A powerful sandstorm blowing over the Gulf has disrupted air traffic in Fujairah and Yemen, and closed schools and sent hundreds of people to hospital with respiratory problems. The visibility dropped to less than 500 metres in the northern emirates forcing the Fujairah airport to shut down until conditions improved. Officials also warned of fog at night and early on Monday. For regular weather update click here Cold Shamal (north-westerly) winds have set the conditions for fog formation. A drop in the wind speed last night was expected to contribute to the condensation of water vapour in the air. \"We expect a rise in humidity at night and early in the morning,\" said a forecaster. In Oman, a sandstorm warning until Wednesday prompted some schools to close early while social media was abuzz with speculation about the consequences of the bad weather. Oman has seen two tropical cyclones — Gonu, the first in 2007, caused extensive financial as well as material damage worth over one billion Omani riyals and nearly 50 people died. In 2010, another tropical cyclone Phet, landed in Oman causing widespread flooding and $780 million in material loss. Twentyfive people were officially reported dead. The Saudi ministry of education announced yesterday the closure of schools in the north, east and south-west of the desert kingdom, a day after similar measures were taken in Riyadh, the official SPA news agency reported. Hundreds of people, mostly children with asthma and allergies, were treated for breathing difficulties triggered by the storm. The storm also affected other Gulf countries, where weather centres warned drivers to use extra caution due to poor visibility and strong winds. In Yemen, the sandstorm disrupted air traffic at Sana\'a International Airport. At least two flights into the Yemeni capital were cancelled on Monday.