Shelter is about wind and wet, not just rain
A healthy horse in a full winter coat is well insulated against straightforward cold. What strips condition from a grass-kept horse is the combination of wind and rain: wind flattens the coat's insulating layer of trapped air, and rain soaks through it, so a horse standing in a gale of driving rain loses heat many times faster than one standing in still frost. That is why the shelter a year-round outdoor horse truly needs is, first of all, a windbreak — something solid between the horse and the weather's direction. In summer the need reverses but does not disappear: the same horse needs shade from hard sun and a place where moving air and shadow give relief from biting flies, because a horse harassed all day grazes less and rests less than one left in peace.
Natural shelter versus a field shelter
Do not overlook what the field already offers. A thick, well-grown hedge, a belt of trees or the lee of a bank or barn wall gives deep, draught-free shelter over a wide area, and horses use natural shelter intelligently, lining up along it as the wind shifts. Many hardy natives winter out contentedly behind nothing more than a serious hedge. A built field shelter earns its place where natural cover is thin, where the field is exposed, or where several horses compete for the same sheltered strip — the hedge's best spot belongs to the boldest horse, while a three-sided shelter with a wide opening gives the whole group somewhere to stand. Whichever you rely on, the test is the same: on the worst night of the year, can every horse in the field get fully out of the wind and rain at the same time?
The dry lying area and winter ground
Standing shelter is only half the requirement. Horses do most of their deep sleeping lying down, and a horse will not lie for long on sodden ground — so a field that turns to a bog offers rest nowhere, and tired, mud-caked legs are the visible result. The outdoor horse needs a patch of ground that stays reasonably dry through winter: the raised floor of a field shelter, a hardcore standing area, or the free-draining top corner of the field, kept usable by moving hay feeding around so no single area is trampled into a swamp. Watch the gateways and the shelter entrance hardest; they fail first, and a horse choosing between deep mud and no shelter will often choose to stand in the weather.
The backup plan every grass-kept horse still needs
Living out is a system, not an absence of one, and the system needs a reserve. Sooner or later every field-kept horse needs to come in: for an injury that needs a dry box and rest, for illness, for the farrier in a storm, or simply because a fine-coated horse is losing condition faster than feed can replace it. That might be a stable of your own, a neighbour's spare box you can call on, or a shelter built well enough to bed down and shut — but it must exist before the night you need it. The same goes for water: a supply you can keep open in a hard frost, checked daily. Judge shelter horse by horse — the thoroughbred type shivering in November needs more than the cob beside him — and write down what each horse's arrangement actually is. KinsLe's Family Sharing keeps those turnout and shelter details on each horse's shared profile, so whoever checks the field in bad weather works to the same plan you would.