of reducing induced drag is the use of winglets, as seen on most modern airliners. Winglets increase the effective aspect ratio of the wing, changing the pattern and magnitude of the vorticity in the vortex pattern. A reduction is achieved in the kinetic energy in the circular air flow, which reduces the amount of fuel expended to perform work upon the spinning air. After NASA became concerned about the increasing density of air traffic potentially causing vortex related accidents at airports, an experiment by NASA Ames Research Center wind tunnel testing with a 747 model found that the configuration of the flaps could be changed on existing aircraft to break the vortex into three smaller and less disturbing vortexes. This primarily involved changing the settings of the outboard flaps, and could theoretically be retrofitted to existing aircraft. Visibility of vortices The cores of the vortices can sometimes be visible when the water present in them condenses from gas (vapor) to liquid. This water can sometimes even freeze, forming ice particles. Condensation of water vapor in wing tip vortices is most common on aircraft flying at high angles of attack, such as fighter aircraft in high g maneuvers, or airliners taking off and landing on humid days. Aerodynamic condensation and freezing The cores of vortices spin at very high speed and are regions of very low pressure. To first approximation, these low-pressure regions form with little exchange of heat with the neighboring regions (i.e., adiabatically), so the local temperature in the low-pressure regions drops, too. If it drops below the local dew point, there results a condensation of water vapor present in the cores of wingtip vortices, making them visible. The temperature may even drop below the local freezing point, in which case ice crystals will form inside the cores. The phase of water (i.e., whether it assumes the form of a solid, liquid, or gas) is determined by its temperature and pressure. For example, in the case of liquid-gas transition, at each pressure there is a special "transition temperature" such that if the sample temperature is even a little above , the sample will be a gas, but, if the sample temperature is even a little below , the sample will be a liquid; see phase transition. For example, at the standard atmospheric pressure, is 100 °C = 212 °F. The transition temperature decreases with decreasing pressure (which explains why