A "crystal sky” over the city on Thursday.

Blue sky with white clouds, dubbed a “crystal sky” by residents, was seen over the city on Thursday.

A “crystal sky” is when air pollutants are few and visibility high.

Air quality on Thursday was “excellent,” with the average concentration of PM2.5 around 10 micrograms per cubic meter and PM10 around 23 micrograms as of 6pm, according to the city’s ecology and environment bureau website.

The World Health Organization standard is for an average concentration of PM2.5 and PM10 should less than 25 and 50 micrograms per cubic meter, respectively, in a day.

The city’s air quality is expected to be excellent to good during the rest of the week, the bureau said.

The weather will be hot over the next three days with thunder showers on Friday. The maximum temperature from Friday to Sunday will be 34 to 35 degrees Celsius, according to the Shanghai Meteorological Bureau.