Heat capacity of the air is dependent on the relative humidity (RH) of the air. Moist air is able to carry more heat (higher heat capacity), and therefore, a less quantity of air is required for taking away the same amount of heat. Hence, usually a certain RH is maintained in the cooling air when the spinning material is insensitive to moisture. If the polymer is sensitive to moisture, then RH has to be balanced so that there is no significant crystallization in the spinning filament due to the moisture present in the cooling air.

Also, the profile of the temperature within the cross-section of the filaments is not uniform and varies from outer surface to the inside core. The profiles can be estimated using a mathematical models. Figure 2.21 shows distribution of the temperature in the cross-section of a typical melt spun filament. As can be expected, the outer periphery of the filament is at a lower temperature than the centre of the filament due to the cooling effect of the air. Also the temperature is lower at the face of the filament which faces the incoming cooling air. In a multifilament spinning, the filament near to the zone from where the cooling air is coming is at a lower temperature compared to those which are far away. This effect is shown in Figure 2.22. This is because the temperature of the cooling air keeps on rising as it passes through initial rows of the filaments. Therefore the structure within the filament may be different from periphery to centre, from cooling face to opposite face, from initial rows of filament to final rows of filaments. Those at higher temperature are likely to have lower orientation due to polymer network relaxation and lower stresses.