Weather forecasts, usually a casual topic of conversation, have become a topic of great interest since the city experienced its worst heatwave in over seven decades. But the reasons why the city¡¯s weather forecasts this summer have often proved inaccurate and how weather forecasting is conducted are matters that still confuse most people.

¡°It was forecast that the rain would occur in some parts of the city, but it turned out to sweep the whole city,¡± a local resident complained to the Xinmin Evening News.

Meteorologists say such inaccuracy is unavoidable as microscale weather prediction still remains a problem worldwide.

¡°A weather system usually covers a region of several hundreds of miles in diameter, an area that makes Shanghai look like a point in comparison,¡± explained Dai Xinfu, chief weatherman of the Shanghai Weather Forecast Centre.

¡°Moreover, the city¡¯s weather is influenced by the atmospheric systems of both the northern and southern parts of the country, making it even more changeable,¡± Dai added.

Weather forecasting, like all scientific predictions, is based on the analysis of information. Data collected from ground sensors, weather balloons and satellite images are the major information sources for precise weather forecasts.

¡°At present, that information is shared among every meteorology station in the world every three hours under the co-ordination of the World Meteorological Organization,¡± said Yao Zhizhan, director of the Shanghai Weather Forecast Centre.

In addition to this regular information, local weather forecast centres also have their own observation systems to acquire more specific regional weather data.

According to Yao, a total of 50 automatic weather loggers and 100 rain gauges are installed in the city, and Doppler radar is also employed to observe meteorological changes.

Data collected from these sources are analyzed by meteorologists, with the help of computer models, to generate weather forecasts.

The city¡¯s weather forecast centre provides four regular forecasts every day. In addition, alarms for disastrous weather, including typhoons, rainstorms, abnormally high and low temperatures and heavy fog, are sent out 30 minutes in advance of their occurrence.

This summer, the forecast centre has released five ¡°black¡± high temperature alarms, indicating that the mercury climbed above 38 degrees centigrade.

¡°Local residents can get these up-to-date forecasts through TV and radio programmes, mobile phone messages, and the electronic displays in public areas,¡± said Yao.