This requires you to finish a stage in a daily, weekly or monthly Community Event where the conditions are "Dry". The symbol for these conditions is a sun - just a sun, no cloud. However, the achievement description doesn't say that the stage must also take place in the middle of the day. It unlocked for me after finishing the third stage of a Weekly Event. The first three stages were all in Dry conditions but the first was in the morning and by the time the third stage took place it was around midday.
The achievement will unlock at the end of the stage, you do not need to wait until the event is over and the next one starts like some of the other Community Event achievements.