The climate of Karachi is quite warm throughout most of the year. Average lows in the winter range in the low 60s and average highs range in the upper 70s to low 80s. During the summertime, the heat can be oppressive, with temperatures ranging from daily lows of in the low 80s to highs in the low 90s. Add to this high humidity levels, and you've got a recipe for some quite warm and sticky days. To beat the heat, your best bet is to travel to Karachi during the late winter or early spring. Precipitation remains low for most of the year, though most of the rain that falls in Karachi occurs during the months of July, August, and September which coincides with the monsoon season.  

You can get up-to-date weather forecasts for Karachi from the website of the Pakistan Meteorological Department.