What would cause condensation with everything being equal on both sides of the cover, including air temperature?
If it has vents in it then both sides can become equal.
Without vents as the cold air heats up and rises it can not get away because the cover traps it.
The hot air draws water out of the ground which is trapped by the cover so your mower is in hot wet air all the time .
When the temperature drops, the water condenses on the cover & your mower.
Overnight more cold air is drawn in from under the cover and the cycle continues.
Look at car covers.
All the outdoor top quality covers have vents in them to prevent this happening.
I use motorcycle covers on my mowers because they are vented.
To prevent condensation covers should go on before dewfall in the evening, come off , get turned inside out before the dew has dried off in the morning then get replaced when dry & the mower is a little warm to prevent UV damage then come off in the cool of the afternoon to go back on before dewfall.
The ground under the mower / car/ bike must be sealed to prevent the heat evaporating moisture out of the ground which gets trapped by the cover.
Inside a shed, under an awning or in a spot that is always shady things are different.
Without the cover the mower gets wet overnight from dew but during the day the dew dries off because of the suns heat and the air circulation so it is only wet during the night when it is cool so any corrosion will be very slow as corrosion is a Redox reaction and as such is almost nill when cold and increases exponentially as the temperature rises so when the air under the cover is both hot and moist corrosion happens at its peak rate.
This is why things rust so much faster in the tropical enviroments than they do in artic enviroments
