You would need 1) the sun to actually be shining, 2) no cloud cover and 3) probably be naked. Most of us won't get enough vitamin D from being outside for an hour.
1. The key UV for vitamin D production by our skin are UVB rays, optimal between 10am and 2pm, except in the winter at 40+ latitude when the rays travel longer through the atmosphere due to the low solar angle. For instance in Washington state your skin cannot produce vitamin D from November to February.
2. UVB do pass through clouds. (But not through glass windows.)
3. Arms and face exposure are sufficient if you spend 10, 20, or 30 minutes a day depending on UV index of the day (the higher the UV index the less time you need to trigger vitamin D production) and on your skin color. There is some kind of "reservoir" effect that limits production of vitamin D to a maximum, so sun exposure time beyond the above does not have any benefit in terms of vitamin D production.
