To clarify - I seal hives fairly often - probably 50% of the time for crop spraying, and 50% of the time for my own convenience - such as mowing, house repairs/ maintenance, - and of course as an immediate step should robbing ever break out.
Mowing doesn't seem to bother them very much - it's strimming which sends them ballistic. I guess the high-pitched whirring must sound like a super-sized hornet to them.
When crop spraying is taking place, or when I need to nip robbing in the bud, they stay 'locked-up' all day. When mowing, the hives are opened just as soon as I've finished.
When strimming, I allow at least 10 minutes, maybe a little longer - then I start opening the hives at the far end first, and work my way along the lines of hives. I've never once had any kind of problem when opening hives which would require smoke or a veil. In contrast, when I've left hives open, I've been attacked when spraying weedkiller 20 feet from a hive. It was probably the high-pitched 'fizz' from the knapsack-sprayer lance which they objected to, as they attacked the arm and hand which was holding the lance.
Keeping a hive locked-down - even in summer - isn't a problem providing the hive has adequate ventilation, such as an open mesh floor, and a crown board (inner cover) with good sized holes which can be covered in mesh.
If the sun is really baking hot, then it would certainly pay to fit some kind of awning over a locked-down hive, in order to provide shade. Excessive sun isn't normally an issue in Britain, but might be elsewhere ...
LJ