env.B

WHY do rains always fall at my special day?!

At a long-awaited picnic day, the weather gets cloudy and soon it rains. At a day I have a date with nice guy after a long time, it pours with rain. While I am in the school taking a lot of classes, the weather is wonderful. But at weekends, when I planned to go out, the weather is terrible.

Most of you may have this kind of experiences a lot. Why does it happen? From this curiosity, our research has started.

Have you ever heard of...

a WEEKLY CYCLE of the weather?

According to various studies, existence of weekly cycles in precipitation is proved. And it is also proved that the weekly cycles of precipitation are affected by weekly cycles of aerosols. Weekly cycles of the concentration of anthropogenic aerosols have been observed in many regions around the world. The phase and the magnitude of these cycles, however, vary greatly depending on region and season. Now following is a detailed explanation of the weekly cycles with an example of China.

Weekly changes in aerosols

[Fig.1] weekly changes in the PM10 anomalies as the average values of the 29 stations.

- Reference: Aerosol weekly cycle over China

To delineate the characteristics of the day-to-day changes in PM10 concentration during a week, the average values of the observed PM10 concentrations at individual stations were calculated for each day of the week over all the summer days during the period 2001-2006.

The resulting daily PM10 anomalies show a well-defined weekly cycle they increase from Sunday through Wednesday and decrease during the later part of the week.

Weekly changes in weather

[Fig.2] Weekly changes in frequency of light rains (≦ 5mm/day) during the months of June-August over 2001-2006.

- Reference: Aerosol weekly cycle over China

According to the investigation of frequency of light rain days, A weekly cycle of light rain frequency is evident, showing a maximum on Sunday and a minimum on Wednesday, where the difference between the maximum and minimum is approximately 3%.

The higher concentration of PM10 on Wednesday seems to reduce the precipitation frequency, which is consistent with the second aerosol indirect effect that the aerosol loading caused by humans, particularly the fine particles, suppress the warm rain processes.

Aerosols affect the clouds and precipitation in a number of ways (e.g., direct, first and second indirect, semi-indirect and so on). In case of China, aerosols have a cloud lifetime effect (second indirect effect) on the frequency of light rains. More effects of aerosols would be handled at the other pages.

Aerosol and Cloud Interaction