when I write, when is it appropriate to describe a person's appearance, the clothes they wear, what time it is, the sky outside, and their current location?
When you feel like it, or when the story needs these details.
Many locations don't need describing, you just have to say where the character is and readers will picture the scene. An office, a kitchen, a dentist's waiting room. We can already supply the details.
I don't always describe POV characters. Other characters' reactions to them hints whether they're good-looking or not, whether they're liked or disliked, what their rank is (social or professional). Quick example from an old story just for fun's sake:
Macauley of all people was waiting for him, a cup of tea in one hand and a sandwich in the other. Heaven forbid that Macauley should be denied his midnight snack -- he'd fade away to a shadow. North climbed shakily out of the cockpit and lurched toward him, relieved to be on solid ground again.
"Good of you to drop in, Laddie," Macauley shouted above the rising din of the patrol car's thrusters. "The rest of us can go home now that you're here."
Two cops at a crime scene, North is the superior officer who just got flown in to the location, he's terrified of flying and heights. Macauley is the kind of slob who eats while watching other people examine dead bodies. This is all the description they get when they're introduced.
When the POV character meets other people, describing physical features can get boring, so I try to focus only on what's unusual or on what's important. But again just for fun example's sake here are 3 short snippets taken from another old story that do supply physical description:
An oil lantern came into view, held by a tall, well-dressed noble with dark eyes, a hooked nose and lips that looked as though they'd never smiled. He wore his arrogance like an impenetrable cloak. His cold, unblinking gaze studied every inch of the cell before settling upon me.
This is a significant major character so I gave him a splash of paint to help the reader see him.
I discovered two men standing there, watching me. They wore long black cloaks, tricorn hats and scarves that covered their faces so that only their eyes were visible. Both carried flintlock pistols, cocked and pointed at my belly. They looked more like highwaymen than anything else...
This is a dangerous moment for the story's hero, the description gives the men a sinister appearance.
I lifted my arms while the servant fastened the belt about my waist. Then I noticed Patrizia, talking to a handsome, fair-haired fellow who leaned indolently against a door frame, his arms folded across his chest. He pretended to listen to what she was saying, but I sensed his entire attention was focused on me at that particular moment.
This "handsome, fair-haired fellow" appears later in the story, at which point he will be easily recognizable, his description here is planted as a visual clue.
So, 4 examples with 4 different reasons to describe or not describe the characters. Sometimes the reader needs to "see" stuff, sometimes the story needs info. Sometimes a personality trait is enough and the reader doesn't need to see much physical description.
-Derek