Reading weather maps

The isobars of a weather map are lines that connect areas of equal atmospheric pressure. The pressure is usually noted on every other isobar so you can see the pattern of pressure distribution. The point with the highest pressure is called a High. It is usually marked with a capital letter “H”. This high pressure center is surrounded by concentric circles of isobars, the isobars closest to the center of the High will have a higher number, than the ones further out. The area with the lowest pressure on the map is called a “low”, this is designated by a capital “L”.

The atmospheric pressure will attempt to reach an equilibrium, so air will flow from a high pressure area toward a low pressure area. The horizontal flow of air is called wind. The wind blows from High to Low, but not directly. Because of the earth’s rotation (Corriolis force) the wind will blow more parallel to the isobars. The wind blows clockwise around a High in the northern hemisphere. The Wind blows counterclockwise around a Low in the northern hemisphere. In the southern hemisphere the wind rotational direction is reversed. A lot of lines close together means strong winds. Areas with widely spaced isobars will have little or no wind.

On a weather map, you will notice some lines that have semi-circles or triangles on either side, or both. These indicate the boundaries for various types of fronts:

Weather fronts1. Cold front: Rainfall can be torrential and wind speeds can be high. Represented on a weather map as a (blue) line with triangles bordering one side. The direction that the triangles point is the direction in which the cold front is moving.

2. Warm front: Often brings a gradual increase in rainfall as the front approaches, followed by prompt clearing and warming after the front passes. If the warm air mass is unstable, the weather might be characterized by prolonged thunderstorms. Represented on a weather map by (red) lines with semi-circles on one side. The side that the semi-circles are on represent the direction in which the warm front is heading.

3. Occluded front: Formed when a cold front overtakes a warm front. Associated with a variety of weather events (possibly thunderstorms) depending on whether it is a warm or cold occlusion. The passing of an occluded front usually brings drier air (lowered dew point). Represented on a weather map by a line with semi-circles and triangles both on the same side. Whichever side they’re on is the direction the occluded front is going in.

4. Stationary front: Indicates a non-moving boundary between two different air masses. Long continuous rainy periods that linger for extended periods of time in one area and move in waves. Represented on a weather map by a line with semi-circles bordering one side and triangles along the opposite side, indicating that the front is not moving in any direction.

These mark the boundary between warmer air on one side and colder air on the other. If you are close to a front and you know the front is moving towards you, you can expect a change in weather (e.g. cloud formation, precipitation, thunderstorms, and wind) when the front boundary passes over you. Their path can be distorted by mountains and large bodies of water.

Links:

  • Global Forecast System – NCEP/NOAA.
    GFS
  • European Centre for Medium-Range Weather Forecasts.
    ECMWF
  • Navy Operational Global Atmospheric Prediction System.
    NOGAPS
  • The Meteorological Office.
    Met Office
  • Weather Online.
    Expert Charts

Medicane

Nearly each year usually in the fall period, when the Mediterranean Sea is still warm, a depression system comes to develop the characteristics of a tropical storm, with cloud patterns wrapped around an eye, intense thunderstorm activity, strong winds at sea level surface and a higher groundlevel temperature within the clouds than outside. This type of storm might possibly intensify into a tropical storm, also called a “Mediterranean hurricane” or medicane.

Xandra is the second tropical-like mediterranean storm after Qendresa who hit hard the island of Lampedusa on november 7. It is the first time in ten years that two systems occur within a month of each other. With a minimum pressure of 992 hPa, Xandra on her course across the Mediterranean Sea has been delivering heavy rainfall to Portugal and most of Spain, affected parts of Morocco, triggered flash floods in south east France before finally hitting the italian coast near from the area of Rome.

As for the waves, forecasts never stopped changing during the two weeks before. After a couple days of wonky east swell, rain and strong winds, a clean four to six foot south swell finally hit on december 01 mid afternoon. It was not Indo perfection on this tricky shorebreak but glad I was free for the short two hours window before the backwash finally ruined it.
However it looks like the models are turning on again, showing bright colors we’ve never seen before. More is yet to come?

Indian winter

Dirk, Petra, Ruth, Qumaira, Stéphanie, Tini, Ulla, continuous flow of deep lows over the North Atlantic these days has finally set up a week of swell in the med. You can also score great waves here, like anywhere else, tracking the swell where it goes. There’s usually a lot of driving involved and a bit of luck as well. Nothing worthwhile ever comes easy.
Normally, we do trips to tropical places, this one was a surf trip at home: long road ahead, heat, sun and pumping waves, all this in the midst of the indian med winter.

Cleopatra

It’s hard to be excited about the last days of swell while many people are affected by the aftermath of “cyclone Cleopatra” which has severely hit the island of Sardinia last week. High water temperatures in the Mediterranean sea have created the conditions for a low pressure system to take up enough energy and organize itself into an extratropical storm. This combined with a slow moving center produced extreme rainfall over a localized area. The equivalent of six months of rain affected Sardinia in less than twenty four hours causing massive flooding, significant damage and the death of 16 people. Italy declared a state of emergency after what turns out to be one of the largest natural disasters over there.
However, this kind of storm is not unusual, even in the Mediterranean. Dramatic weather conditions happen each year especially in the fall/winter season. Meanwhile, a clean swell also pulsed a bit with fun uncrowded waves to enjoy somewhere.

Too big or not too big?

A promising frontal system with gusty SSE winds was expected earlier this month. It finally set up a large swell that rather hit Spain and the western breaks of the french mediterranean. Despite all maps showing 8ft+ on our shores days earlier, it ended up small wrecked by the wind. However here you know the best forecasts result in shitty waves all the time so you learn to appreciate what you have. You can’t really judge from the photo, but it’s 3 foot double-ups and closeouts. Good from afar, far from good, but it was the only game in town.

Storm in a teacup

Interesting to see how pumping waves in the Mediterranean Sea are often associated with weird borderline climatic conditions. Storms lashing especially the French Riviera are rare, but not unheard of. There’s usually a few prominent cases every year, of which only a few deliver actual swell up to our southeastern shores. The Mediterranean provides relatively little surface area making it tough for tropical like storms to develop, however it happens and when it does, storm in a tea cup is really what it’s all about.

This one was generated last week by a 992 HPa depression a hundred miles or so west of Corsica island. This configuration is a best-case scenario as it turns on solid south swells, two magic words for hundreds of desperate guys in this part of Europe.

Ghost sessions

As of today, ex-tropical cyclone Nadine has lasted approximately 19 days. Nadine has probably opened the door for a few depressions in the Mediterranean. This is likely to continue since she re-developped a second time into a tropical cyclone on September 24, and is forecasted to move north northwestward in the eastern Atlantic for the next days. So there was a few surf lately in the Mediterranean caused by a couple of moderate wind storms, nothing great, just typical med sessions.

More than anywhere else, surfing in the Mediterranean requires a lot of reactivity. You must always be on the lookout to enjoy these sporadic swells. Right place, right time, conditions come and go in a blink of an eye quite literally. We had another demonstration of how fast and unpredictable this can happen just the other day.

The wind was blowing steadily since hours, producing a decent windswell but pretty bad waves because of a strong onshore everywhere. There was a little hope that the wind could drop a bit in the late afternoon. So I decided to wait for that, hoping that the swell wouldn’t completely disappear in the meantime. I was quietly installed in the back of my van, laying in the convertible sofa bed installed specially so as to keep an eye on the conditions outside.

An hour or so has passed in the muffled sounds produced by gusting winds over the car. Quite suddenly, without any apparent reason, the wind died. This was quite unexpected and to give an idea, these webcam videos show how conditions were like, before, and half an hour later.

Click to Watch VideoClick to Watch Video

In a matter of minutes, the wind picked up again, increasing in strength but in the opposite direction. It only took a few time for the wind to switch 180°, changing the conditions from depressing to almost not bad with now the offshore growing a nice white mane on top of the breaking waves.

That’s when a heavy thunderstorm broke out. Dark clouds followed by intense rain, hail and lightenings hit the area very quickly. The situation could have felt like the end of the world. The storm was big enough to cause a short blackout reported in the local news, but fortunately there was no damage. It was also probably the reason of the rather strange wind behaviour. Such manifestations of nature are not so common, but the chain of events wasn’t finished yet. What happened just after was also funny.

So I was chilling out in my van waiting for the wind to calm down as most meteo models had predicted, and I didn’t expect what was about to happen at this point. The parking was empty and there was barely no one on the beach. The moment I noted the wind had changed and the storm hit, it was glassing off and waves got far better. It all happened rather quickly, but cars started right away to fill up the parking and guys in wetsuits were already into the water. The situation was unreal. While I thought I was alone, all those guys showed up in the middle of this dramatic weather almost at the same time. They just came out from nowhere and did that at the precise moment when the wind turned offshore!

Needless to say, it wasn’t the session by myself I had imagined in my van. Sometimes, I really wonder how med surfers get their information. Anyway, it wasn’t the session of the year but the surf wasn’t too bad.