Roller-Blading on Weather Maps

Let's focus our attention on a large region like the Middle West and distill the set of weather observations on each station model down to a single number which, for sake of argument, is the air temperature at each airport. Such a distilling of data results in a map like the one shown here (please keep this window open). At first glance, you probably can't see the entire forest for the trees. In other words, organized patterns are difficult to pick out from the thicket of temperatures.

But there's a method to my madness! Just like the contours of constant elevation on the map of Hawaii gave us a sense of organization regarding the dramatically changing topography on the island, drawing isotherms, which are contours of constant temperature, will help us to identify organized patterns of warmth and chill over the Middle West. To optimize your learning potential, I suggest that you first print out a hard copy of the map so that you can follow along. For a printable version of the map, click here and then hit CTRL-P.

To successfully isopleth the 50-degree isotherm, imagine that you're a competitor in a roller-blading contest and that you're wearing number "50". You can win the contest only if you roller-blade through gates marked by a flag numbered slightly less than than 50 and a flag numbered slightly greater than 50.

To begin the process of drawing isotherms, we first hit the virtual roller-blading paths along the beach, where a special contest is underway. In order to successfully negotiate the course, roller bladers can only skate through special pairs of "temperature gates". For each competitor, "legal" gates are relatively easy to spot; a blue marker carries a temperature that's slightly less than than the contestant's number and its twin red marker shows a temperature that is slightly greater than the contestant's number. Failure to roller-blade through a legal pair of temperature gates during the race will result in immediate disqualification! Understanding this simple rule for this contest will help you to draw isotherms on your map.

On most maps of temperature, isotherms are drawn for every multiple of ten degrees Fahrenheit. Sometimes, isotherms that are multiples of five degrees Fahrenheit are included for greater detail (had we drawn elevation contours every 250 feet on the topographical map of Hawaii, it would have greatly enhanced the detail). I want you to begin by drawing the 50-degree isotherm, applying the spirit of the rule governing my quirky roller-blading race. Feel free to isopleth on your hard copy, or you can draw directly on the contouring tool with the handy-dandy digital pencil. Try to "skate" your pencil through two neighboring "gates" marked by temperatures in the upper 40's and the lower 50's. Would you like to see a few of the gates just to get you started? Go to the pull-down menu and select the 50-degree isotherm. Now click the "Show Hints" check-box. A few "gates" will then appear to help you chart your course. If you make a mistake, don't get nervous. Simply select "Erasing" to make your mistake go away.

Got the hang of it? Great! Now I'll offer the following protocol for isoplething just so you can earn "style points":

  1. Like an expert roller blader negotiating the course of gates, you should trace your isotherm in a neat and smooth course instead of instead of a jagged, jerky path (initially trace lightly in pencil just in case you accidentally get off course).
  2. An isotherm should begin and end at an edge of the map, or, alternatively, loop around and close on itself (like the circular contours of constant elevation around Mauna Loa and Mauna Kea).
  3. An isotherm should never branch or fork and "skate" two different courses at once. Such a branching results in immediate disqualification.
  4. Typically, you must draw isotherms on weather maps where there are "gaps" in the data (it's an undeniable fact of life that there are a limited number of observation sites). To draw isotherms through a "gap" in the data, you must interpolate (estimate the value of) the temperature between two given data points. For example, if you're drawing a 45-degree isotherm and you come upon two data points marked 49 degrees and 44 degrees, you'll want your isotherm to pass closer to the point marked 44 degrees. In other words, use a little common sense.

I'll modestly add to this set of rules in just a minute. For now, go ahead and draw the 50-degree isotherm (you will be asked to perform similar tasks on the quiz, so make sure you know how to isopleth).

When you're done, make sure you select the 50-degree isotherm in the pull-down menu below the map. Then click "Draw Line" to watch the virtual pencil "skate" through the proper gates and correctly draw the 50-degree isotherm.

Now compare your isotherm to this solution. Need more practice? Try drawing the 80-degree isotherm. When you're done, select the 80-degree isotherm in the pull-down menu below the map and click "Draw Line". Please note that the isotherm the virtual pencil traces is a closed contour. For the record, it encloses a pocket of hot air over the Ohio River Valley. How did you do? If you feel a little shaky about isoplething, don't worry ... all you need is a little more practice. So "jump back on the horse" and choose another isotherm to draw. Then check your work to make sure you're on target. Repeat until you feel confident that you're up to speed on isoplething.

By now, you've probably surmised the fifth and final rule for isoplething:

  1. Isopleths should be drawn at equal intervals. There are exceptions, of course. During the cold season, for example, it is sometimes prudent to draw the 32-degree isotherm on a surface temperature analysis. As you know, a reading of 32 degrees marks the melting point of ice (not the "freezing point" as Hale Stone often calls it ... most water contains impurities and freezes at temperature below 32 degrees).

Now that your computer has generated all of the isotherms, there's more we can do to visually organize the temperature field over the Middle West. To borrow from another Chicago song title, Color My World. Read on.