 One of the key skills from the lesson is to be able to identify troughs and ridges on analyses of sea level pressure. And sometimes doing so is pretty obvious. It's pretty clear where the troughs and ridges are. But sometimes it's not so obvious. And so I want to walk you through a couple of examples where it's not immediately obvious whether you have a trough or a ridge. And in these situations you have to use the definitions of troughs and ridges to your advantage. By definition, a trough is in a long gated area of relatively low pressure. And a ridge is in a long gated area of relatively high pressure. So we use those definitions to our advantage to find out whether we have a trough or a ridge. So first we'll look at this axis here that runs from North Dakota westward. And we want to figure out if this is a trough or a ridge axis. So you can do a check. First you look at the point where the axis intersects an isovar. And you know the value of pressure right on the isovar because the isovars here are labeled. Where my cursor is, the value along the axis would be 1,012 millibars because it intersects the 1,012 millibar isovar. So now we want to check what the pressure would be on either side of the axis. If we go up here, the pressure is going to be something between 1,012 millibars and 1,016 millibars because it's between those two contours. So the pressure would be higher there. And if we go down here to the south and to southern Montana, we find the same thing. It would be something between 1,012 and 1,016. So again, it would be a higher value which means that the pressures at the axis are relatively low. That's in a long gated area of relatively low pressure. This is a trough. We can look at another example here at this axis that runs from Illinois southwest through Texas. We want to find out if it's a trough or a ridge. So we do the same type of check. We look at where the axis crosses an isobar where we know the value. Here the value of pressure right on the axis would be 1,028 millibars. We do a check on either side. If we go over here farther to the west in Texas, the pressure would be something between 1,028 and 1,024 millibars. So it would be something lower. If we go over here on the other side, again, it's something between 1,028 and 1,024. So something lower. So this is in a long gated area of relatively high pressure. That would be a ridge by definition.