First off temporary fluxuation in humidity levels are fine, but over the long run you want to mimic their natural environment as best as possible.
Humidity needs to be measured within the enclosure, not taken from the weather report or even from the room where your gecko resides.
If night time temperatures in your living area are not to your liking and you can afford an air conditioner I would recomend that you first go that route, an air conditioner will reduce the humidity levels in your home somewhat and allow for easier humidity and heating control (it is always easier to add heat and humidity than take it away).
If the addition of an air conditioner alone does not reduce the humidity level within the enclosure purchase a dehumidifier for the room that your gecko is housed in, this will (if properly sized) do the trick if the room is properly sealed from the exterior high humidity levels.
To sum it up, measure humidity within the enclosure, if they are high reduce the humidity in the room the enclosure is placed by air conditioning, dehumidifier, or both. Either air conditioners or dehumidifiers work best in a room that is shut off from the outside (closed doors and windows).
Maurice Pudlo