I usually plan ahead. Sometimes I'll just pull a couple burgers out of the freezer and fire up the gasser for them. Anything more and I prefer to light charcoal. I made my first charcoal fired pizza last week and had to start the dough the previous day so it took planning. Lots of evenings it's just a nice salad. Lunch is often leftovers or a sandwich.
Outside conditions don't usually matter. Darkness is more an issue than cold.