5 Humidity levels for growing orchids
Most orchids require 60 % to 80 % humidity. These humidity levels are necessary for the plants to perform at their best and reward you with blooms that stay perfect for the longest possible time.
Although these levels may appear high, they are in fact well within the comfort zone for people which is 40 % to 70 % relative humidity.
In the summer time the natural humidity is usually sufficient to meet the needs of your plants, except on bright, sunny, dry days.
Air conditioning in the summer and artificial heat in the winter, especially from forced air heating and electrical baseboard heating, will dry the air well below the need of your orchids.  
Plants placed in the path of air conditioning or in the path of forced air heating or next to a radiator or next to a heat source such as a refrigerator can get quickly desiccated, loose their buds and even their leaves in just a few days.
Consider investing a few dollars in a hygrometer to help you evaluate your conditions.  
If your humidity levels are consistently too low, consider buying a humidifier.  A cool mist humidifier can cost as low as $ 20 to $ 25 and will help considerably in maintaining the health of your plants.
Another way of increasing humidity is by setting your plants on trays filled with pebbles or gravel and with water, as long as the plants are not in contact with the water.
For this to be effective the trays must be wide enough so that the leaves of your plants are over the tray (from where the humidity will raise).
A tray for a single plant will not be of much help as the little humidity rising from it will disperse very fast.
Ideally you want to have a dozen or more plants grouped together as they will create a micro climate with higher levels of humidity.
In my opinion, the humidifier is the better solution. But if you use one make sure the mist does not blow directly on your plants as this will eventually wet them and promote bacteria and fungus growth that may well kill your plants.