Doreen Allison, of Milton, asks :-

Why does the wind drop at sunset or in the evening after blowing a gale all day?

Lisa Murray, a meteorologist with MetService, responded.

Your observation of the wind dropping mainly applies to two scenarios which are described below.

The first case is a summer sea-breeze. When the Sun rises, the land heats up faster than the sea so it heats the air above it which expands and becomes less dense so rises. That air is replaced by cooler air from above the sea, giving rise to a cool onshore breeze. There is an equivalent off-shore breeze at higher altitudes to replace the air below. At night, the heating effect stops, the land cools quicker than the sea, the air above the land cools quicker and so the off-shore breeze stops. These cool off-shore breezes are the bane of many towns near the sea, e.g. Christchurch in still summer weather has the cold easterly start about 10am and drop off after sunset.

The second case is when we have strong wind during the day, where it is easy for the air to mix and this causes surface gusts. When the surface air cools down at night it forms a temperature inversion, in that the air near the surface (the lowest 100m or so) is cooler than the air above and this temperature inversion dramatically reduces the amount of mixing that occurs between different vertical layers of the atmosphere. As a consequence, once the inversion sets up (after sunset), it is much harder for fast-moving air above the ground to mix down to the surface, where it could appear as a gust of wind.

If there is a low pressure area or storm in the region the winds will blow day or night. Late autumn and winter can bring the strongest storms of the year. These storms have cloud and temperature structures that can often overrule the tendency for inversions to set up at night. The much stronger winds near the storms, coupled with a tendency to avoid inversions near storms, mean that windy nights are more common in the cold session.

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