He says he sees things on rainy days as doesn't show when it's fair weather.

他说他在雨天看到在晴天却看不出来的东西。

youdao

For example, in fair weather, the air pressure is generally high, the air is still, and often full of dust.

例如，在晴朗的天气里，气压通常很高，空气是静止的，而且经常含有灰尘。

youdao

Their wearisome heads went up and down at the same rate, in hot weather and in cold, wet weather and dry, fair weather and foul.

不管天气是冷是热，是干是湿，是好是坏，它们那疲惫的机头都会以同样的节奏上下摆动。

youdao

Having made his economic reputation in fair weather, Mr Brown must now cope with more troubled times.

ECONOMIST: The credit crunch

Nothing builds a fair weather band wagon like a decade of dominance.

FORBES: The Giants Have Better Fans

When he looks at the latency on an exchange, he wants to know more than how it functions in fair weather conditions, he wants to know how it works in something like the flash crash.

FORBES: Nasdaq makes technology superiority claim in its drive to acquire NYSE