On a clear day, a pilot in controlled airspace can choose a VFR or IFR flight plan, and the FAA regulations were devised in a way which accommodates both VFR and IFR operations in the same airspace.

在晴天，在管制空域内的飞行员可以选择目视飞行规则或者仪表飞行规则的飞行计划，而且联邦航空局的条例是以这样一种方式设计的：在同一空域同时适用目视飞行规则和仪表飞行规则的操作。

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On a clear day you can see France.

天气晴朗时你可以看见法国。

《牛津词典》

On a clear day you can see the coast.

天晴时你能看见海岸。

《柯林斯英汉双解大词典》

It can do this between sunrise and noon on a clear day with an average outside temperature of thirty-two degrees Celsius.

VOA: special.2009.04.20

to get a view all over the city, on a clear day, of course.

俯瞰整个城市，当然这要在一个晴朗的日子。

神奇的博物馆 - SpeakingMax英语口语达人

This was clear on Wednesday hardline separatists called a general strike to protest the Indian leader's two day visit to the region.

VOA: standard.2009.10.28

On a clear day, you might even be able to make out the Welsh coast.

BBC: Day trips from Dublin

The view, on a clear day, is of the Columbia Icefield 75km to the south.

BBC: The natural wonders of Banff and Jasper National Parks

On a clear day you can see to the Lizard Peninsula more than 60 miles away.

BBC: Historic Mount Edgcumbe's uncertain future