Making the weather more accessible

Pressure, highs and lows

on February 11, 2015

In the past year I have seen the media explain normal weather conditions as some very strange things. Recently, the “Weather bomb” has become a new definition, which is more understandable then some media phrases as the word bomb as a news headline could alarm a few people, but it did seem to be thrown around rather a lot. Anyway, I thought I’d clear a few things up when it comes to pressure.

The weather around the world is affected by pressure, which is measured by a barometer in millibars (mbar) or hectopascals (hPa). The standard pressure level is 1013mbar. The pressure continuously tries to balance out around the globe. In large scale terms, high pressure tends to be based close to the equator and low pressure close to the poles. This is because high pressure and low pressure systems surround the two polar jet streams as illustrated in the diagram below.

Diagram to illustrate global circulation (Climate4you 2015)

Diagram to illustrate global circulation (Climate4you 2015)

High and low pressure systems develop where the jet stream meanders. In the northern hemisphere, the low pressure systems form to the north of the polar jet stream in troughs and high pressure systems form to the south of the jet stream in ridges.

High pressure systems bring settled weather to the  UK, this can be dry and sunny or cloudy, as this week has shown. Winds travel in a clockwise direction around a high pressure system (in the northern hemisphere) which can lead to cold or warm air from the continent being brought into the area. In comparison, low pressure systems bring unsettled and windy weather to the UK. The winds travel in an anti-clockwise direction and usually have frontal systems associated with them with can bring heavy and persistent bands of precipitation. An example of how these systems appear on a synoptic map is shown below as well as how pressure is illustrated by isobars; lines of equal air pressure.

Synoptic map showing low pressure across the UK and high pressure across the Mediterranean (KNMI 2015)

Synoptic map showing low pressure across the UK and high pressure across the Mediterranean (KNMI 2015)

Just to make things even more confusing, there can be different types of high and low pressure systems. The most significant variant of a high pressure system would be a high a Blocking High. This works in the same way as a normal high pressure system. However, it is more rigid in it’s form and therefore low pressure systems fail to move it out the way or break it’s barrier. These systems tend to last 7-10days.

When it comes to low pressure systems, the most notable is probably a Bomb Low (also now known as a Weather bomb). A Bomb Low is when the pressure at the center of the low drops by more than 24mbar in 24 hours; this means there are often very strong winds associated with them. Another low pressure system that can lead to notable weather is a Polar Low. Polar Low’s are short lived low pressure systems, but they can be very intense. The form in cold conditions when the sea below is warmer. These systems are often associated with heavy snow and strong winds.

Over the next few weeks I’ll try and explain frontal systems and air masses in more detail, so that you can read the crazy headlines and translate it into what the weather is actually going to be like!! E.g. “Arctic blast to hit the UK” last week would be “Normal conditions for the UK in the middle of winter.”

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