Maps and weather

About maps and weather

Maps and weather gathers the symbols on two kinds of chart: the surface weather map, which shows the air, and the topographic map, which shows the land. Both draw the world with lines that join points of equal value. A Met Office teaching pack describes isobars as lines of equal pressure “similar to contours”, and the USGS defines each contour as a line of equal elevation. Each page says what one symbol means, from the agencies that draw the charts.

Weather map symbols covers the surface analysis and the station plot. The fronts come first: NOAA’s Weather Prediction Center draws a cold front with blue triangles and a warm front with red semicircles on the side the front is moving towards, and the WMO’s manual on weather charts gives the same colours, with purple for an occluded front and alternate red and blue for a stationary one. Then come the H and L pressure centres, isobars, and the station model, the cluster of numbers round a small circle, which WPC says is standardized by international agreement. The present weather symbols are dots for rain, commas for drizzle and asterisks for snow, among others.

Topographic map symbols covers the US Geological Survey’s symbol sheet, with Natural Resources Canada’s and the Ordnance Survey’s versions beside it. The colours of a USGS map sort its symbols: brown for contours, blue for water, green for vegetation, and black for roads, trails, railroads and other features people built. The pages cover contours and depression contours, benchmarks and spot elevations, the north arrows and the scale bar, and the symbols for trails, railroads, power lines, streams, marshes, buildings, cemeteries and mines.

Neither system forecasts the weather or gives directions. The weather pages describe what a chart shows; the map pages what a symbol stands for, and the map’s own legend has the last word, since the USGS notes that maps in the same series can show the same feature with slightly different symbols.