Relative humidity and air temperature

[canaby]

English Language Tests, Intermediate level

ESL/EFL Test #173 "Simple Present, Simple Past, Simple Future", question 6

The relative humidity usually ......... when the temperature goes down.

(a) falls
(b) fell
(c) will fall

English Language Tests, Intermediate level

ESL/EFL Test #173 "Simple Present, Simple Past, Simple Future", answer 6

The relative humidity usually falls when the temperature goes down.

Correct answer: (a) falls

Your answer was: incorrect
The relative humidity usually will fall when the temperature goes down.
_________________________

give its explanation

canaby

[Mister Micawber]

.
The percentage of water vapour in the air usually decreases when the air temperature decreases. Warm air holds more water than cool air.
.
_________________
Native English teacher at Mister Micawber's

[Don Geraldos]

But is there anything grammatically wrong with saying:

'The relative humidity usually will fall when the temperature goes down.'?

A little tortuous, perhaps, but grammatically no different from saying:

''The relative humidity will usually fall when the temperature goes down.'