The Observatory says radioactive material from Fukushima nuclear plant is unlikely to reach Hong Kong.
Predicting the probable trajectory of spewed materials from the damaged plant more than 3,000 kilometres from Hong Kong, the weather scientists said the city would be under the influence of a westerly air flow from China until Friday.
'All the radioactive materials in Fukushima will be blown east towards the Pacific Ocean by the westerly and they will not reach Hong Kong,' said Leung Wing-mo, assistant director of the Observatory in charge of radiation monitoring.
Edmund Leung Kwong-ho, chairman of the city's Energy Advisory Committee, said any pollutants that travelled far would be greatly diluted during the journey.
'Substances billowing out from a stack will normally have their concentration diluted by 30,000 times for every 5km they travel,' he told government officials.
But Leung also said it was possible that radioactive pollutants might be present in rain.
Three explosions at reactors at the Fukushima plant in northern Japan damaged by the post-quake tsunami have raised fears that radioactive material might spread to other parts of Japan or even further.
Tokyo, about 250km from Fukushima, has reported an increase in radioactivity. But Taiwan, between Hong Kong and Japan, has not recorded any unusual readings.
The Hong Kong Observatory, which runs a network of 10 stations to monitor radiation levels, said there was no sign of unusual fluctuations in radiation levels in Hong Kong.
It said ambient readings had been normal - around 0.1 microgray per hour, equivalent to the health impact of a radiation dose of 0.0001 millisieverts.
A Hongkonger is exposed to an average of three millisieverts per year naturally.
Nevertheless, the Observatory has increased the frequency of air sampling, checking for radioactivity every day instead of once each few days. All samples are analysed at the Observatory's laboratory.
Local nuclear specialists also played down fears about imminent radiation threats.
But they agreed there had not been enough information about the nuclear crisis, making it difficult for observers to assess the real situation.
'The information released has been very confusing, and sometimes conflicting,' said Professor Woo Chung-hoo, a nuclear specialist at Polytechnic University. 'Whether the worst will happen will hinge on whether the reactor container remains intact. But even in the worst case, a repeat of Chernobyl is unlikely,' he said.
City University president Way Kuo, an expert on nuclear safety, said cold weather in Japan would probably hinder dispersion of leaked radioactive substances. Cooler air was likely to keep them closer to ground level.
A long way to blow
The HK Observatory says winds are blowing radioactive material out into the Pacific. Tokyo is this many kilometres from Hong Kong: 3,000