Weather: a combination of meteorological elements (such as temperature, humidity, etc. ) and weather phenomena (such as clouds, fog, etc. ) in the atmosphere of a certain region at a certain instant or a certain period of time. Which of the following is not the weather? ['Nanjing will be cloudy tomorrow with a maximum temperature of 2 ℃ and a minimum temperature of 0 ℃.', "It's snowing outside.", 'There are many foggy days this winter.', 'It frosted yesterday.']