Skip to 0 minutes and 7 secondsI'm Sylvia Knight, from the Royal Meteorological Society. And I'm just going to take a few minutes to go through the isoline drawing exercise. Because it was quite a tricky one, some people might not have had the opportunity to have a go at it. Other people might have had a go, and found that they got stuck or weren't sure about whether they did the right thing or not. So well, for what it's worth, here is my solution. OK, so you were presented with a map, a map of weather stations. So each of these little hollow blobs on the map represents a weather station. And the number by it is the temperature that weather station recorded.

Skip to 0 minutes and 41 secondsAnd the task was to draw contours of constant temperatures, so isotherms at 10.5 degrees, 8.5 degrees, 6.5, 4.5, and 2.5. So if we think about the 10.5 degree one first, that's going to be the line that separates everywhere that's warmer than 10.5 degrees from everywhere that's colder than 10.5 degrees. So if I start by just shading the blobs that are warmer than 10.5 degrees, we can see that they're 3 here. And it should be fairly obvious what sort of line you've got to draw to separate those from everywhere else. So here's my solution. And you can see that I've just drawn one line to separate that 11 off, and another one to enclose those two.

Skip to 1 minute and 23 secondsNow, you could've joined those two up, maybe with carrying on a meeting of those two circles, like that. But I think it looks better if you do it like this. So we've got two areas where the temperature's warmer than 11. If you think of it in terms of a map of land surface height, that those would be your hilltops, where the land surface values are highest. Or in terms of the temperature, it's two little areas where the temperatures are warmest. So if we think about the 8.5 degree contour next, we now need to draw a contour that separates everywhere that's warmer than 8.5 degrees.

Skip to 1 minute and 57 secondsSo now colouring in everywhere that's 9 or 10 degrees as well, we've now got to draw a line that separates everything that's coloured from everything that isn't coloured. And that gives us a line that looks a bit like this. So now looking at the 6.5 degree contour, we've just got these four areas here, which are 7 or 8 degrees. But our contour needs to separate all the coloured dots from everywhere that's not coloured. So it doesn't just stop here. It needs to carry on and follow down here, so that everywhere that's colder than 6.5 is on one side. And everywhere that's warmer is on the other side.

Skip to 2 minutes and 33 secondsNow, notice that even though those contours are already close together, they mustn't touch. Because if they touched, then that would imply it was 6.5 degrees and 8.5 degrees at the same place in the same time, which obviously isn't possible. So they just go very, very close to each other. So if you were travelling from that dot to that dot, you would start at 5 degrees. When you went through that contour, it would be 6.5 degrees. When you went through that one, it would be 8.5 degrees, until eventually you reach 9 degrees right there. OK, so now we're going to do the 4.5 degree contour. Same idea again. We'll colour everything that's 5 or 6, in yellow.

Skip to 3 minutes and 9 secondsAnd then draw a contour which separates everything that's coloured from everything that isn't coloured. And again, the contours get very close to each other. A little bit too close, maybe, in there. Maybe they should be separated a little bit more there. But you get the idea again that we've got everything that's warmer than 4.5 degrees on one side. And everywhere that's colder, on the other side. Now, we could've done that, we could've joined that contour up there and then just drawn an isolated circle around those. That would've been equally valid. OK, and now looking at the 2.5 degree contour, there's just these few places where the temperature is colder than 2.5 degrees.

Skip to 3 minutes and 48 secondsSo you just need to draw a line to separate those off. Again, that line could join up around here. We really don't know, because we haven't got any temperature information in the middle here. So again, anything that you do around here is equally valid. And there we have our temperature map. And you can see, we've got the warmest temperatures here, and the colder temperatures in a band around it. So moving onto the pressure map, and you'll see that we've got the same sort of thing again. We've got these little blobs, which mark the weather stations. And the blobs are in exactly the same places as they were for the temperature map.

Skip to 4 minutes and 24 secondsSo those little circles are in the same places as these little circles. But now we've got some very different information presented to us. The numbers are the pressure values, and everything in shorthand. So where it says 06.5, that's actually short for 1,006.5 millibars or hectopascals. Or here, 13.5, that's short for 1,013.5 so they just haven't bothered writing down the first couple of numbers each time. So we've got the pressure values. Whether the blob is shaded in and how it's shaded in, is actually a representation of how much cloud there is. But we're not going to worry about that too much at this stage. And then you've got these little tails on the blobs.

Skip to 5 minutes and 8 secondsAnd that's showing you where the wind's coming from. So here we've got northerly winds. In this area, we've got westerly winds, moving around to southerly winds in this sort of area. And that's useful. Because we know that the wind follows the pressure contours. So if we're going to be drawing pressure contours, we know that they've got to be roughly parallel to those wind arrows. OK, so it's the same sort of idea again. We're starting off by drawing the 1,016 millibar pressure contour. So the only places where the pressure is higher than that are these four places here. So it's relatively straightforward to draw a pressure contour that separates those colour blobs off from everything else.

Skip to 5 minutes and 50 secondsAnd roughly follows the wind direction. Same thing again, to draw the 1,012 contour. Again, it's got to come around here, separate everything that's coloured from everything that isn't. The fact that there isn't a number here doesn't mean there wasn't pressure. It just means it wasn't recorded. But it's a fair guess that it's going to be more than 1,012, and that's why our pressure contour goes off there. And now we're doing the 1,008 contour in the same sort of way. So now we've got a pressure contour that comes around here. And it's worth noting that pressure contours are always as smooth as you can make them.

Skip to 6 minutes and 26 secondsYou tend to get very sharp shifts in pressure contour over a very short area of space. So yes, so pressure contours are always nice and smooth. And just like any other contour, they can't stop in the middle of the map. They either close off into a circle. Or they go all the way to the edge of the map. And then last, but not least, we've got the 1,004 contour here. There's our pressure map. And it shows you that the lowest pressure is in the centre here. And the highest pressure, the pressure then goes out, as you go towards the edges.

Skip to 6 minutes and 59 secondsAnd hopefully that's ringing some bells, and you're recognising that this is, therefore, a low-pressure weather system, with the lowest pressure in the middle, and the high pressure as you go out. And you can see from the wind direction, that the wind is blowing around this in an anti-clockwise direction. So this is a low-pressure weather system, a depression in the northern hemisphere. And if we go back and look at the temperature map that corresponded to it, here's our warm air in the middle, the warm sector. And here are the areas where the temperature's falling off fast. So those are where our fronts are. So here we've got our warm front. Then we've got the cold front in this area here.

Skip to 7 minutes and 39 secondsAnd there's a clue about that on the pressure map too. You'll see that the wind's direction doesn't move round constantly in a circle. But it shifts vary dramatically from a northerly direction to a westerly there, to a southerly. And where those shifts occur, that's another clue that that's roughly where the fronts are. So we've got our fronts there and there again. Then if we look at the last map that you were given-- the weather map-- again, the little dots are in exactly the same places. But now you've got some information about whether it was raining or drizzling or heavy rain at these stations. It's not a contour drawing exercise in the same sense.

Skip to 8 minutes and 20 secondsBut here I've just roughly shaded in where it's raining most heavily, in these areas here, where it's just raining, in these areas here. And then it's drizzling all in this area here. So again, you get the picture that you've got the heavy rain on the fronts. Drizzle or almost drizzle, in the warm sector air, in the middle, and then nothing much going on around the outside of the depression. So hopefully you found that useful. It's quite nice to be able to look at a weather map and to be able to look at weather observations like that and to be able to interpret what's going on.

Skip to 8 minutes and 56 secondsAnd hopefully it's made you think a little bit about the key features of a depression again.

Sylvia's solution

In this video Sylvia demonstrates the solutions to this Week’s practical activity; the isoline drawing exercise in Step 3.12.

We have also provided the solutions for the exercise on a PDF sheet for you to download.

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This video is from the free online course:

Come Rain or Shine: Understanding the Weather

University of Reading