I don't think that there is a "cool dry" climate anywhere, year round.
AH yes, I know.
Kind of high up on the dry side of the volcanoes in Hawaii.
The rain falls mostly on the wet side.
Once over the summit, the air is relatively dry.
I was surprised to learn that there's quite a bit of (imported) cactus in Hawaii - at least on the big island.
The winds are pretty consistent, year round, so each of the islands has a dry, and a wet side.
When the wet air over the ocean blows over an island, as it rises, it cools and it rains.
Once over the summit, it warms, and the relative humidity decreases quite a bit.
When you have large dry areas of land, like desert, it tends to be warm-hot in the day, and cold at night, because dry land doesn't hold as much heat. Water holds heat. Dry land does not.
That said, Death Valley is still somewhat warm at night. But the difference between the daytime and nighttime temperature is pretty dramatic.
The question is so vague from your teacher. Answer: a climate that has cool temperatures with relatively low precipitation.
There are many cool dry climates, the coolest and driest is Antarctica, so you need to be more specific.
Denver
About -3 degrees celsius, with no rain
Mount Kilimanjaro. the further up you go, the cooler it gets.
The would be cool...
