Weather 101

Air has weight, and where more of it is piled up the pressure is higher. Air flows from high pressure toward low pressure, like water running downhill. That flow is wind.

Wind is always named for where it comes FROM. A north wind, or a northerly, blows from the north toward the south. On a map the particles and arrows move the way the air travels, so a northerly’s arrows point south.

Watch out: currents are the other way round. A current “setting east” flows toward the east.

Isobars are lines joining places with the same air pressure, like contour lines on a hiking map. Pressure is in hectopascals (hPa), also called millibars; average sea-level pressure is about 1013 hPa. Charts usually draw a line every 4 hPa.

The closer the isobars, the steeper the hill and the stronger the wind. Tightly packed lines mean a blow; widely spaced lines mean light air.

Because the Earth spins, the air does not go straight from high to low. It is turned (the Coriolis effect) until it runs almost along the isobars. In the northern hemisphere wind goes anticlockwise around a low and clockwise around a high. South of the equator it is the reverse. Near the surface friction tips it slightly inward, toward the low.

A sailor’s trick, Buys Ballot’s law: in the northern hemisphere, stand with the wind at your back and the low is on your left.

At sea, wind is measured in knots: one nautical mile per hour, a little faster than a mile per hour (1 knot ≈ 1.85 km/h ≈ 0.5 m/s).

The Beaufort scale, from 0 (calm) to 12 (hurricane force), was written by sailors to describe what the sea looks like at each wind speed. It is still the quickest way to picture a number.

Gusts are the short bursts above the average wind. They are often a third to a half stronger, and more in showers. The gust is what heels the boat and breaks things, so plan for the gust, not the average.

Wind waves (the “wind sea”) are made by the wind blowing right where you are: short, steep and messy. Swell is wind waves that have travelled away from the storm that made them: long, smooth and regular. Most seas are a mix of both.

Wave height in a forecast is the significant wave height: roughly the average of the highest third of the waves, close to what an experienced eye would call the height. Expect bigger ones. About one wave in a few thousand reaches twice the significant height.

Period is the time between two crests, in seconds. Long period (10 s and more) means long, gentle swells. Short period means steep, close waves that slam. Two metres at 12 seconds is a lazy roll; two metres at 5 seconds is a beating.

Waves grow with three things: wind strength, how long it has blown (duration), and the stretch of open water it blows over (fetch). That is why a strong wind off a nearby shore gives a flat sea.

A weather model is a program that takes millions of observations (from satellites, balloons, ships, planes and buoys), builds a picture of the whole atmosphere, and steps it forward with the laws of physics, a few minutes at a time.

It works on a grid. Each box holds one value for wind, pressure and so on. NOAA’s GFS publishes on a grid about 25 km across. ECMWF, the European model, computes at about 9 km, but its free open data, which our map draws, comes on the same 25 km grid. Anything smaller than a few boxes (a narrow bay, a headland, a single squall) a global forecast cannot see. Newer AI models such as Google DeepMind’s WeatherNext learn from decades of past weather instead of solving the physics step by step.

Models run several times a day, at 00, 06, 12 and 18 UTC. Each forecast has two times: the RUN time, when the model started, and the VALID time, the moment the forecast is for. “12Z run, +36 h” means started at 12:00 UTC, valid 36 hours later.

Z (Zulu) means UTC, the clock every forecast uses. Convert to your own time before you plan.

The atmosphere is chaotic: a tiny error in today’s picture grows every hour. So forecasts are very good for a day or two, good to about five days, and after that show the trend more than the detail.

The best free check is to compare models. When GFS, ECMWF and WeatherNext agree, confidence is high. When they disagree, the honest answer is “not sure yet”: wait for the next runs and look again.

Forecasters also run a model many times with slightly different starting points, an ensemble. A tight bundle of answers means confidence; a spread means uncertainty.

A front is the border between two air masses. On a chart a cold front is a blue line with triangles, a warm front a red line with half-circles; the symbols point the way the front moves.

Ahead of a cold front in the northern hemisphere the wind is often southwest and the pressure falls. As it passes: a line of cloud, squalls and heavy showers, gusts, and the wind veers (turns clockwise) to the west or northwest. Behind it, pressure rises, the air is cooler and clearer, and the wind can stay gusty.

Veer means the wind shifts clockwise (south to west); back means anticlockwise (west to south). A falling barometer means a low or a front is coming: a drop of more than about 3 hPa in three hours is a warning.

Closer to shore, the sea breeze is a small front of its own: on a sunny day the land heats, the air over it rises, and cool sea air rushes in, often by early afternoon.

Tap a card to flip it. These are the words you will meet in forecasts and on the map.