What causes white smoke from diesel engine when its hot outside?

White smoke is unburned fuel, usually caused by low combustion temps in the engine.

It should clear up after the engine warms up. If it takes a long time or the smoke persists when the engine is warm, it could be retarded injection. If you have a cold start lever pull it to the first notch and see if the smoke disappears. If not it could be an injector problem, or an injection problem (injection pump TDI... etc)