Synoptic scale: understanding global weather drivers

Understanding weather begins at the global level, what meteorologists call the synoptic scale. The word "synoptic" comes from Greek and means "seen together" or "comprehensive." In meteorology, it refers to large-scale atmospheric patterns and interactions that influence the weather over wide areas.

Figure 1: synoptic chart over the Atlantic ocean and Europe, 6 August 2025

Synoptic scale level is the first level to consider when assessing weather before heading at sea. It gives an overview of the large scale drivers such as high and low pressure areas, front positions, clouds, winds, air and sea temperatures...

These patterns are captured in synoptic charts, which are displaying:

• Atmospheric pressure (with isobar lines, H being high pressures and L (T in German, C in Croatian) being low pressures,
• Weather fronts (see picture below): 
• cold front shown as blue lines with triangles pointing towards its moving direction, 
• warm front shown as red lines with half-circles pointing towards its moving direction,
• stationary fronts as alternating cold and warm fronts, usually aligned and not moving, 
• occluded with pink alternating triangle and half-circles, indicating that two cold fronts have merged, pushing up the warm air above the sea surface. 
• Temperature (as colour gradients), as an added layer,
• Wind (direction shown by arrows, animations or arrows with barbs), and
• Cloud cover from satellite imagery,
• Precipitations thunderstorms from radar imagery.

Figure 2: weather front representation, Croatian weather services, forecast for 7 August 2025. 

On the synoptic chart above, a cold front is moving eastward over the west coast of England and Scotland (blue triangle lines), catching over an warm front along the east coast of England and Scotland (red semi-circle line), resulting in an occluded front (pink alternating triangles and semi-circles). On the east, the top right is showing a short stationary front with aligned red warm front and a blue cold front on the upper right corner of the picture.

Most of these elements come from direct measurements or satellite data, but fronts are identified by expert meteorologists interpreting data layers. A comprehensive synoptic chart that has been fully analysed and integrated is hard to find online since they require human analysis. Check the Data Sources section for the best synoptic charts for the Northern Adriatic. As you can see on the links we provide in the site, the representations are not always consistent!

Air masses: anticyclones and depressions

While the global air circulation model helps anticipating weather, real weather is shaped by interactions between air masses. Particularly where cold polar air meets warmer air from the Ferrel cells, turbulence and undulations develop along their boundaries. This leads to fronts and perturbations.

Two key systems dominate:

• Anticyclones: high-pressure systems formed by sinking cold and dry air. These bring clear skies and light winds.
• Depressions: low-pressure systems caused by rising warm, humid air. These bring clouds, rain, storms and stronger winds.

Use isobar charts as on figure 1 to identify these systems. Platforms like Windy allow overlaying pressure with wind speed and direction. For practical sailing, consider selecting wind gusts rather than average wind, as gust winds often represents the actual conditions you may face at time and therefore you should be ready for.

Seasonal interpretation matters:

• In winter, high pressure is often cold and stable, especially inland, resulting in clear skies but freezing nights.
• In summer, high pressure means sinking, warming air, bringing hot and stable dry and sunny conditions.

On July 13 over Istria, for example, wide-spaced isobars and no fronts meant stable, uneventful synoptic conditions. However, global calm doesn't guarantee local safety. On June 19, a sudden Nevera storm hit the Northern Adriatic despite calm synoptic conditions—over 30 boats were pushed ashore. This is to highlight that synoptic scale drivers are not sufficient to anticipate local weather conditions in the Adriatic, because the Adriatic is a small area sea, where local drivers are to be seriously be considered along in combination with synoptic drivers!

Navigating pressure systems

When high- and low-pressure systems sit near each other, quick decisions may be needed, especially at sea. Two sailor rules of thumb:

• Avoid the center of a low-pressure system. Winds rotate counter-clockwise (in the northern hemisphere), so being in the northeast quadrant keeps you on the escaping edge.
• If crossing a cold front, choose a path that avoids the center of the low. Distance from the centre minimises impact.

Fronts and their passage

Meteorologists identify fronts by analysing temperature contrasts, cloud formations, wind patterns, and upper-air flows (like jet streams). Here's how different fronts behave:

Cold fronts:

• Identified by a blue line with triangles pointing in the direction of movement. A cold air mass wedges under a warmer, lighter one, forcing it sharply upward, like a bulldozer.

Expect:

• Sudden cooling as warm air is replaced;
• Wind shift (usually veering to W/NW after SE/Jugo);
• Falling then rising pressure;
• Towering cumulus and cumulonimbus, intense but short rain and thunder.

Example:

• in Bora setups, wind may swing abruptly from SE to NW.

Warm fronts

Marked by a red line with semicircles. Warm, lighter air slowly rides over a receding cold air mass.

Expect:

• Gradual warming and rising humidity,
• Winds backing from east to SE to south,
• Steady drizzle or layered cloud buildup,
• Slow pressure fall and gradual rise post-front.

Note: warm fronts often precede a cold front.

Occluded fronts

These occur when a cold front overtakes a warm front, lifting warm air completely off the surface. Symbolised by alternating triangles and semicircles on the same side, coloured in pink.

Two types:

• Cold occlusion: air behind is colder than ahead and behaves like a cold front,
• Warm occlusion: air behind is milder and behaves like a warm front.

Expect:

• Persistent cloud cover and rain,
• Confusing wind shifts,
• Lower visibility.

Occluded fronts mark the final stage of a mature low-pressure system.

This overview of synoptic drivers is the first layer of weather understanding. Knowing how to interpret pressure systems, fronts, and synoptic charts gives you the broad strokes. For sailing in the Northern Adriatic, though, mesoscale and microscale phenomena are often more immediately relevant. We'll explore those next.