Isobars: lines on a synoptic chart that join areas of equal atmospheric pressure
Front: the point where two air masses meet; it typically brings changeable weather conditions
Atmospheric pressure: the weight of air pressing down on the Earth
19.1 Weather Conditions
Isobars Isobars are lines on a weather map that connect areas of equal atmospheric, or barometric, pressure. The millibar (mb) is the unit of measurement of atmospheric pressure. Each isobar represents a difference of 4 mb of atmospheric pressure. Millibars are also referred to as hectopascals.
A
B
: Fig. 19.1 A synoptic chart of Ireland and Western Europe ACTIVE LEARNING
1. What is the name given to the study of atmospheric conditions?
2. What is a synoptic chart?
3. What is the name given to the unit of pressure for isobars?
4. What do tightly packed isobars tell us about weather conditions in that area?
A wind is caused by air moving from areas of high pressure to areas of low pressure and vice versa. There are clear links between wind patterns and isobars.
• Isobars that are closely grouped together indicate that there are strong winds as a result of rapidly changing atmospheric pressure (see the area marked A in Fig. 19.1).
• Isobars that are spaced widely apart indicate that conditions are calm as there is a slow change in atmospheric conditions (see the area marked B in Fig. 19.1). This means winds are light.
• Winds typically blow almost parallel to isobars, giving winds a swirling motion (think of the shape of hurricanes).
Fronts
A weather front is the point where two air masses meet. Fronts move with air masses and bring quickly changing weather conditions. Three types of front can be identified on a weather chart:
320 EARTH
A
Z
A short question on weather maps has appeared every year since 2006 on both the Higher Level and Ordinary Level papers.
