高 as a verb talking about the weather
The construction Adj. + 于, where the adjective X is actually a stative verb means &quot;more X than&quot;.
Stative verbs, which in English are normally translated as adjectives, are verbs that describe qualities, and can be modified by intensifiers as 很，太，非常, etc.
低 means &quot;low&quot;, it is a stative verb, therefore 低于3度 means &quot;lower than 3 degrees&quot;. If you want to keep some morphological correspondence with English, you can render is as &quot;to drop&quot; (= to go lower than).
As for your example sentences:

最高气温很少高于40度 here 高 (high, tall) is a stative verb, or adjective, like 低 (in fact it's its antonym), therefore this is a correct sentence, where 高于40度 means &quot;higher than 40 degrees&quot;.

最高气温很少超过40度 I think 超过 is not the best verb here, but the sentence sounds fine to me.

最高气温很少达40度 here 达 alone doesn't sound okay; I would say 达到

最高气温很少降到40度 the word 降到 means “to fall, to drop&quot; so this sentence  is borderline nonsensical (= the highest air temperature rarely drops to 40 degrees). You can use 升到 &quot;to rise&quot; instead: 最高气温很少升到40度