Well, here's a thought on keeping the clay warm when temps outside are such that they cause the clay to become cold and difficult to knead...
You can try a number of different yet similar things, such as heating up a pan of water, not too hot, put the clay in a sealable baggie and take the whole ting outside with you. When not using the clay while wiping off what you've just dine, put the clay in the baggie andin the pan of water.
Another possibility would be to get a small container that is used to keep food either cold or warm, buy one of those items, whatever they are called that allow you to either heat them up or make them cold, heat that up, put it in the container and keep the clay in there and take that outside with you.
Those are just a couple ideas, but anything along that line ought to to the trick I would think.