ABC of Weather: Isobars

The above picture (courtesy of Meteorologica) shows the British Isles complete with mapped isobars. These are the many blue lines shown on the map and denote lines of equal air pressure. The term isobar is derived from iso, from the Greek meaning equal and bar, one of the unit names given for air pressure measurement.

We have High pressure (aka an anticyclone) over the UK at the moment: Tuesday's map* showed it was to the south-east with the pressure well over 1000 millibars (1027 in fact). High pressure means a period of calmer weather, which we've actually been enjoying over the past couple of weeks or so. You can tell the weather is calmer from the map if the space between the isobars is relatively wide.

However, whilst the weather has been quiet and sunny, the winds on the whole have been bitterly cold because they've been from the north or east. As air circulates in a clockwise direction around an anticyclone in the northern hemisphere, you can tell which way the wind is blowing simply by finding the centre of the High and following it round to its position over the British Isles. Luckily there's also been sunshine, so it's been fairly pleasant outside for gardening once the early morning frost has gone, though that cold wind has meant I've had to resort to my fingerless gloves from time to time.

Today, our weather is set to change to dull and wet as a Low (aka cyclone) is due to come in from the south-west tomorrow. It means the air pressure will probably fall to below 1000 millibars, the mapped isobars will grow closer together and their alignment over the British Isles will change as the wind picks up and veers round to the south-west.

Maps like the one shown are one of the basic tools used by weather forecasters. The data for the maps are collected from dozens of weather stations throughout the UK and the air pressure measurements are corrected to sea level values (as air pressure varies with height) before the computer produces a map like the one above. When they're shown on the TV forecast, I get much more out of that simple visual cue on how our weather will be over the next day or so, than any amount of description from the presenter. There's nothing like the tightly packed isobars of a remnant hurricane heading our way to get me rushing out into the garden to batten down the hatches!

* = I have no idea what the map will be showing when you read this as it appears the HTML code I'm using to show you the map updates the map every few hours or so rather than being a snapshot in time!

How's the weather with you today?

For many more Interesting posts in the way of I, do visit the ABC Wednesday blog.