leidora world weather map by month

• 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.
• Glaciers and ice sheets read Glacier: the model holds them at a fixed 10 m of water instead of tracking snow, so a number there would mean nothing.

• 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 Clouds.

• 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. Day length and sun height are what Sunlight shows.
• It rarely falls much below 60%, because cloud still lets diffuse light through: 90% and above is reliably sunny, 75–90% mostly sunny, below 65% often grey, below 50% the heart of a monsoon.
• On the CLOUDS layer a sunny month leaves the map clear, and the cloudier it is, the denser the grey. The cells are coarse, so a small island or a sunny coast can take the value of the sea or hills beside it.

• 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.

Where it comes from. The 27–32 °C afternoon is what beach tourists called ideal in a survey of about 850 northern Europeans (Rutty & Scott 2010). The rest is Leidora’s own: the weights, the sea, rain and sun ranges, tuned against places with a known beach season, and the coastal correction, measured against weather stations.

How pleasant a month’s weather is for exploring a city on foot, 0–100, on every land cell. It is the Holiday Climate Index for cities (HCI:Urban, Scott et al. 2016), built from surveys of what tourists say they prefer, with two changes for a world map (below).

Classes (the paper’s own)

• Ideal 90+, Excellent 80–89, Very good 70–79, Good 60–69, Acceptable 50–59, Marginal 40–49.
• Unacceptable, below 40: too hot, cold, wet or stormy to recommend (grey on the map).

What goes into it

• Comfort, 40%: the afternoon high as it feels with the humidity. Best at 23–25 °C felt, gentle on the cold side, steep above 30 °C.
• Rain, 30%: full marks when dry, nothing above 12 mm/day.
• Sky, 20%: cloud cover; a few clouds (11–20%) rate best.
• Wind, 10%: a light breeze rates best, a gale below nothing.

Changed from the paper. Humid heat counts as hotter and dry heat is no longer counted as cooler than the air (half the US weather service’s heat index, at the humidity of the afternoon). And a month cannot score more than its temperature allows, so a clear, dry, −40 °C January no longer rates as well as a mild one: a felt afternoon of −6 °C or colder, or one as hot as Dubai’s summer, scores at most 30, Unacceptable.

Monthly averages read a little kinder than day-by-day scoring: Paris runs about 10 points above the paper’s own figures in every month, so the order is reliable and the labels are on the generous side. Air pollution is not part of it.