The ongoing spell of heatwave is predicted to tighten its grip on Delhi and no relief is in sight for at least six days, the India Meteorological Department said on Tuesday. The maximum temperature at the Safdarjung Observatory, considered the official marker for the city, is expected to touch the 40-degree mark on Wednesday and soar to 42 degrees Celsius by Friday. The base station had recorded a maximum temperature of 39.6 degrees Celsius on Wednesday last week, the highest this season so far. The weather stations at Ridge, Najafgarh, Pitampura and Sports Complex recorded a heatwave on Tuesday, with their maximum temperature settling at 40.4 degrees Celsius, 40.2 degrees Celsius, 40.6 degrees Celsius and 40.7 degrees Celsius, respectively. The Safdarjung Observatory recorded a maximum temperature of 38.7 degrees Celsius, four notches above normal.