Settled weather is calm weather. There is hardly any wind.
In summer this weather may give long sunny days. You know it is settled if it is hazy and if you get a red or a deep yellow sunset with a light purple sky above.
Settled weather is caused by air gently sinking down. Weather forecasters call this a high pressure.
In winter, settled weather is often not so nice, as it brings dull frosty days known as gloom.
