So I bought a Turtle Wax product and gave it a go today. It was quite warm and the car was warm, not hot, but warm to the touch. Many of these products claim they can be used in direct sunlight but, on today's experience, I would counsel against it. It was hard work - not so much removing the dirt because you shouldn't apply much pressure - but there was quite a bit of streaking which took some buffing.
Optimum No Rinse would be a better option in these conditions.