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rodcarey created the topic: Synoptic charts and measuring QNH
Just wonder where on the synoptic chart you take the QNH measurement Ie in the middle of either a low or high is the QNH measurement, each gradient line after that has a value which is on the line. I have attached a sample for Melbourne, the high is 1024, and the next gradient line is 1016. Melbourne sits north of the Gradient line, between 1016 and 1024. I would assume the gradient line 1016, and the area between this and 1024 is all a QNH of 1016, and once you hit the gradient line around the high, then it would become 1024?
bobtait replied the topic: Synoptic charts and measuring QNH
Because it's called a gradient, it's useful to imagine that you are looking at contours on a map of a hill that is 1024ft high. As you leave the top of the hill, the spacing of the contours is 4 but only every second contour line is given a value to avoid making the map too cluttered. So the line between the 1016 and 1024 would be 1020. It is quite valid to interpolate between the contours, although, in practice, it would be normal not to go further than half way between two lines (2Hpa).