• Air temperature 2 m above the ground, averaged over all 24 hours of every day in the month: nights pull it well below what a day feels like.
• For what the day and the night feel like, use Daily Max and Nightly Min.
• A cell is an average over its whole area, so a cell spanning coast and hills reads cooler than the beach town in it.

• The average afternoon high and night-time low: each day’s true maximum and minimum from ERA5-Land’s daily statistics, averaged over the month.
• The gap between them is the day–night range: small on coasts and in the humid tropics, large in deserts and mountains.
• On coasts the grid tends to read the afternoon high 2–3 °C too low compared with weather stations (it mixes in sea air). The map shows the grid’s value; the beach score corrects for it (see below).

• Average rain and snow (as water) per day, in mm. Multiply by 30 for the monthly total: 2 mm/day is about 60 mm a month.
• Rough guide: under 1 mm/day is dry, 1–3 occasional showers, 3–6 regularly wet, above 6 a rainy season.
• It says how much falls, not how often: 2 mm/day can be a short tropical downpour every afternoon or a grey drizzle all day.

• Snow on the ground as water equivalent (mm): what the snow would make if melted.
• Real snow is several times deeper: fresh snow roughly 10×, old packed snow 3–5×. 50 mm here is very roughly 15–50 cm on the ground.
• A monthly average over a whole cell, so it says little about a particular ski slope.

• The sun’s energy reaching the ground, in W/m², averaged over all 24 hours including the night.
• It depends on clouds and on the season and latitude: short winter days and a low sun give low values even under clear skies. About 50 is a northern winter; 250–330 a desert summer.
• For “how sunny is the sky” regardless of season, see Clear Sky.

• The share of the sunshine a cloudless sky would deliver that actually reaches the ground (ERA5, 0.25°, about 28 km cells).
• It measures clouds and haze only, so a clear October day scores as well as a clear July day, though the sun is weaker.
• It rarely falls much below 60%, because cloud still lets diffuse light through: above 90% is reliably sunny, 75–90% mostly sunny, below 75% often overcast.

• Monthly average sea surface temperature of the nearest sea cell within 60 km (ERA5).
• It is the open water nearby: shallow bays and lagoons run warmer, upwelling coasts colder.
• Rough guide for swimming: 18 °C bracing, 22 °C pleasant, 25–28 °C warm.

How good a month is for a beach holiday, 0–100, on coastal cells only. Inland cells, and coasts with no sea temperature within 60 km, are left blank.

Classes

• Excellent 90+: hot, sunny, dry, warm sea: the best the coast offers.
• Great 80–89: near ideal; one thing slightly off.
• Very good 70–79: a good beach month with a compromise, such as a cooler sea.
• Good 60–69: beach days likely, but not reliably: a shoulder season.
• Fair 45–59: possible on the better days; plan something else too.
• Poor 30–44: mostly too cool, wet or cloudy.
• Unsuitable, below 30: not beach weather (grey on the map).

What goes into it

• Air, 30%: the afternoon high, corrected up by 1.8–2.7 °C on coasts to match weather stations. Full marks at 27–32 °C, best in the middle, falling off quickly below and above.
• Sea, 35%: the sea temperature above. Full marks at 22–28 °C, best around 25 °C.
• Rain, 20%: no penalty up to 1.5 mm/day (brief showers), then falling steadily.
• Sun, 15%: Clear Sky, full marks from 90%.

The parts are multiplied, not added (a weighted geometric mean), so one bad part pulls the whole score down: perfect air cannot make up for a cold sea, which is why upwelling coasts such as Namibia or Peru score low despite sunny, dry summers. Example: Salento in October, with 22 °C afternoons, a 23 °C sea and some rain, scores in the 60s: Good, not Excellent.

A cell is about 11–22 km across, so the score describes the coast’s climate, not one particular beach. Wind, waves and water quality are not part of it.

• Composite metric (0–100%)
• Combines temperature, precipitation and sun
• Optimal: 24–28 °C with low rain