你的问题似乎是一个概念,所以我回答概念上。为了论证假设平面与不透水地面面积,喜欢摇滚,一个不透水墙。After it has rained for some time with a certain intensity over that area — everywhere the same —, there will be a certain height of water on the ground, which will be the same height all over this flat area. This height is measured in millimeters and reported. You can now go and divide this area up in square meters, or soccer fields, or square milimeters; the height of the water would obviously not change. That's the beauty of this measurement: It is independent of any particular area. These are unrealistically ideal assumptions. The intensity or duration differed from place to place, the ground is not level, water seeps away etc. So the statement actually says: "This station measured a rain event which would cover any area with x mm of rain if it had rained everywhere the same amount as it did here." The duration is irrelevant with respect to the amount but must be mentioned together with the measurement because it makes a huge difference in meaning and impact: 100 mm in a month are just wet while 100 mm in a day are catastrophic. In your case, "last Friday", one may assume a 24 hour calendar day. It's also often not possible to make a reliable statement for a larger area, especially in the case of local severe weather events. One can make an estimation based on measurements from different stations in the area and qualitative observations, but the only reliable accurate statements which can be made are factual measurements at weather stations; everything else is inferred, as in your example about all of Hong Kong. I can imagine though that among Meterologists the sentence "Hong Kong reports 3 mm of rain" is taken to mean "The [Hong Kong Observatory][1] reports 3 mm of rain", and equivalently for other places. [1]: https://www.hko.gov.hk/abouthko/aboutus.htm