Half of the world's population is affected by Asian monsoons , but monsoons are difficult to predict. American researchers have put together a 700-year record of the rainy seasons, which is expected to provide guidance for experts making weather predictions. Every year, damp air masses,known as monsoon,produce large amounts of rainfall in India, East Asia, Northern Australia and East Africa. All this wet air is pulled in by a high pressure area over the Indian Ocean and a low pressure area to the south. According to Edward Cook , a weather expert at Columbia University in New York., the complex nature of the climate systems across Asia makes monsoons hard to predict. In addition, climate records for the area are too recent and not detailed enough to be of much use. Therefore, he and a team of researchers spent more than fifteen years traveling across Asia, looking for trees old enough to provide long-term records. They measured the rings, or circles, inside thousands of ancient trees in more than 300 places. Rainfall has a direct link to the growth and width of rings on some kinds of trees. The researchers developed a document they are calling a Monsoon Asia Drought Atlas. It shows the effect of monsoons over seven centuries, beginning in the 1300s. Professor Cook says the tree-ring records show periods of wet and dry weather. "If the monsoon basically fails or is a very weak one, the trees affected by monsoons at that location might put on a very narrow ring. But if the monsoon is very strong, the trees affected by that monsoon might put on a wide ring for that year. So, the wide and narrow ring widths of the chronology that we developed in Asia provide us with a measure of monsoon variability." With all this information, researchers say they can begin to improve computer climate models for predicting the behavior of monsoons. "There has been widespread famine and starvation and human dying in the past in large droughts. And on the other hand, if the monsoon is particularly heavy, it can cause extensive flooding." said Eugene Wahl, a scientist with America's National Oceanic and Atmospheric Administration. "So, to get a sense of what the regional moisture patterns have been, dryness and wetness over such a long period of time in great detail, I would call it a kind of victory for climate science." It is difficult for experts to predict Asian monsoons because   _  .
it is hard to keep long-term climate records. they are formed under complex climate systems. they influence many nations. there is heavy rainfall in Asia.