The 7-day cycle of human activities may lead to the "weekend effect" in climate variables and air pollutants. The weekend effect is defined as the average value (e.g., the diurnal temperature range) for Saturday through Monday minus the average value for Wednesday through Friday. A composite of the ground observations over northeastern China presents that, during 26-year (1980-2005) summers, the weekend effect in the diurnal temperature range increased by 1.2°C. Conversely, the weekend effects in the relative humidity, cloud amount, and light rain (≤5 mm day-1) events decreased. These changes are due to a shifted phase of the weekly cycle of the meteorological variables. The long-term change in weekend effects have a high correlation coefficient (|r| 0.8) with the decrease in relative humidity over the region, which is likely induced by regional warming. The results suggest that regional warming is a possible factor in a transition of dominant aerosol effects in the weekend effect.