm evening temperatures.

Jan Null, founder of Golden Gate Weather Services and Adjunct Professor at San Francisco State University, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ggweather.com.