How to Read a Synoptic Weather Chart

Every wind, swell and weather change you care about is written on the synoptic chart before it ever reaches you.

It looks intimidating at first, a tangle of curved lines and letters, but the basics take only a few minutes to learn.

Once it clicks, a glance at the chart tells you roughly where the wind will blow from, how hard, and when it will change.

For anyone who sails, fishes, surfs or paddles, that is a genuinely useful skill, so here is how to read one.

Highs and lows

The big letters are the starting point, with H marking a high pressure system and L marking a low.

A high, or anticyclone, generally brings settled, stable weather with lighter winds near its centre.

A low, or depression, is the troublemaker, bringing cloud, rain, stronger winds and the fronts that sweep through with them.

In broad terms, find the high for calm and the low for wind, then look at how the two are arranged.

Isobars are the key to wind

Isobars are the curved lines joining places of equal air pressure, a bit like the contour lines on a topographic map.

The single most useful rule is that the closer the isobars are packed together, the stronger the wind will be.

Widely spaced isobars mean light winds and an easy day, while a tight bunch of lines means it will honk.

So before anything else, look at the spacing over your patch of coast and you already know if it is a big-wind day.

Which way will the wind blow

Wind flows almost parallel to the isobars rather than straight across them, so the lines also show you the wind direction.

In the Southern Hemisphere, wind circles anticlockwise around a high and clockwise around a low.

A handy check is Buys Ballot's law, which says that if you stand with your back to the wind, the low pressure is on your right.

Put those together and you can trace, from the chart alone, roughly where the wind over your break or bay is coming from.

"The isobars tell you the strength and the direction; once you trust them, you stop guessing and start planning your sessions."

Reading the fronts

Fronts are the boundaries between airmasses, and they are drawn as lines with symbols along them.

A cold front is a line with triangles, and it usually brings a sharp wind change, a temperature drop and a burst of showers.

A warm front is a line with semicircles, and it tends to bring more gradual cloud and rain ahead of milder air.

The symbols point in the direction the front is moving, so you can see which way the change is heading and when it may reach you.

Putting it together for the coast

In Australia the position of the high largely sets your coastal wind, so watch where it sits relative to your location.

A high sitting to your south often means easterly winds, while a high sliding off to the east opens the door to a northerly ahead of a front.

The strongest winds usually sit near cold fronts, deep lows and in the westerly stream that runs across the far south of the country.

Track how the whole pattern shifts from one daily chart to the next, and you will start to anticipate changes rather than react to them.

Common mistakes

Reading a single chart. One map is a snapshot, so compare a few in sequence to see which way the systems are actually moving.

Ignoring the isobar spacing. The letters grab your eye, but the spacing of the lines is what really tells you the wind strength.

Forgetting the hemisphere. Northern Hemisphere rules are reversed here, so always apply the Southern Hemisphere circulation.

Trusting the chart alone. A synoptic map shows the big picture, so pair it with a local wind forecast for the fine detail.

Common questions

What do the numbers on the isobars mean?

They are air pressure values in hectopascals, and the change from one line to the next shows how quickly pressure is rising or falling.

How do I know how strong the wind will be?

Look at the isobar spacing, because tightly packed lines mean strong wind and widely spaced lines mean light conditions.

Which way does wind blow around a low here?

In the Southern Hemisphere the wind circulates clockwise around a low and anticlockwise around a high.

Do I still need a wind forecast?

Yes, as the chart gives you the big picture while a local forecast fills in the timing and strength for your exact spot.

Put it into practice with the Seabreeze Wind forecast