Hong Kong sweltered in a heatwave and choking air pollution yesterday as a storm approached the south China coast. The Observatory issued the standby storm signal at 4.45pm yesterday after a tropical depression entered the 800-kilometre alert zone. The storm was centred at about 640 kilometres southeast of Hong Kong at 7pm yesterday and was moving in a northwestly or westerly direction at 20km/h. It was forecast to head towards Zhanjiang in Guangdong or Hainan if its track was unchanged. The Observatory said last night that it was too early to say whether a higher alert would have to be issued today, as it would depend on the track and intensity of the storm. But it forecast the weather to become cloudy with squally rain today, with temperatures between 29 and 31 degrees Celsius. Winds will pick up tomorrow, with some showers. The weather is forecast to remain unstable next week. The very hot weather warning remained in force throughout yesterday. The Observatory recorded this year's highest maximum temperature so far of 33.1 degrees at its headquarters in Tsim Sha Tsui. In other places, such as Sha Tin, the mercury rose to 36.4 degrees. The lack of wind meant pollutants in the air were not dispersed, and the city was blanketed in a grey layer of particles during the day. The air pollution index hit 150 in Tap Mun and Tung Chung, where the ozone level was also very high. In Central and Mong Kok, the roadside air quality started to deteriorate in the evening, with the index rising to above 100.