<Table> <Tr> <Td> </Td> <Td> The examples and perspective in this article deal primarily with the Northern Hemisphere and do not represent a worldwide view of the subject . You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate . (August 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> The examples and perspective in this article deal primarily with the Northern Hemisphere and do not represent a worldwide view of the subject . You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate . (August 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> A snow day in the United States and Canada is a day that school classes are cancelled or delayed by snow, heavy ice, or extremely low temperatures . Similar measures occur in response to flooding, tornado watches, and severe weather (storms, hurricanes, dense fog, heavy snow, etc .). The criterion for a snow day is primarily the inability of school buses to operate safely on their routes and danger to children who walk to school . Often, the school remains officially open even though buses do not run and classes are canceled . </P> <P> More and more schools are closing due to extreme heat, "heat days". Heat days are like snow days . Some districts (San Diego Unified) provide schools with an operational procedure that is used as a guideline to address what actions schools should take at different levels of rising temperatures . </P>

When do schools have to close due to cold weather
find me the text answering this question