How it works — Turnout

Turnout gives two answers: what to put on, and how risky the grass is. This page shows exactly how each one is worked out. Every number here is read from the same code the app runs, so it can't fall out of step.

The rug

Find the coldest hour you're out

Only the hours inside your turnout window count. Of those, it takes the coldest feels-like temperature. Feels-like includes wind chill, and wind is what a horse in a field actually stands in. It uses the coldest hour rather than the average because the rug goes on at the start and has to cope with the worst of it.

Adjust it for your horse

Each horse feels the cold differently, so it knocks degrees off (or adds them) to get an effective temperature: how cold the day feels to this horse.

Read the rug off the ladder

Then the checks

Hot days

Horses cool mostly by sweating, and sweat only cools if it can dry. So as well as the temperature it uses the comfort index vets use: the temperature in °F plus the humidity in %. Whichever is worse sets the level for the hottest hour you're out.

On a hot day it also takes off a rain sheet, because warm rain on a bare horse helps cool them. Frost on the paddock picture means the air gets to 0° or below while they're out; sweat drops mean it's hot.

Where the rain figures come from

Weather forecasts come from a handful of big national models. Turnout asks Open-Meteo, which is a free way of reaching them, and it names the one it wants rather than taking pot luck:

Hourly rainfall at one spot is the least reliable thing any forecast produces — far softer than temperature or wind. So the app also asks two other models the same question, and tells you when they disagree. Three forecasts agreeing on rain at 11:00 is worth acting on; one of three is worth a waterproof in the car and no more. The rug itself always follows the best model for where your field is, never a show of hands.

Checked against the rugging charts

On a calm, dry day, where feels-like and air temperature are the same, the answers match the turned-out columns of the usual UK rugging charts at every temperature from −14° to 20°, for both unclipped and fully clipped horses. A trace clip always lands between the two columns. A test checks this every time the app is built.

It will sometimes pick a heavier rug than a chart. That's on a windy morning, because it uses wind chill and the coldest hour, where a chart uses the air temperature. The ladder stops at 300g, so below −10° for your horse it adds a warning to use a neck cover or liner as well.

Try the rug

Change anything and the answer updates. It's the same code the app uses, and the pony changes just as it does in the app: rain when it's wet, snowflakes when snow is forecast, lightning for thunderstorms, frost at 0° or below, sweat when it's hot and a heat haze when it's very hot.

Feels-like while out 5° Humidity 70%

Clip Type Age Forecast

Field shelter

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Grass and laminitis

Why the weather matters

Grass makes sugar whenever it's light, but it can only use that sugar to grow when the soil is warm enough, about 6°. When the two come apart, sugar is made and not spent, and it builds up in the leaf. That's why a frosty, sunny morning is the classic danger. Stressed grass in a dry spell stops growing too, so it hoards sugar in the same way.

Score the grass, 0 to 100

For the day of your turnout, it adds up points for each thing pushing the sugar up:

Plus a little for the time of year, because spring and autumn are when cases cluster:

Then the score is banded:

When the grass is lowest

After a mild night, early morning is genuinely the low point: the grass used up yesterday's sugar overnight.
After a cold night nothing was used overnight, so the day starts high and climbs. Shortening turnout helps more than changing when it starts.
After a frost there is no low point. The grass can't use its own sugar until the ground warms, often for days. It won't suggest a "better time" on these days, because there isn't one.

Then your horse, which matters more

Who the horse is matters far more than what the weather is doing. Each answer in your horse's profile carries a weight:

The two are combined like this:

Try the grass

Set up a day and a horse and see how the rating is made.

Coldest overnight 2° Warmest in the day 13° How bright 70% Rain in the last week 10mm

Month Where Laminitis before? Diagnosed Grazing

For this horse

What it can't know

It can't see your horse. It doesn't know if they're sweating under a rug, shivering, or have dropped weight. Slide a hand under the rug at the shoulder: warm and dry is right, damp or sweaty means too much.
Over-rugging is more common than under-rugging. Horses cope with cold and wet better than we expect, and when unsure it leans lighter on warm days.
The forecast can be wrong, and your field has its own weather: a hollow is colder, a hilltop windier.
The rugs are calibrated against UK rugging charts and the grass model is built on temperate, cool-season pasture like rye and fescue. It works anywhere with seasons like that, north or south of the equator. In hot climates, warm-season grasses store sugar differently, so treat the grass rating there as a rough guide only.
The laminitis rating is a weather guide with an allowance for your horse. It is not a veterinary assessment and can't tell you whether a horse is about to founder. If you suspect laminitis (pottery short steps, heat in the hooves, a strong digital pulse, rocking back onto the heels) ring your vet the same day.