Practical Weather – Pressure Maps. How to use them to predict what the weather will be like

“We should not take forecasts as absolute truths,” explains marine meteorology “guru” Riccardo Ravagnan, head of Meteomed, a specialized rib of 3BMeteo and Italy’s leading provider of applied recreational boating meteorology.

Who better than he, in the run-up to the summer season, can explain how to take advantage of his knowledge and the tools that Meteomed makes available to boaters to prevent the weather “drafts” that increasingly affect our seas? In six stages, for six months, we will share with you meteorology pills to keep on hand at all times.

Weather. The first tool to consult: pressure maps.

What, then, should be the first tool a mariner should consult? Ravagnan says it is definitely pressure maps, the gateway to meteorology. “Pressure maps are the first step in our Meteorologist’s Checklist for Safe Boating, a practical guide that we at Meteomed have developed to accompany every sailor (beginner or expert) and available for those who request it by writing to info@meteomed.it.

What are pressure maps?

A pressure map (or isobaric chart) is a cartographic representation showing the distribution of atmospheric pressure in a given area. Lines joining points of equal pressure are called isobars and are used to identify centers of high and low pressure, such as anticyclones and depressions.The set of these lines, drawn on a map, generate a pressure map.

Pressure maps allow us to understand the synoptic context, that is, to get a snapshot of the atmosphere on a scale as large as the Mediterranean basin. Using these maps, we are able to understand where the main centers of action, i.e., areas of high and low pressure, are located and how they are moving. That’s where everything comes from: wind shifts, disturbance fronts, and areas of instability or flat calm.

Weather tools. How do they read?

First, the pattern of the isobars must be observed : if they are very close together, they indicate a strong pressure gradient and therefore intense winds, while if the lines are spaced apart, the wind will be weak. The pressure gradient is a measure generally expressed in millibars of the change in atmospheric pressure over a given horizontal distance. It is important to understand and be able to read it because the gradient determines the direction of the wind, which flows from high-pressure areas to low-pressure areas, following a trajectory perpendicular to the isobars.

Then we need to locate the centers of high and low pressure, but most importantly we need to understand where the fronts are, which are dynamic structures that bring with them sudden changes in weather: a front can turn a clear day into a stormy afternoon.

In essence, the pressure map tells us what to expect and why; according to Ravagnan, “We no longer have to take predictions as absolute truths, but learn to read them in relation to context. The goal is to develop a critical sense, based on knowledge and observation. The era of ‘weather icons’ is over: today we need understanding. This is the only way we can say that we abide by the principle of diligence also provided for in the regulations on safety at sea.”

And after the maps…

The second step is to read the free general Mediterranean bulletin, which Meteomed publishes every week. It is a very high-value tool, because meteorologists read the same maps that anyone can look at, but they interpret them with an expert eye, pointing out unstable phenomena and critical areas.

Through this first stage, we learned how to use the famous checklist to gain awareness and the bulletin for professional confirmation. Learning to use them together means developing a real shared language with the atmosphere.

Federico Lanfranchi