Question 61: While someone was observing Sawm (Fast) during the month of Ramadaan, there appeared heavy fogs in the sky after `Asr (Afternoon) Prayer. Being in a desert, he thought that the sun had set. After breaking his Sawm, the sun appeared. What is the ruling on his Sawm? Should he complete his Sawm for this day? Is his Sawm rendered invalid? Should he make up for that day? Or should he offer Kaffaarah (expiation)?
Answer: If the reality is as you mentioned, then his Sawm will be invalid and he has to continue his Sawm until sun sets… read more here.