An immense band of low clouds covered most of central Europe in mid-November, 2012. The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite captured this true-color image on November 15 at 12:30 p.m. in London, England (12:30 UTC). At that time low clouds stretched from the French Alps in the south to the North Sea, and from the Irish Sea in the west to Belarus in the east. Local news sources report that the Eiffel Tower in Paris, France was partially obscured by heavy fog, and flights in the region were disrupted due to poor visibility. According to the BBC, Heathrow airport in London, England cancelled 78 services while London City Airport cancelled at least 20 flights through the day. The Woolwich Ferry service in London was also suspended during the morning. Fog is simply clouds formed at ground level. Like all clouds, fog forms when the local air reaches its dew point—the temperature at which water vapor condenses into tiny droplets. Sunlight typically warms the air and burns off fog, but as autumn advances and nights lengthen, fog becomes more common, especially in low-lying valleys. If unperturbed by winds, such fog can persist for days.