这里为什么分别用 at night 与 in the night?

有一读者问:初中英语新教材有这样几句话:

In the Northeast it will be fine. The temperature will stay above zero in the daytime, but at night it will fall below zero again. In the Northwest, there will be snow in the night.

请问 at night 与 in the night 有何不同?

在这里用 at night, 是由于与其前的 in the daytime 相对之故;用 in the night 则是为了强调时段的概念。