Interpreting Contour Plots
Another important objective from the lesson, and a really very valuable skill, is the ability to
extract some information from contour maps. You'll see contour maps on TV weathercasts and
on weather websites. So it's very helpful to be able to gather some information from them.
This is a map of surface temperature at 23Z on Monday, January 6, 2014. So if we do the time
conversion and subtract 5 hours to get Eastern Standard Time. That's 1800 hours or 6 o'clock in
the evening, 6:00 PM in the eastern part of the United States. We subtract 8 hours, to get the time
on the West coast to get the Pacific time. And that means it's 3:00 PM Pacific time.
So this is a map showing temperature in the late afternoon and early evening, across the United
States. We can see, there was some very, very cold air. There was an outbreak of cold air in the
Upper Midwest and the Great Lakes. Temperatures were well below 0. And we can estimate the
actual temperatures pretty much anywhere on the map, if you know how to read a contour map.
We know from the lesson that these lines are isotherms. They represent lines of constant
temperature. And we have to know what our contour interval is, in order to interpret the map.
Here our contour interval is 5 degrees. We see a 30 degree isotherm here in Northern Texas. The
contour immediately to its north is 25 degrees. The contour immediately to its south is 35
degrees. So each contour represents a change of 5 degrees, compared to the isotherm that it’s
adjacent to.
So based on that, we can find the temperature, at least estimate it anywhere on the map. If we
picked a spot here in central Ohio where the cursor is, we know the temperature has to be
between minus 5 degrees and minus 10 degrees, because it falls between those two contours. So
it's colder than minus 5, but it's warmer than minus 10, in central Ohio.
We also can use our knowledge to our advantage, to see what the temperature is, even when the
contours are not labeled. We see this unlabeled contour here in Northern Minnesota, along the
Canadian border. And we know that temperatures along that contour are minus 20 degrees,
because our contour interval is 5 degrees.
This is the minus 15 degree contour, running through Central Minnesota. The next contour,
representing colder air, would be minus 20 degrees. A point inside that circle would have a
temperature lower than minus 20 degrees but higher than minus 25 degrees, since no minus 25
contour actually exists on the map in that area. So we can say that it's lower than minus 20, but
we know it has to be higher than minus 25.
Another thing you'll find is that, some websites have some strange conventions when it comes to
shading contour maps. This particular map, we'll focus on the shaded areas. They represent wind
speeds at about 30,000 feet above the Earth's surface.
And the strange thing about the convention on this website is that, the shadings repeat
themselves. So not all areas shaded in light gray, for example, represent the same range of wind
speeds. So knowing the contour interval is, again, the key to being able to interpret the map.
On these particular maps, the contour interval is 20 knots. And the shadings start when wind
speeds reach 70 knots. So the light gray shading here starts at 70 knots. Then the dark gray starts
at 90 knots. The white starts at 110 knots.
But then the colors repeat themselves. We go back to light gray. But that means 130 knots. Then
this transition to dark gray means 150 knots. And the transition back to white is 170 knots.
So knowing that, we can find out or at least estimate the wind speed at pretty much any point
within these shaded areas. If I needed to know the wind speed here, say, in Central Missouri, I
know that it has to be somewhere between 70 knots and 90 knots, because it falls between the
contours of 70 knots and 90 knots, in that light gray shaded area. But if I needed to find out the
wind speed here off the east coast of the United States in the white shaded area, I would have to
count contours to know what the wind speed is there.
We start with the outer light gray shading. We have 70 knots, then 90 knots when we transition
to dark gray, 110 knots when we transition to white, 130 knots when we transition back to light
grey, then 150 knots and 170 knots when we start the white shading. And since we're inside the
white shading, the wind speed is greater than 170 knots, but it's less than 190 knots, since there's
no contour or shading representing 190 knots.