barometer.today shows air pressure for any city: the reading now, the change since yesterday, and the week ahead. It also has a world isobar map, animated hour by hour from a day back to four days ahead. This post explains what isobars are and the algorithms that we use to draw them. The code is Python with numpy, scipy, contourpy and shapely, and anyone drawing contour maps from gridded data will meet the same problems. What an isobar is Air pressure varies from plac...