You can work out of doors at any time of the year if you go prepared.  I offer a few tips on how to be prepared.  What you will need will depend on the medium you are using, drawing outside is relatively simple equipment-wise but painting is a bit more complicated.

Drawing outside is a pleasure and gives so many advantages such as the quality of the light, seeing in context and in three-dimensions.  All you really need to draw outside is a good hard-backed sketchpad and two – three drawing pencils in different grades of B. 

A more developed sketch or drawing might require sitting rather than standing and this case you might want to bring a simple folding camping stall and perhaps a lightweight drawing board.  Drawing outside in the summer will require water and suntan lotion, you may also want to wear a hat because when you are looking down the back of your neck can get burnt without realising! 

Painting outside requires more equipment, the media that lends itself best to working on location is watercolour.  You can buy ‘field kits’ for watercolour which are tiny boxes of paints and brushes and these are ideal.  You will also need to take a bottle of water and a container to pour water into and a few brushes (on location I usually limit myself to three brushes or even just one to make life simple).

Acrylics and oils are less good on location I think, acrylics because they dry so quickly and oils because of the various mediums, solvents etc. you might require.  I tend to work slowly in acrylics and oils so a much longer sitting would be required in these mediums, something I would only consider on the warmest of days without any risk of strong winds or rain.  However many artists do enjoy working outside in oils and acrylics and there are solutions to these issues. 

Here is a list of equipment that I think you need on location:

A good quality hard-backed sketchpad (this will save on a drawing board and will travel well)

A good set of soft pencils for drawing (B, 2B, 4B)

Folding camping stool (optional)

Watercolour field set of paints

Small bottle of water

Small container to hold water

Camera (optional) for reference photographs

Sun cream

Packed lunch and water

If you are going outside in the winter then I would add hat, fingerless gloves and warm waterproof coat to the list!