rise to a different altitude to catch wind in a returning direction. With experience, luck, and the right conditions, some pilots are able to control a precision landing at the destination. On rare occasions, they may be able to return to the launch site at the end of the flight. This is sometimes called a box effect, when winds at altitude flow in the opposite direction of surface winds. Hazards The dangers of the sport include excessive (vertical or horizontal) speed during landing, mid-air collisions that may collapse the balloon, and colliding with high voltage power lines. It is the last of these, contact with power lines, that poses the greatest danger. One of the most common causes of serious ballooning accidents in the US is power line strikes. One reason for the high frequency of such incidents is the fact that pilots often attempt to land their balloons on or near roads in order to reduce the amount of off-road driving necessary to recover the balloon. However, in most rural areas where balloons fly, roads usually have power lines running along with them. Of the 11 accidents involving fatalities recorded by the NTSB between 1997 and 2007, 4 involved contact with power lines, 3 involved falling after hanging onto the outside of a rising balloon, 3 involved striking an object on landing (boulder, wall, or tree), and 1 involved an equipment failure (an eyebolt). Winter flight The ability to fly hot air balloons in the winter is limited mostly by the ability of the participants to withstand the cold. The balloons themselves fly well in cold air. Because the temperature difference between inside and outside the balloon, not the absolute inside temperature, determines the lift it develops, a much lower internal temperature is sufficient to fly in cold weather. However, if the liquid propane in the fuel tanks is too cold (0 °C/32 °F or less) it does not generate sufficient vapor pressure to adequately feed the burner(s). This can be overcome by charging the fuel tanks with inert gas such as nitrogen or by warming them, with electric heat tapes for example, and insulating them against the cold. Tethering Sometimes, especially at balloon festivals or other special events, balloons are flown while tied to the ground with ropes (tethers). This enables quick rides to many passengers, instead of long rides drifting with the wind away from the event