Krakow is Poland's second-largest city and the John Paul II International Airport has direct flights to many European cities including London, Paris, and Frankfurt. Visas are not required for visits of less than 90 days for citizens of many countries, including the United States. Public transport is cheap and clean, and many sights are within walking distance of the main square. A Krakow Tourist Card gives free access to the city's main museums and costs 100 złoty (around $26) for 48 hours. Electricity supply is 220 volts and a standard European two-pin plug or adapter is needed. Spring and fall are ideal times to visit, away from the oppressive summer heat and the cold winter's bite.