# Weather 101 > A free, hands-on weather course: wind, isobars, highs and lows, waves and period, weather models and fronts, then a quiz. No jargon, no maths. Page: https://www.freethewind.com/wx-101 Weather is not magic and it is not just for meteorologists. Eight short lessons, a few things to slide and tap, and a quiz at the end. By the last page you will read a forecast map like a sailor. ## Lesson 1: Wind is air going downhill *Basics.* Know where wind comes from, and which way a forecast arrow points. 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. Remember: Wind is named for where it comes from. Currents are named for where they go. ## Lesson 2: Highs, lows and isobars *Basics.* Read wind strength and direction straight off a pressure chart. 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. Remember: Tight isobars, strong wind. Lows spin anticlockwise in the north, clockwise in the south. ## Lesson 3: How strong is strong? *Basics.* Turn a number in knots into what you will see and feel. 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. Remember: Plan for the gusts. The average wind is the easy part. ## Lesson 4: Waves, swell and period *Basics.* Tell a comfortable sea from a nasty one, at the same height. 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. Remember: Height tells you how big. Period tells you how nasty. ## Lesson 5: What a weather model is *Basics.* Know what GFS and ECMWF are, and read a forecast’s run and valid time. 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. Remember: Every forecast has a run time and a valid time. Fresher runs are usually better. ## Lesson 6: How sure is the forecast? *Going further.* Judge how much to trust a forecast five days out. 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. Remember: Models agreeing is confidence. Models disagreeing is information too. ## Lesson 7: Fronts and what they bring *Going further.* Recognise a cold front coming and know what it will do to the wind. 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. Remember: Falling pressure, then a veer and gusts: that is a cold front passing. ## Lesson 8: Speak weather *Going further.* The words forecasts use, in plain language. Tap a card to flip it. These are the words you will meet in forecasts and on the map. - **Knot**: One nautical mile per hour, about 1.85 km/h. The unit of wind and boat speed at sea. - **Gust**: A short burst of wind above the average, lasting seconds. Plan for it. - **Isobar**: A line joining places with the same air pressure. Close together means strong wind. - **hPa / millibar**: The unit of air pressure. 1013 is average; below 1000 is a fair low. - **Low (depression)**: An area of low pressure: wind, cloud and rain. Wind spins anticlockwise round it in the north. - **High (anticyclone)**: An area of high pressure: usually settled, light winds near its middle. - **Trough**: A stretched-out area of low pressure; often showers and a wind shift along it. - **Ridge**: A stretched-out area of high pressure; a calmer spell between lows. - **Front**: The border between two air masses. Cold fronts bring squalls and a wind shift. - **Veer**: The wind shifting clockwise, for example south to west. - **Back**: The wind shifting anticlockwise, for example west to south. - **Squall**: A sudden strong wind, often under a dark cloud line, with rain. Minutes, not hours. - **Fetch**: The stretch of open water the wind blows over. Longer fetch, bigger waves. - **Swell**: Waves that travelled away from where they were made: long, regular and smooth. - **Period**: Seconds between two wave crests. Longer is gentler. - **Significant wave height**: Roughly the average of the highest third of the waves. Some will be bigger. - **Lee**: The side sheltered from the wind. A lee shore is the one the wind blows toward: avoid it. - **Sea breeze**: An onshore wind on sunny afternoons, as the land heats faster than the sea. - **UTC / Zulu (Z)**: The world’s forecast clock. 12Z is 12:00 UTC, whatever your time zone. - **Run time**: When a model started its forecast. Newer runs know more. - **Valid time**: The moment a forecast is for. Always check it against your plan. - **GFS**: NOAA’s Global Forecast System, the American global model. Free and run 4 times a day. - **ECMWF**: The European Centre’s global model, often the most accurate a few days out. - **Ensemble**: One model run many times from slightly different starts, to measure uncertainty. Remember: You now speak enough weather to read any forecast. ## Final quiz 1. A “westerly” wind blows… From the west / Toward the west / Along the west coast 2. On a pressure chart, isobars packed close together mean… Light wind / Strong wind / Rain for sure 3. In the northern hemisphere, wind circles a low… Clockwise / Anticlockwise / It blows straight into the middle 4. One knot is about… 1 km/h / 1.85 km/h / 10 km/h 5. Force 6 on the Beaufort scale is a… Gentle breeze, 7–10 kn / Strong breeze, 22–27 kn / Storm, 48–55 kn 6. Significant wave height is 2 m. The biggest waves you might meet are about… 2 m / 4 m / 10 m 7. A 12Z run is valid at 00Z two days later. That forecast is… +12 h / +36 h / +48 h 8. The wind shifts from south to west. It has… Veered / Backed / Died 9. The barometer drops 5 hPa in three hours. That suggests… Settled weather ahead / A low or a front is coming / Nothing; pressure always moves 10. Two models agree for tomorrow but disagree wildly for day 7. Best reading? Both days are equally certain / Tomorrow is solid; day 7 is still open / Pick the model you like The page has the interactive diagrams and checks your answers. --- Free the Wind — Premium marine weather by SeaPeople. Not for navigation. Publisher: [SeaPeople](https://seapeopleapp.com).