LAHORE - Snowfall in hilly areas and change in wind direction has caused considerable decrease in the mercury level in plains, heralding the onset of winter. Weak westerly wave, moving across Northern Areas and parts of Punjab, has given snowfall over hills while change in wind directions caused rushing of cold air to plains that replaced prevailing warm air mass and caused falling of temperature during night time. "The weak westerly wave has given snowfall in Northern Areas and hilly areas of Punjab. Due to the activity of westerly wave, the accompanied cold air has replaced warm air mass in the plains. This has caused decrease in the temperature by 5-6 degree Celsius. Intensity of sunshine and its duration is also less during the second half of November that has caused sudden decrease in temperature," said meteorological expert Shaukat Ali Awan. He said that the temperature would further drop in the coming days, as a fresh westerly wave was moving across Iran. "The fresh westerly wave moving across Iran will affect Northern Areas in next 2-3 days. The cold air mass is prevailing over the region and the fresh westerly wave will not allow the warm air to caste its impact. After passage of westerly wave, cold air will rush that will cause further decrease in temperature in hilly areas and plains," Shaukat Awan said, adding, the decrease in temperature has heralded the onset of winter. He said that the prevailing cold and dry weather was causing diseases. He said that special attention should be given to children and elderly people to save them from prevailing cold and dry weather. According to the experts, a trough of westerly wave is moving across northern areas of the country. On Friday, maximum and minimum temperature in the City was recorded 23 C and 7 C respectively. Relative humidity in the morning was recorded 65 per cent, which decreased to 45 per cent in the evening. Local meteorological department has forecast mainly dry weather with mist in the morning for the City during the next 24 hours. Met office has forecast mainly dry weather for most parts of the country. However, light rain is expected over upper NWFP, Kashmir and Northern Areas.