Aden is hot and humid most of the year. The least bad months are December through February, when daytime temperatures hover around 28°C and the air is dry. From May to September the heat and humidity are unrelenting, with sea fog rolling in along the harbour many mornings. Rain comes in short bursts in March and again in July and August, when the wadis can flood without warning.
Any travel decision is dominated by the security situation rather than by the weather. Check current advisories — Yemen has been in armed conflict since 2015 and most foreign governments advise strongly against travel.