A high or an anticyclone is an area of high atmospheric pressure that forms as cold air presses down on the Earth. It is marked ‘H’ on a weather chart, which stands for high pressure or anticyclone. Anticyclones bring settled weather. Winds blow towards the centre in a clockwise direction (see Fig. 19.6(b)) and bring weather conditions such as clear, cloudless skies, light winds and no rain.
(b)
HIGH
:) Fig. 19.6 (a) A high-pressure system moving across Ireland (high pressure = clear skies) and (b) a high pressure/anticyclone
ACTIVE LEARNING
1. What type of weather is associated with each of the following?
(a) A cold font (b) A warm front (c) An occluded front
2. What is a cyclone and how would you identify it on a synoptic chart?
3. What type of weather is associated with an anticyclone?
4. Use Fig. 19.7 to answer the following questions:
(a) What is the direction of the most common wind to blow across Malin Head?
(b) In general, are westerly or easterly winds stronger?
(c) What was the most frequent wind at Dublin Airport?
(d) Which wind was the least frequent?
(e) What percentage of weather was calm at Valencia?
322 EARTH 19.2 Weather Charts Malin Head
Wind Radar Charts Met Éireann uses wind radar charts to show the most frequent winds blowing in an area. The charts show:
• The direction from which the wind blows
• The percentage of time it blows from that direction, using a ‘Scale of frequency’ bar
• The percentage of calm weather, which is read from the figure at the centre of each weather station chart
Wind Rose Diagrams
Wind rose diagrams are used to show wind speed and direction in an area over time. Each ‘arm’ of the diagram shows the direction the wind was blowing from, while the different colours indicate the different speeds. In Fig. 19.8, we can see that the most frequent wind direction was southerly, but the strongest winds came from the west and north-east.
Valentia Observatory : Fig. 19.7 A wind radar chart 0 5 10 15 20 25 % Scale of frequency Belmullet Clones Claremorris
