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, St├ęphanie, 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.

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.

Hurricane 2012

Ex-tropical storm Christine in med
Say hello to ex tropical storm Christine, which almost turned into a TMS (Tropical like Mediterranean Storm) in the Mediterranean on september 3rd. Christine generated heavy rain and caused flash flooding in Malta, but unfortunately no swell for us.

Coming next: Leslie, Michael… and Nadine!
Is 2012 the next hurricane record after 2005 and 2006?
Are we all going to die in 2012?
To find out, stay tuned to Free-Surf Photo.