Using the Weather and Radar Tools

Prerequisites

Before this lesson you should:

• Have worked through Weather Forecasting, so that you know the Beaufort scale, the structure of the Shipping Forecast, fronts, isobars and Buys Ballot's law.
• Have read Chartplotters, GPS and Introduction to Radar, so that you know in outline how radar works and what range, gain, sea clutter and rain clutter are.
• Know COLREGs Rules 5 (lookout), 7 (risk of collision) and 19 (restricted visibility) from the COLREGs lesson.

This is a practical lesson. It teaches you to use four free tools on this site: the weather tools, the Weather School game, the radar simulator and the Weather & Radar ID game. Keep the tools open in another tab and try each step as you read.

Learning Objectives

By the end of this lesson you will be able to:

• Use the Beaufort explorer to relate a forecast force to wind speed, sea state and the sail plan for a cruising yacht.
• Decode any Shipping Forecast area forecast with the forecast decoder, and explain each term.
• Run a go / no-go check for a day sail and explain the reasons behind the verdict.
• Progress through the Weather School levels, including reading the sea state and decoding forecast terms against the clock.
• Set up a radar picture: range, gain, tune, sea clutter and rain clutter.
• Measure range and bearing with the VRM and EBL, and convert relative bearings to true.
• Choose head-up or north-up, and read every corner of the radar display.
• Identify land, rain, racons, ships and small craft on the radar screen.
• Use echo trails and a steady EBL bearing to judge risk of collision and estimate the closest point of approach.
• Test yourself with the Weather & Radar ID game until you can score 18 out of 20 in both decks.

Why Practise With Tools?

At sea you will meet weather and radar under pressure: a forecast read at speed on the VHF, a radar picture in fog with a ship somewhere ahead. The RYA syllabus expects a Day Skipper to interpret forecasts and to understand the basic use of radar. Reading about a control is not the same as having turned it. A simulator lets you make every mistake safely: turn the sea clutter right up and watch a yacht disappear, or leave the gain too low and miss a buoy.

The tools here are learning aids with simplified physics. They are not a forecast, and not navigation equipment. Always use the official forecast and your own radar's manual.

Part 1: The Weather Tools

Open the weather tools. Three tabs run across the top: Beaufort scale, Forecast decoder and Go / no-go. Each tab has its own web address ending in #beaufort, #forecast or #go-no-go, so you can bookmark the one you use most.

The Beaufort explorer

The Beaufort scale describes wind strength from force 0 (calm) to force 12 (hurricane force) by its effect on the sea. The diagram below summarises the key thresholds.

Click to zoom

To use the explorer:

1. Drag the slider, or tap any number from 0 to 12 under it.
2. Watch the animated sea. Wave height, length and steepness, white horses, spray blown off the crests and streaks of foam all build with the force. The sky darkens, the yacht heels and reefs, and the anemometer on the right shows the mean wind in knots.
3. Read the panel beside it: the mean wind, the probable wave height, the Met Office description of the sea, and the sail plan and advice for a 10 m cruising yacht.
4. Read the small print. Wave heights are for the open sea, well away from land. Close to the coast, and with wind against tide, the sea is shorter and steeper.

Try this: set force 4, then force 6. The wind's force on your sails rises with the square of the wind speed, so 24 knots pushes roughly three times as hard as 14 knots. The explorer shows the change in the sail plan from full sail to two reefs.

The forecast decoder

The Shipping Forecast gives each sea area four items in a fixed order: wind, sea state, weather and visibility. The decoder uses that order to label each sentence.

1. Pick an example, or type the forecast you heard: the sea area names, a colon, then the forecast. For example: "Wight, Portland: Southwest 5 to 7, occasionally gale 8 later. Moderate or rough. Rain then showers. Good, occasionally poor."
2. Recognised sea areas show as dark pills above the four parts. A pale pill means the name is not a Shipping Forecast area: check the spelling.
3. Tap any highlighted word for its official meaning. Directions, forces, wind terms (veering, backing, cyclonic), sea states, weather and visibility each have their own colour.
4. Read the "In plain English" line: forces become a knots range, and sea state and visibility terms become metres and miles.

The timing words are the ones most often misread. In a gale warning, imminent means within 6 hours of the time of issue, soon means 6 to 12 hours, and later means more than 12 hours.

The go / no-go check

1. Choose your crew's experience. A new crew is comfortable up to about force 4, a competent crew force 5, and an experienced crew force 6, each with a hard limit one force higher.
2. Set the forecast mean wind, then the gusts. The gusts slider can never go below the mean wind, because gusts are always at least as strong.
3. Set the visibility and how much daylight you will have in hand on arrival.
4. Tick a gale warning, or wind against tide, if either applies.
5. Work through the "Before you go" checklist: forecast, tides, bolt hole, crew, boat and passage plan.
6. Read the verdict and every reason under it. A gale warning always gives NO-GO.

The verdict is a rule of thumb for coastal day sailing in a yacht of about 10 m. The skipper's judgement comes first, and smaller boats, tired crews and exposed coasts all lower the limits.

Part 2: Weather School

Weather School is a game in nine levels. Score 50% on a level to earn one star and unlock the next, 80% for two stars and a perfect run for three. Timed levels give extra points for quick answers and for a streak of right answers. Stars climb you through the ranks from Landlubber to Weather Guru, and are saved in your browser.

The frontal passage simulator

Drag the time slider from "24 h before" to "after". At each stage the sky photograph, the barometer and its tendency, the wind dial and force, the air temperature, the visibility and the weather all change. The barograph at the bottom draws the pressure curve, and a red vertical line marks the moment you have dragged to.

The pattern to learn matches the diagram below: high cloud thickening ahead, rain at the warm front, a murky warm sector, and squalls and a sharp veer at the cold front.

Click to zoom

Watch the barometer as you drag. The rate of fall is the real warning: a fall of more than 1 hPa an hour means strong winds are coming.

Click to zoom

The synoptic chart

Red semicircles mark the warm front, blue triangles the cold front, and the shaded wedge between them is the warm sector. The closer the isobars, the stronger the wind. After you answer a question about a lettered point, the game reveals the wind arrow there, so you can check your use of Buys Ballot's law: with your back to the wind in the northern hemisphere, the low is on your left.

The two newest levels

Sea State Spotter shows the Beaufort explorer's sea set to a random force. Use the waves, the white horses, the spray, the yacht's sail plan and the anemometer to name the force. The wrong answers are always at least two forces away, so the call is fair.

Shipping Forecast Ace shows a line of forecast with one term highlighted, exactly as the decoder does, and gives you 20 seconds to choose its official meaning: a sea state, a visibility, a timing word, a pressure tendency or the speed of a moving system.

Part 3: The Radar Simulator

Reading the display

Your boat is at the centre. A solid line, the heading line, runs from the centre in the direction your bow points. Range rings are drawn at equal intervals, and a bearing scale runs round the edge. The four corners show:

The sweep rotates once every 2.5 seconds, as a real radar at 24 revolutions a minute does, and each echo glows brightest just after the sweep has passed over it.

Setting up the picture

Follow the same order every time you switch on:

1. Range. Press + or −. The ring interval changes with the range: 0.25 mile on 1.5 miles, 0.5 mile on 3 miles, 1 mile on 6 miles. Use the shortest range that shows what matters, and range out every few minutes for distant or fast traffic.
2. Gain. Turn it up until a light speckle just shows all over the screen, then back off slightly. Too little gain hides weak echoes such as GRP yachts; too much buries everything in noise.
3. Tune. Adjust for the longest tune bar. A badly tuned set gives weak, patchy echoes.
4. Sea clutter (STC). Waves close to the boat return echoes, heaviest on the windward side. Add just enough to thin them. Overdone, it removes real targets close to you, which is exactly where a collision would happen.
5. Rain clutter (FTC). Use it only while a shower is on the screen. It shrinks the shower to its leading edge so hard targets inside show. Turn it down afterwards.

The diagrams below show the effect of tuning and how to choose a range.

Click to zoom

Click to zoom

Measuring range and bearing

1. Switch on the VRM, a ring you can move, and lay it on the inner edge of the echo. Echoes are smeared outward by the radar pulse, so the near edge gives the true range.
2. Switch on the EBL, a line from the centre, and swing it through the middle of the echo. The ‹ and › buttons move it half a degree at a time.
3. Check the last letter of the EBL readout. R means a relative bearing (head-up display); T means a true bearing (north-up display).
4. To convert, add your heading: true bearing = relative bearing + heading, taking away 360 if the answer is over 360. An echo at 047 relative when you are heading 100 true bears 147 true.

Head-up or north-up?

Head-up puts your heading at the top, so the screen matches the view from the helm and the bearing scale reads relative bearings. North-up puts north at the top, so the screen matches the chart, the heading line points at your heading, and bearings are true and ready to plot. Use north-up to fix your position and compare the coastline with the chart; many skippers use head-up for collision avoidance because left and right on the screen match left and right on deck.

Identifying echoes

• Land: a strong, solid return with a bright edge facing you. Nothing shows behind high ground: that is a radar shadow.
• Rain: a large, soft, grainy patch with no hard edge.
• Racon: a radar beacon that replies with a Morse letter painted outward from its position.
• Ships: strong, bright echoes. A large steel ship can be seen many miles away.
• Small craft: weak, small echoes that may come and go, especially in clutter. A radar reflector helps a lot.

Echo size depends on the target's material, shape and height, not just its size. Always confirm with your eyes, your ears and AIS if you have it.

Trails, steady bearings and CPA

The simulator draws relative motion: everything moves relative to you. Switch on trails and each echo leaves a line of dots showing where it has been. Extend that line past the centre in your mind:

• If it runs through the centre, the target is on a collision course.
• The distance at which the extended line passes the centre is the closest point of approach (CPA). Use the range rings to judge it.

The EBL gives a second test. Lay it on the echo and watch for a few minutes. If the echo stays on the line while the range closes, the bearing is steady and, under Rule 7, risk of collision exists.

In fog Rule 19 applies to every vessel. For a vessel detected by radar alone forward of the beam, avoid altering course to port, unless it is being overtaken. For a vessel abeam or abaft the beam, avoid altering towards it. A bold turn to starboard, or slowing right down, is usually the answer for a ship closing on the bow.

Missions and free play

The Missions tab has twelve tasks that build these skills one at a time: set the gain, tune the receiver, pick the range, clear the sea clutter, see through a shower, range with the VRM, bear with the EBL, switch from head-up to north-up, identify echoes by tapping them, recognise risk of collision, act under Rule 19, and estimate a CPA. The control a mission is about has a ring round it. Press Check when you think the picture is right: first time earns three stars, and each wrong check costs one. In the live missions the clock runs at 30 times real speed, and the question opens once you have watched for the stated number of minutes.

Free play puts you on a coast approach with buoys, a racon, two ships, a yacht and a moving rain shower. Every control works, and tapping any echo tells you what it is, its true bearing and its range.

Part 4: Test Yourself

The Weather & Radar ID game deals 20 random questions a round from a bank of about 30 per tool, with the answers shuffled. Every picture is the real tool: the Beaufort sea, the decoder, the go / no-go check, the cloud photographs, the synoptic chart, the frontal simulator, and the radar screen with its control panel. Pick the Weather deck, the Radar deck, or Both for ten of each. Some questions show a live tool, such as the forecast decoder or the frontal simulator: try it before you answer.

Worked Example

You are planning a day sail from the Solent towards Poole in a 10 m yacht with a competent crew. You hear the forecast for Wight: "Southwest 4 or 5, occasionally 6 later. Slight or moderate. Showers. Good, occasionally moderate."

1. Decode it. Paste it into the decoder. Wind: south-west force 4 to 5 (11 to 21 knots), force 6 more than 12 hours after issue. Sea state: 0.5 to 2.5 m. Weather: showers, with gusts under each. Visibility: more than 5 miles, sometimes 2 to 5.
2. See it. In the Beaufort explorer, force 5 shows a moderate sea with many white horses, and a reef for a 10 m cruiser.
3. Check it. In the go / no-go check, set competent crew, wind 5, gusts 6, good visibility, three hours of daylight in hand. With the checklist complete the verdict is CAUTION: force 5 is at the top of this crew's comfortable range, so reef before leaving.
4. Sail. Mid-afternoon a shower reduces visibility to under a mile off Hurst. You switch on the radar on the 3 mile range: rings every 0.5 mile. The shower is a soft patch to the west. You add rain clutter and a hard echo appears inside it, 1.8 miles away on the VRM.
5. Assess. You are heading 250 true, head-up. The EBL on the echo reads 015 R: true bearing 265. Over six minutes the echo stays on the EBL and the range closes to 1.1 miles. Its trail points at the centre. Steady bearing, closing range: risk of collision.
6. Act. The ship is forward of your beam and detected by radar alone in poor visibility. You avoid a turn to port, turn boldly to starboard on to 310, sound one long blast every two minutes, and keep watching. The echo starts to draw left and its trail now passes well clear.

Common Mistakes

• Reading the force, not the gusts. Forecasts give the mean wind. Reef for the gusts.
• Misreading timing words. "Later" is more than 12 hours away; "imminent" is within 6 hours.
• Treating the go / no-go verdict as permission. It is a prompt for your judgement, not a replacement for it.
• Leaving the gain where the last person set it. Re-set it after every range change.
• Too much sea clutter. A clear centre looks tidy but can hide the yacht 400 metres away.
• Leaving rain clutter on. It weakens every echo a little once the shower has passed.
• Plotting a relative bearing as true. Check whether the EBL readout ends in R or T.
• Measuring to the middle of an echo. Range is to the inner edge.
• Reading relative trails as a ship's course. They show how it moves relative to you, not where it is heading.
• Trusting radar over your eyes. Small craft, wooden boats and anything in clutter may never show.

Summary

• The Beaufort explorer links each force to wind speed, sea state and a cruising yacht's sail plan.
• The forecast decoder labels wind, sea state, weather and visibility, and explains every term with its official meaning.
• The go / no-go check weighs wind, gusts, visibility, daylight, gale warnings, wind against tide and your preparation.
• Weather School's nine levels cover clouds, fronts, synoptic charts, local effects, forecasting, sea states and forecast terms. One star unlocks the next level.
• Set up the radar in order: range, gain, tune, sea clutter, rain clutter.
• Measure range with the VRM on the inner edge of an echo, and bearing with the EBL. True = relative + heading.
• North-up matches the chart; head-up matches the view from the helm.
• A steady bearing with a closing range means risk of collision. A trail pointing at the centre means a collision course.
• In fog, for a radar-only target forward of the beam, avoid turning to port.
• Test yourself with the Weather & Radar ID game until you score 18 out of 20 in both decks.

Check Your Understanding

1. In the Beaufort explorer, why are the wave heights shown likely to be lower than the waves you meet off a headland at spring tides?

Answer: They are open-sea heights after a long fetch. Near the coast, and especially with wind against a strong tidal stream off a headland, the waves are shorter, steeper and can break, so the sea is worse than the figures suggest.

1. The decoder shows "Northwest 6 to gale 8, soon". When is the gale expected?

Answer: Soon means 6 to 12 hours after the time of issue.

1. You tick "Gale warning in force for the area" in the go / no-go check. What verdict do you get, and why?

Answer: NO-GO. A gale warning means force 8 or gusts of 43 knots or more are expected, and leisure craft should stay in port.

1. How do you unlock the Sea State Spotter level in Weather School?

Answer: Earn at least one star (50%) on the level before it, Forecaster's Challenge. Each level is unlocked by a star on the one before.

1. The whole radar screen is covered in speckle. What has gone wrong and how do you fix it?

Answer: The gain is too high. Turn it down until a light speckle is only just visible, then back off slightly.

1. A yacht is known to be about 0.4 miles away but the centre of the screen is completely clear. What is the likely cause?

Answer: Too much sea clutter control. It suppresses all echoes close to the boat, including real ones. Ease it back until the clutter just thins.

1. In head-up mode the EBL on an echo reads 300 R and you are heading 120 true. What is the echo's true bearing?

Answer: 300 + 120 = 420, minus 360 = 060 true.

1. Why should you measure range to the inner edge of an echo?

Answer: The radar pulse smears each echo outward in range, so the nearest edge is the true distance.

1. Over six minutes an echo stays on the EBL and its range closes from 3 to 2 miles. What do you conclude, and what does Rule 7 say?

Answer: Its bearing is steady and the range is closing, so risk of collision exists. Rule 7 says that risk of collision shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change.

1. In fog, a ship detected by radar alone is closing on your port bow. Which action should you avoid, and what would you do instead?

Answer: Avoid altering course to port (Rule 19(d)(i)), since the ship is forward of the beam and not being overtaken. Turn boldly to starboard or slow right down, keep sounding your fog signal and keep plotting it.