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Old 15-03-2011   #1
Irishsnow
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Default Guide To Reading A Weather Chart



1. Depression (Cyclone/Low Pressure):



A depression is an area of low pressure that steers the weather systems that circulate around it and is often marked by an ‘X’ or ‘L’ in a weather chart. In a low pressure centre, moist air at lower levels rises and cools creating abundant clouds. Depressions (more HERE) often form along the Polar Front and will normally move from a south-west to north-east direction over the north Atlantic. When in the vicinity of Ireland, depressions can bring unsettled weather to the country.

2. Frontolysis:


A weakening weather front is known as ‘frontolysis’. Frontolysis often occurs when a front moves in over an area of where air is descending strongly enough to create a ridge of high pressure. A weakening front is represented on a weather map in the form of a ‘broken’ weather front. In the case above, an upper warm front is dissipating as it moves in towards Scandinavia. An intensifying weather front, known as ‘frontogenesis’ is depicted as an un-formed weather front - usually by a small dot between the coloured symbols that define a warm or cold front on a surface pressure chart.

3. Warm Front:


A warm front is the dividing line between cool air and an advancing warmer, moister air mass. A warm front more often than not brings cloudy weather to Ireland with a spell of light to moderate rain. A warm front is represented on a weather chart as a red line that has soft, rounded ‘D’ symbols attached in regular sequence to it which are generally pointing in the direction in which the warm front is moving. In the example chart above, it can be seen that the warm front over the UK is moving eastwards into the North Sea.


4. Warm Sector:



A warm sector normally lies behind warm front and ahead of a cold front. In our example chart above, Ireland is lying under such an air mass (further explanation HERE) as the warm front has moved through in towards the UK. Because the air has picked up a good deal of moisture on its long journey over the sea, this type of set up usually brings cloudy, breezy damp weather.

5. Anticyclone (High Pressure)



Anticyclones are areas of high pressure in which air descends rather than rises as in an area of low pressure. As a result, anticyclones are normally associated with fine, quiet weather with very little wind. They are often marked with an ‘H’ such as in our chart above.

6. Cold Front:



A cold front forms the boundary between a warmer air mass and a cooler one. A cold front often brings a short spell of moderate to heavy rain over Ireland before it introduces clearer, cooler air from the Atlantic. On a weather chart, a cold front will be drawn as a blue line with blue triangular points attached in regular sequence to it which point in the direction in which the front is moving. As can be seen in the chart above, a cold front to the north-west of Ireland is moving south-eastwards.

7. Isobar:



The lines on weather chart, such as on the above, are known as isobars. Isobars represent the mean sea level pressure (MSLP) values over a given area. On the chart above for example, a MSLP value of 996hPa (—–996—–) occurs along where that particular line runs. The width at which separate isobars run parallel to one another, (commonly known as the isobar gradient) can give an idea of how strong wind speeds are over a particular area. Isobars tend to have a tighter gradient closer to a low pressure system and a wider gradient closer to a high pressure system. A general rule of thumb is the tighter the isobar gradient, the windier it will be. Click HERE for a more detailed explanation of atmospheric pressure.

8. Cool Sector:

Normally, once a cold front has over a cooler, brighter and drier air will set in over Ireland, usually in the form of a Polar Maritime air mass (more HERE). MSLP normally increases once this cooler air moves in although showers can occur.

9. Occluded Front: (Occlusion)



An occluded front or ‘occlusion’ is formed when a cold front has caught up with a preceding warm front and generally represents the dividing line between one cool air mass and another less/more cooler air mass. On a surface pressure weather charts, an occlusion is normally represented by a purple or pink line with the rounded symbols of a warm front, and the triangular symbols of a cold front grouped closely together and attached in regular sequence along it. Occlusions can either be classified as warm, where a less cold air mass is overtaking a colder air mass, or cold, where a colder air mass is overtaking a less cold air mass. Because occlusions tend to the most common frontal feature closest to a fully form depression and thus slower moving, they can sometimes bring prolonged spells of rain over Ireland.

http://www.irishweatheronline.com/ir...her-chart.html
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