20 CHAPTER 1 Weather Analysis: TheTools of theTrade Isoplething Weather Maps: Getting Down and Dirty with Data There are some important differences between drawing contours on your hand and drawing isopleths on a weather map. First, the contours in Figure 1.14b loop around and close on themselves. In doing so they help pinpoint the "mountain peaks." When you isopleth weather data, this won't always happen. Yes, sometimes an isopleth will close on itself and thus identify a pocket of higher or lower value—this is particularly true of air pressure. But more often, an isopleth on a weather map will simply end at the edge of the map, where the data ends. Second, while the three-dimensional surface of the hand provides elevation information at all points, weather data is typically observed only at selected loca-tions. As a result, it is usually necessary to estimate the values between observation sites using interpolation. This technique often requires a little detective work,
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