You can keep your cockatiel outside in an outdoor aviary, you must ensure these conditions if you are to do so. An outdoor aviary needs to be draught free, dry, have an insulated room to shelter from shade and should face the sun.
If you keep an outdoor aviary you can buy different sand to the standard sand that is used in indoor cages. Coarse sand will allow you to be able to rake away the droppings and used for longer before it needs to be replaced. Alternatively you can use pea shingle, which can be hosed down regularly with water.
The mesh that makes up the walls of your aviary should be buried into the floor down to about 35cm to stop rodents from getting in. The gauge for the wire used in most aviaries is 19G.
The roof of the outdoor aviary should offer an open space where the cockatiels can bathe in light rain showers. It should also offer a good amount of shelter in case of heavy rain. Place all the food and water containers underneath the sheltered section. In extreme cold conditions the water in the containers can freeze, it is important to change water frequently or buy water containers with heating elements.