The correct spelling is "Celsius," ending with the letter "s." "Today is 20 degrees Celsius" is NOT correct, but would be easily understood. "The temperature today is 20 degrees Celsius" is correct. "Today the temperature is 20 degrees Celsius" is equally correct. "Today it is 20 degrees Celsius" isn't absolutely correct, but is perfectly acceptable colloquial English. "Today it is 20 degrees Celsius outside" is correct. 20 degrees Celsius is a beautiful, perfect temperature when you can go outdoors without a jacket. -20 is read aloud using the words "minus twenty." "Today the temperature is minus 20 degrees Celsius" means it is bitterly cold, probably winter, cars may not start, and you'd better put on a warm coat befor eyou go outside!