Late afternoon, the harbour of a little island in the South China Sea. Fine, dry, low humidity, blue sky, warm, a day to be out and about. . .
But with the prompt setting of the sun (around 6pm), the temperature plummets -- from 17.3C to what is now 11.9C.
There are colder places hereabouts. For instance, there's somewhere called Ta Kwu Ling. Apparently, it's in the New Territories. It's not yet midnight and the temperature there has already dropped to 4.5C. Maybe there'll be frost there in the small hours?
Why Ta Kwu Ling? Why so cold? Google -> Wikipedia -> Ta Kwu Ling
Hong Kong Observatory: Grass temperature at Ta Kwu Ling
NB: "This webpage displays the grass temperature during the period 5 p.m. to 8 a.m. the following day. It is updated every 10 minutes. The grass temperature during the period 8 a.m. to 5 p.m. is not displayed. The measurement may not truly reflect the grass temperature due to direct exposure of the sensor to the sun."
Since frost is so rare in Hong Kong, it is a matter of some import -- people will drive out in the middle of the night to places such as Ta Kwu Ling or even climb one of the many peaks in the New Territories to view the frost, touch it, photograph it. You might do too if you grew up in a place which is mostly warm.