<p>Eastern China (EC), located on the downstream region of Tibetan Plateau (TP), is a large area that has become vulnerable to frequent haze. In addition of air pollutant emissions, meteorological conditions were a key <q>inducement</q> for air pollution episodes. Based on the study of the Great Smog of London in 1952 and haze pollution in EC over recent decades, it is found that the abnormal <q>warm cover</q> in the middle troposphere, as a precursory <q>strong signal</q> hidden, could connect to severe air pollution events. The convection and diffusion in the atmospheric boundary layer (ABL) were suppressed by a relatively stable structure of <q>warm cover</q> in the middle troposphere, leading to the ABL height decreases, which were favourable for the accumulation of air pollutants in the ambient atmosphere. The warming TP built the <q>warm cover</q> in the middle troposphere from the plateau to the downstream EC region and even the entire East Asian region. The frequent haze events in EC is connected with a significantly strong <q>warm cover</q> in the interdecadal variability. It is also revealed that a close relationship existed between interannual variations of the TP's heat source and the <q>warm cover</q> hidden in the middle troposphere over EC.</p>