An air show is an event at which aviators display their flying skills and the capabilities of their aircraft to spectators in aerobatics. Air shows without aerobatic displays, having only aircraft displayed parked on the ground, are called "static air shows". Some air shows are held as a business venture or as a trade event where aircraft, avionics and other services are promoted to potential customers. Many air shows are held in support of local, national or military charities. Military air firms often organise air shows at military airfields as a public relations exercise to thank the local community, promote military careers and raise the profile of the military. Air show "seasons" vary around the world. Whereas the United States enjoys a long season that generally runs from March to November, other areas often have much shorter seasons. The European season usually starts in late April or Early May and is usually over by mid October. The Middle East, Australia and New Zealand hold their events between January and March. However, for many acts the "off season" does not mean a period of inactivity, they use time for maintenance and practice. The type of displays seen at an event are constrained by a number of factors, including the weather and visibility. Most aviation authorities now publish rules and guidance on minimum display heights and criteria for differing conditions. In addition to the weather, pilots and organizers must also consider local airspace restrictions. Most exhibitors will plan "full," "rolling" and "flat" display for varying weather and airspace conditions. The types of shows vary greatly. Some are large scale military events with large flying displays and ground exhibitions while others held at small local airstrips can often feature just one or two hours of flying with just a few stalls on the ground. Air Displays can be held during day or night with the latter becoming increasingly popular. Shows don't always take place over airfields; some have been held over the grounds of stately homes or castles and over the sea at coastal resorts.