Explanation:
When there is a sudden temperature change after a calm and cloudy weather then it is most probably due to the low pressure at the site. When there develops a low pressure at a site then to counter balance it the air from the high pressure area moves to the low pressure area and the moisture in the air also gets carried along to the mountainous cold region.
When the moist air flows over a cold region then the moisture of the air condenses and the atmosphere becomes cloudy and foggy with large quantity of precipitation.