Each year a tropical weather system known as the intertropical front crosses Chad from south to north, bringing a wet season that lasts from May to October in the south, and from June to September in the Sahel. Variations in local rainfall create three major geographical zones. The Sahara lies in the country's northern third. Yearly precipitations throughout this belt are under ; only occasional spontaneous palm groves survive, all of them south of the Tropic of Cancer.