An extension of a high pressure, as depicted by a "finger" of isobars extending away from a high pressure, is called a ridge or a wedge. The equivalent for a low pressure is called a trough. Because troughs are likely to bring bad weather (the opposite of a ridge) they are depicted by thick black lines on surface pressure charts. Ridges have no equivalent symbol.
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What is the extension, as typified by isobars protuding from the centre, of a low pressure system?