Final answer:
At -40 degrees Celsius, the capacity of air to hold water vapor is extremely low due to the very low vapor pressure at this temperature. The vapor pressure of ice is 0.6 torr at -23°C and would be even lower at -40°C.
Step-by-step explanation:
The amount of water vapor that air can hold depends on its temperature. At -40 degrees Celsius, the air can hold significantly less water vapor compared to warmer air because the vapor pressure of ice (which has a relationship with the amount of water vapor that air can contain) is very low at this temperature. The vapor pressure of ice at temperatures just slightly above -40 degrees is on the order of torr. For example, at -1°C, the vapor pressure is about 4.2 torr, while at -23°C, it drops to only 0.6 torr. Since -40°C is even colder, the vapor pressure, and thus the capacity of the air to hold water vapor, would be correspondingly less.
While an exact number for the vapor pressure at -40°C wasn't provided, it can be inferred that it is lower than at -23°C. Hence, one can conclude that the amount of water air can hold at -40°C is extremely low, allowing for only a minuscule amount of water vapor to coexist with the air before condensing or subliming.