For many years, the classic case was the obliging typhoon that moved across the Philippines with its center passing directly over the fully-equipped weather observatory in Manila. It happened on October 20, 1882. The wind which came ahead of the center was of destructive violence, reaching above 120 miles an hour in a final mad rush from the west-northwest before the calm set in. It was not an absolute calm. There were alternate gusts and lulls. The way the winds acted led the observer to think that the center was about sixteen miles in diameter. He said:
“The most striking thing about it was the sudden change in temperature and humidity. The temperature jumped from 75° to 88°. The air was saturated at 75° but the humidity dropped from 100% to 53% in the center and then rose to 100% again as the center passed. When the wind suddenly ceased at the beginning of the calm and the sun came out, many people opened their windows but they slammed them shut right away, because the hot, dry air seemed to burn the skin.”
For more than fifty years after this, there were arguments about the reasons for these changes in temperature and humidity. Some scientists claimed that they were caused merely by the heating of the sun in a clear sky and that the air which preceded and followed the center was cooled and saturated by the rain. Some of the Jesuit scientists at Manila did not agree. One weatherman showed, for example, that if they took air at 75° and 100% humidity and heated it to 88°, the humidity would fall only to about 61% and that the air at Manila at that time of year had never had such a low humidity (53%), even when the sun was shining.
The general conclusion was that the air descends in the eye of the tropical storm. At least, they were convinced that it descended in the Manila typhoon. When air descends, it is compressed, coming into lower levels where the pressure is higher. This compression causes its temperature to rise and the air then has a bigger capacity for moisture. In other words, the air becomes warmer and drier. There never has been full agreement on this question. Certainly, in some cases, the air is not warmer and drier in the center.
In later years, typhoon centers passed over other observatories and had various effects. However, one struck Formosa on September 16, 1912, and the calm center passed over the observatory long after the sun had gone down. In this case, the temperature jumped from 75° to 94° and this could not be explained by the direct heat of the sun. But there were different results in other cases and in one instance the temperature fell a little.