How Do You Draw Rainfall on a Map? Join the Places That Get the Same Amount

Rainfall is measured at gauges, which are points, and a map needs values everywhere, so a line joining points of equal rainfall is drawn by interpolating between the measured values. That line is a contour in exactly the sense that a height contour is, and the resulting map is read the same way, with closely spaced lines showing a steep gradient and widely spaced ones a gentle change. The name belongs to a family of terms for lines of equal value, alongside ones for temperature, pressure, depth and many other quantities, all built on the same Greek root for equal.

Everything between the gauges is an estimate and several methods exist:

• •Drawing by eye, which uses the mapper's knowledge of the terrain
• •Simple averaging between nearby gauges, weighted by distance
• •Dividing the area so each gauge governs the ground nearest it
• •Statistical methods that model how values vary with distance
• •Methods that include elevation, since rainfall rises with height
• •Radar and satellite data, which measure between the gauges directly

The network supplying the measurements is not distributed to suit the mapping, which introduces a systematic bias that has to be managed. Gauges are placed where somebody can reach them and where an organisation has a reason to measure, which means near settlements, at airports and at low altitudes, and mountains are consistently under-sampled. Since rainfall generally increases with elevation and varies sharply across a mountain range, the areas contributing most of the water to a river system are the areas measured least. Correcting for that is a standard part of hydrological work and is why elevation is built into the interpolation rather than treated as an afterthought.

The family of terms is large and recognising the pattern makes technical maps readable. Lines of equal temperature, pressure, depth, salinity, magnetic declination and travel time all have their own names built the same way. Lines of equal earthquake intensity map how strongly a shock was felt rather than how large it was. Lines of equal date of a recurring natural event, such as the first appearance of a leaf or a migratory bird, map the progress of a season across a country. In every case the map is built from point observations and interpolation, so the same caution about what lies between the measurements applies to all of them.

The output feeds decisions with substantial consequences, which is why the interpolation method matters rather than being a technicality. Water supply planning needs to know how much falls on a catchment. Flood modelling needs the distribution of a specific storm, not an average. Agricultural planning, crop insurance and irrigation design all use long-term maps. Reservoir operators use them for inflow forecasting. And climate monitoring compares maps across decades to detect change, which requires the method to be consistent over time, since a change of interpolation technique can produce an apparent trend that is entirely an artefact.

Joining points of equal rainfall turns point measurements into a continuous picture, read exactly as height contours are, with everything between the gauges estimated by interpolation. Gauges sit where people can reach them, so mountains are under-sampled precisely where most of the water falls. Comparing maps across decades requires the method to stay constant, or the change itself produces a false trend.