Latitude and longitude: finding places and plotting a storm track

Imagine a hurricane warning that gives the storm's centre as two numbers: seventeen degrees north, sixty-four degrees west. Where on Earth is that? That spot is out in the Caribbean Sea, a little south of the Virgin Islands. Now keep both numbers the same, and change one letter, so north becomes south. A natural guess is that the storm shifts a little, still somewhere out at sea. That guess is wrong, because the new spot is on another continent, hundreds of kilometres from any sea. Two numbers can pin down any spot on Earth, once you know what each one measures and the direction it runs. Hold on to that storm, because it comes back once north and south are clear.

Start with the line round the middle of the Earth, halfway between the North Pole and the South Pole. That line is the Equator. Lines of latitude run east to west, parallel to the Equator. They stack up the globe like the rungs of a ladder. Latitude is how far north or south of the Equator a place is, measured in degrees. The Equator is nought degrees. The numbers grow as you move towards either pole, until they reach ninety degrees at each pole. That is why a latitude always carries a letter, N for north or S for south. Twenty degrees north and twenty degrees south are the same distance from the Equator, on opposite sides of it. Now a question of your own. A town sits at thirty degrees south. Is a town at thirty degrees south nearer the Equator or the South Pole? Nearer the Equator. It is thirty degrees from the Equator, but sixty degrees from the South Pole, which is at ninety. Five lines of latitude have their own names. The Equator, at nought degrees, is the first of them. The Tropic of Cancer runs at about twenty-three and a half degrees north. The Tropic of Capricorn mirrors it, at about twenty-three and a half degrees south. Much further from the Equator, the Arctic Circle runs at about sixty-six and a half degrees north. The Antarctic Circle mirrors it, at about sixty-six and a half degrees south. The Equator also splits the Earth into two halves, called hemispheres. North of it is the northern hemisphere, and south of it is the southern hemisphere. Here is a new one. A research station stands in the far north of Canada. The station is at seventy degrees north. Which named line is it beyond? The Arctic Circle. Seventy is more than sixty-six and a half, so the station lies north of the Arctic Circle. Now the words for direction. A place is north of the Equator or south of it, never above it or below it, because north is only at the top of a map by habit. One examiner's report says students must be encouraged to use the correct terms, north of the Equator rather than above it. The slip it names is writing 'above the Equator'. The habit that earns the credit is a compass word every time: north, south, east or west, never above, below, left or right. Latitude has a near neighbour, altitude. Latitude is distance north or south of the Equator, in degrees, while altitude is height above sea level, in metres. A mountain top on the Equator has a high altitude, but its latitude is still nought degrees. One examiner's report notes a significant proportion of students appeared to confuse latitude with altitude, mixing up how far from the Equator with how high. The fix is to check the unit. Latitude comes in degrees and altitude comes in metres, so a number of degrees is never a height. Now put that to work. A city on a map carries the label two thousand three hundred metres. Is two thousand three hundred metres a latitude or an altitude? An altitude, a height above sea level. A latitude would be given in degrees, with its letter.

Latitude alone only gives you a ring round the globe. A second number has to say where on that ring a place sits. Lines of longitude, also called meridians, run from the North Pole to the South Pole, like the segments of an orange. The starting line is the Prime Meridian, at nought degrees, which runs through Greenwich in London. Longitude is how far east or west of the Prime Meridian a place is, measured in degrees. Longitude grows to one hundred and eighty degrees on each side. One hundred and eighty east and one hundred and eighty west are the same line, on the far side of the globe. The Prime Meridian splits the Earth into two more halves, the eastern hemisphere and the western hemisphere. Try this one. London is at about fifty-one and a half degrees north, nought point one degrees west. Which two hemispheres is London in? The northern hemisphere, from its letter N, and the western hemisphere, from its letter W, if only just. London's position also shows the rule for giving one: latitude first, then longitude, each with its letter. Tables and graphs do not always keep that order, so read the headings before you copy a number across. You may also meet minutes. Some atlases split each degree into sixty minutes, so fifty-one degrees thirty minutes is fifty-one and a half degrees. Now back to the hurricane at seventeen degrees north, sixty-four degrees west. Flip the letter to south, and keep sixty-four degrees west. From seventeen north to seventeen south, how many degrees of latitude is that? Thirty-four degrees. It is seventeen degrees down to the Equator, then seventeen more beyond it. That jump carries the storm across the Equator and onto land, into the middle of Bolivia in South America, hundreds of kilometres from any sea. That is why a position without its letters is no position at all. Drop one letter and it could be in two places. Drop both and it could be in four.

Reading a position off a map follows the same order. Find the line of latitude, then the line of longitude, and read where they cross. Picture a map of the fictional land of Owlobia, which straddles the Equator. It has lines of latitude every five degrees and lines of longitude every ten degrees. The town of Hootley sits exactly on the thirty degrees east line. Going south from the Equator towards the five degrees south line, Hootley is two-fifths of the way there. Your job is the town's position, latitude first, then longitude, with both letters. Try it on paper before I show you the answer. Latitude first. Hootley is south of the Equator, so its letter is S. The gap between the Equator and the next line is five degrees. Two-fifths of five is two, so Hootley is at two degrees south. Then longitude. Hootley sits right on a drawn line, so its longitude is thirty degrees east. Put together, Hootley is at two degrees south, thirty degrees east, with both letters on. That fraction step is how you place anything between the lines. Find the step between two drawn lines, then take the fraction of that gap. Now a new question on the same map. The town of Perchley is at five degrees north, twenty degrees east. One option is that Perchley is north-east of Hootley. Another is that it is north-west. The last is that it is south-west. Which direction is Perchley from Hootley? Perchley lies north-west of Hootley, across the Equator in the northern hemisphere. Now the harder half: why is Perchley west of Hootley, and not east? Both towns are east of the Prime Meridian, but Perchley's twenty degrees east is the smaller number. A smaller number east sits closer to the Prime Meridian, which lies to the west. A full description puts it together. Perchley is north-west of Hootley. It is seven degrees further north, from two south to five north. It is also ten degrees further west, from thirty east to twenty east. One examiner's report on a question like this says the answers were mostly correct, showing that the use of latitude and longitude lines on maps is well understood. So if Hootley's position came out right for you, you are in good company. Keeping both letters on is what keeps it right.

A storm track is the line that joins a storm's positions in date order. Some courses ask you to plot one from a table. Here is a hurricane's centre at noon each day, with invented positions. Day one is twelve degrees north, fifty degrees west. Day two is fifteen north, fifty-five west. Day three is nineteen north, sixty west. Day four is twenty-four north, sixty-three west. Day five is thirty north, sixty-four west. The track graph has latitude up the side, from ten to forty degrees north. Longitude runs along the bottom, with seventy degrees west at the left and fifty degrees west at the right. Days one to three are already plotted, and the sixty degrees west line is drawn. Day four is at sixty-three west. One choice puts it three degrees to the left of the sixty west line. The other puts it three degrees to the right. So does sixty-three west go three degrees left of that line, or right? Three degrees to the left. West of the Prime Meridian, longitude grows as you go further west, and on this graph west is to the left. Three degrees to the right would be fifty-seven west, a different place. Left and right only find the spot on the paper. When you describe where the storm is, it is still north, south, east or west. Then go up to twenty-four degrees north, just under halfway from twenty to thirty, because the lines are ten degrees apart. Day five sits one degree further left, at sixty-four west, right on the thirty north line. Join the points in day order and label each day, and the track is finished. One examiner's report on a plotting question like this says most incorrect responses plotted a point at sixty-one or sixty-six degrees west. The wrong points were out along the longitude axis. Check the axis first. Before you plot, put a finger on the longitude axis and see the direction its numbers grow in. Describing a track uses one of the eight compass points, with the degrees moved as evidence. From day one to day three, the storm went from fifty to sixty degrees west, and its latitude rose from twelve to nineteen. Which of the eight compass points fits that move? North-west. The westward move was ten, from fifty to sixty, and the northward move was seven, from twelve to nineteen, so it travelled both ways by similar amounts. Now a fresh stretch of track, from day three to day five. Work out the change in each number, then the compass point that fits best. Have a go at this one before I show you. It moved four degrees west, from sixty to sixty-four, but eleven degrees north, from nineteen to thirty. So it curved round to head north. A full description gives both parts: north-west for the first three days, then curving round to the north, with the degrees as evidence. Why storms curve like that belongs to the chapter on tropical storms.

The named lines of latitude also help you say where something is on a world map. Picture a simplified world map with the Equator, both Tropics and both polar circles drawn on. It shades the main seas that freeze over every winter. The shading covers the Arctic Ocean round the North Pole, and the seas off the north coasts of Canada and Russia. It also covers Hudson Bay in Canada, and a ring of sea all round Antarctica. A first part asks you to name the line at about twenty-three and a half degrees south. That is the Tropic of Capricorn. A second part asks you to describe where the frozen seas are, and here are two answers to weigh. Answer A says the frozen seas are at the top and the bottom of the map. Answer B says most of them lie north of the Arctic Circle, round the North Pole, with the rest in a ring all round Antarctica, on either side of the Antarctic Circle. Which answer is stronger, and why? Answer B is stronger. It uses named lines of latitude as reference points, while top and bottom are not directions at all. To finish it, add what else the map shows. No frozen seas lie between the Tropics, and on this map Hudson Bay is the exception, reaching well south of the Arctic Circle. That answer says where, and never why. Describing a whole pattern like this gets its own video, Describing patterns and distributions on a map.

Time to pull it together, with a few questions to answer out loud.

In what order do you give a position, and what must each number carry? Latitude first, then longitude, and each one carries its letter, N or S, then E or W.

Now a place you have not met. It sits halfway between the ten and twenty degrees south lines, right on the forty degrees west line. Halfway from ten to twenty south, on forty west: what is its position? Its position is fifteen degrees south, forty degrees west.

A switch now, to a track graph with west to the left. Which sits further left: seventy-two degrees west or sixty-eight degrees west? Seventy-two degrees west, because west longitudes grow towards the west.

And a last one to stretch you. A fishing village on the coast and a hut four thousand metres up a mountain both sit at twelve degrees south. Which of the two is further from the Equator? Neither. Their latitude is the same, because it counts distance from the Equator, not height, and the hut's four thousand metres is its altitude.

And the hurricane: at seventeen degrees north it was out in the Caribbean Sea, but at seventeen degrees south it sits in the middle of Bolivia. One letter carried it across the Equator, so both letters always go on if a position is to say where.

If you could plot that storm track again on your own, give this video a thumbs-up, so you know you've got it. If it hasn't landed yet, save it, give it a day, and look again.

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