Despite the record cold and snow in the Northern Hemisphere, global warming is here to stay, say climatologists. Extremes of weather - short-term meteorology - are part of the big climate picture.

Snow removing equipment are busy at work clearing the runways after a heavy snowstorm at Washington Reagan National Airport in Washington, D.C.,the United States. [Xinhua]

Now that the idea of global warming is widely accepted (though of course skeptics remain), the latest "snowcaplyse" on the US East Coast and recent blasts of unusual freezing weather make some people wonder what's going on.

On the occasion of the International Weather Festival tomorrow, scientists worldwide are expected to reassure the public that the world really is getting warmer, despite these extraordinary "blips on the screen."

Climate experts say that part of global warming is more frequent extreme weather, such as unusually heavy snows and frigid weather in China, North America and Europe.

Shanghai is also experiencing changes of climate.

Climate refers to the average, variations and extremes of weather in a region over long periods of time, usually based on a weather record of more than 10 years, says Kong Chunyan, senior engineer of Shanghai Central Weather Observatory.

Weather, however, describes temperature and humidity in a small area for a short time.

Since the last century temperatures have been rising continuously worldwide, but there are periods, even years, that do not conform to patterns in particular areas.

Along with global warming, scientists have noted more frequent extreme weather, including high temperatures, storms, typhoons, hurricanes and snowstorms.

"Global warming is a climate trend, which doesn't necessarily indicate that the temperature increases day by day or month by month," says Kong, "Instead, it keeps growing in fluctuation."

Fluctuations can be extreme and frequent: the heat wave in Europe in 2003, Hurricane Katrina in North America in 2005, the recent heavy snows in the Northern Hemisphere.

Shanghai is also experiencing more frequent extreme weather within the past few decades, according to Chen Zhiqiang, vice president of the Shanghai Central Weather Observatory.

The number of annual high-temperature days (with highest temperature over 35 degrees Celsius) has increased greatly.

The average annual high-temperature days within the recent 10 years reached 20.7 while from 1971 to 2000 there were only nine.

Every year there are around 40 days with fog, 50 days with thunder and the number of storms are also increasing.

Weather is a complicated system that includes the atmosphere, the ocean, the biosphere, the cryosphere and the geosphere. There is a subtle energy balance at work. Extreme weather can be triggered by any subtle climate changes, says Kong.

Shanghai is a typical giant city with a "heat island effect."

The temperature difference between urban and suburban area in Shanghai has increased from 0.5 degree Celsius in 1960 to 1.2 degrees in 2000. The tall buildings concentrated in the urban area weakened the land wind that can help drive heat and air pollution out of the city.

The sharp temperature difference may also cause heavy rain and thunder when warm or cold air passes by. Both the sides of the Huangpu River are high-risk regions for extreme weather. And various high structures in the urban area also make local weather difficult to predict.

Sustained extreme weather can cause problems in agriculture, transport, communication and power supply. Giant cities like Shanghai are usually vulnerable in the face of such problems, according to Chen. Since the city is closely linked to other areas in development, weather disasters in other provinces may cause problems.

"All we can do is to put in place an effective and efficient counter-measure system to minimize possible weather damage," says Chen.

The forthcoming World Expo 2010 Shanghai requires more accurate, pinpoint forecasts.