What do you mean 40-20 today?

Today, the weather is over 40-MINUS 20, which means that the high temperature of 40 degrees Celsius has dropped to 20 degrees Celsius. According to relevant data, the temperature difference in some places is large, reaching 40 degrees at noon and dropping to 20 degrees in the afternoon, while in some places it is a continuous high temperature warning. Red warning of high temperature above 40 degrees, high temperature warning signal is divided into three levels, which are represented by yellow, orange and red respectively.