You've packed the sunscreen, the flask and the dog. The forecast says 20°C and sunny. Then you step onto the beach and get sandblasted by a force five, squint into a wall of sea fret, and watch six-foot waves explode over the promenade. Coastal weather is different. Reading the forecast properly — beyond the temperature — is the difference between a glorious day out and a miserable one.

Start with the wind, not the temperature

Temperature is the headline, but wind is the plot. On the coast, a 15 mph wind can make 20°C feel like 12°C, especially when you're wet from paddling or sweating up a cliff path. Look for sustained wind speed and gusts. Gusts are what knock you off balance on an exposed path and whip up sand. If gusts are more than 10 mph above the average, plan for shelter.

In the UK, forecasts often give wind in mph for land and knots for marine. One knot is roughly 1.15 mph, so 20 knots is about 23 mph. The Beaufort scale is your friend: force 3 (7–10 knots) is a gentle breeze, force 5 (17–21 knots) means small trees sway and you'll need a windproof layer. Force 6 and above? Save the cliff walk for another day.

Direction makes all the difference

A 20 mph wind from the south-west feels completely different depending on which way the coast faces. Onshore winds blow from sea to land — they bring cooler air, bigger waves and sand in your sandwiches. Offshore winds blow from land to sea — they can make the water deceptively calm near the shore, but they also carry inflatables out to sea. Never take a blow-up kayak or paddleboard out in an offshore wind.

Check the forecast's wind arrows against a map. If you're heading to a north-facing beach and the wind is from the south, you'll be in shelter. That same wind will be brutal on a south-facing cove. A sea breeze can also develop on hot, still days: by early afternoon, cool air rushes inland, creating a sudden onshore wind of 10–15 mph. Plan your paddle before noon.

Swell height and period: what the waves will do

Swell height is the average height of the highest third of waves, measured in metres. A 1-metre swell is manageable for most beach walks. Two metres means waves will be breaking high on the beach and over promenades. But height alone isn't enough. Look at the swell period — the time between wave crests, in seconds. A 1-metre swell with a 15-second period carries far more energy than a 1-metre swell with a 6-second period. Long-period swell can close off a beach, surge up slipways and make rock pooling dangerous.

Also check the sea state description: slight, moderate, rough. For a seaside walk, moderate is usually fine on a high path. For swimming or rock hopping, wait for slight. And remember the tide: a low tide rock platform can be underwater in an hour.

Visibility and sea fret

Visibility is measured in metres or kilometres, and it matters for two things: safety and scenery. A forecast of "good" visibility means you'll see the cliffs, the lighthouse and the islands. "Moderate" (2–5 km) means hazy horizons. "Poor" (under 1 km) means fog, mist or sea fret — that cold, damp blanket that rolls in on a warm day. Sea fret is common on the north-east coast in spring and early summer. If visibility is under 500 metres, stick to low-level promenade walks and avoid cliff edges. You won't see the view anyway.

Tides, rain and timing your day

Tides are not optional. A beach that's huge at low tide can vanish at high tide, trapping walkers against cliffs. Check high and low tide times plus the tidal range. Spring tides (around new and full moon) have the biggest range — more beach exposed, but also faster incoming water. Neap tides are gentler. Aim to walk on a falling tide, and always know your escape route.

Rain in a coastal forecast is often showery. A 40% chance of rain might mean a brief downpour, not a washout. Look at the precipitation timeline: a dry morning with showers after 3pm is perfect for a beach day. Also check UV — sea breezes don't stop sunburn, and reflection off water and sand can double the dose. And don't ignore temperature: a 12°C day with 30 mph wind is a winter coat day, not a t-shirt day.

Your five-minute coastal forecast check

Before you leave, run through this list:

• Wind: sustained speed, gusts, direction relative to the coast.
• Swell: height in metres, period in seconds, sea state.
• Visibility: under 1 km? Stick to low, familiar paths.
• Tides: high and low times, spring or neap, range.
• Rain: timing of showers, not just the daily chance.
• Temperature and UV: wind chill, sun protection, layers.

Get these right and you'll enjoy the coast in any season — not just the sunny days. The sea doesn't care about your picnic plans, but a good forecast helps you work with it.