<P> Tropical cyclones are known to form even when normal conditions are not met . For example, cooler air temperatures at a higher altitude (e.g., at the 500 hPa level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certain lapse rate is required to force the atmosphere to be unstable enough for convection . In a moist atmosphere, this lapse rate is 6.5 ° C / km, while in an atmosphere with less than 100% relative humidity, the required lapse rate is 9.8 ° C / km . </P> <P> At the 500 hPa level, the air temperature averages − 7 ° C (18 ° F) within the tropics, but air in the tropics is normally dry at this height, giving the air room to wet - bulb, or cool as it moistens, to a more favorable temperature that can then support convection . A wetbulb temperature at 500 hPa in a tropical atmosphere of − 13.2 ° C (8.2 ° F) is required to initiate convection if the water temperature is 26.5 ° C (79.7 ° F), and this temperature requirement increases or decreases proportionally by 1 ° C in the sea surface temperature for each 1 ° C change at 500 hpa . Inside a cold cyclone, 500 hPa temperatures can fall as low as − 30 ° C (− 22 ° F), which can initiate convection even in the driest atmospheres . This also explains why moisture in the mid-levels of the troposphere, roughly at the 500 hPa level, is normally a requirement for development . However, when dry air is found at the same height, temperatures at 500 hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres . At heights near the tropopause, the 30 - year average temperature (as measured in the period encompassing 1961 through 1990) was − 77 ° C (− 132 ° F). A recent example of a tropical cyclone that maintained itself over cooler waters was Epsilon of the 2005 Atlantic hurricane season . </P>

The surface salinity in the central parts of the world's oceans is determined by