corresponding to the so-called standard conditions, i.e., = 1 atm = 1013.25 mb = 101325 Pa and = 293.15 K (which is 20 °C = 68 °F). The relative humidity is a comfortable 35% (dew point of 4.1 °C = 39.4 °F). This corresponds to a partial pressure of water vapor of 820 Pa = 8.2 mb. In a vortex core, the pressure () drops to about 80% of the ambient pressure, i.e., to about 80 000 Pa. The temperature in the vortex core is given by the equation above as or 0.86 °C = 33.5 °F. Next, the partial pressure of water in the vortex core drops in proportion to the drop in the total pressure (i.e., by the same percentage), to about 650 Pa = 6.5 mb. According to a dew point calculator, that partial pressure results in the local dew point of about 0.86 °C; in other words, the new local dew point is about equal to the new local temperature. Therefore, this is a marginal case; if the relative humidity of the ambient air were even a bit higher (with the total pressure and temperature remaining as above), then the local dew point inside the vortices would rise, while the local temperature would remain the same. Thus, the local temperature would now be lower than the local dew point, and so the water vapor inside the vortices would indeed condense. Under the right conditions, the local temperature in vortex cores may drop below the local freezing point, in which case ice particles will form inside the vortex cores. The water-vapor condensation mechanism in wingtip vortices is thus driven by local changes in air pressure and temperature. This is to be contrasted to what happens in another well-known case of water condensation related to airplanes: the contrails from airplane engine exhausts. In the case of contrails, the local air pressure and temperature do not change significantly; what matters instead is that the exhaust contains both water vapor (which increases the local water-vapor concentration and so its partial pressure, resulting in elevated dew point and freezing point) as well as aerosols (which provide nucleation centers for the condensation and freezing). Formation flight One theory on migrating bird flight states that many larger bird species fly in a V formation so that all but the leader bird can take advantage of the upwash part of the wingtip vortex of