I paint landscapes outdoors, in oils, on panels about the size of a paperback book. I have done it for four years now, and this year I decided to keep a diary of the season, partly so I'd remember what I learned and partly because when I started I could not find anyone to tell me the plain, practical things: what to carry, how to keep the wind from taking the painting, what to do when the light changes and the painting you started is no longer the painting in front of you.

"Plein air" just means "open air." It's painting in front of the subject instead of from a photograph or from memory, in the actual light, with the actual wind. The French Impressionists made it famous, but people had been doing it long before them and are doing it all over the country now. There is nothing mystical about it. It is mostly carrying things, and looking.

I work in the city during the week and drive upstate most weekends between April and October, to a county of dairy farms and small rivers about two hours north. I'm not a professional and I don't sell much. This is not that kind of page. I'm forty-six years old and I'm trying to learn to see.

The first year I carried a full French easel, a box of twenty tubes, a jar of turpentine and four canvases, and by the time I'd walked to a spot and set up I had lost the light and my enthusiasm. The whole point of what follows is less.

The pochade box. A small wooden box — mine is about 10 by 12 inches — that holds the palette in the bottom and the panel in the lid, so when you open it the painting is upright in front of you and the paint is under your hand. It screws onto a camera tripod. Box, tripod and a shoulder bag: that's the whole easel, and I can be set up in three minutes. I bought mine from a cabinetmaker who builds them; you can also make one from a cigar box, and several people I've met have. "Pochade" is French for a quick sketch, which is what comes out of it.

Panels. Not canvases. I use hardboard cut to 6x8 and 8x10 inches and gessoed with three coats, sanded between; a sheet of quarter-inch hardboard from the lumberyard makes about thirty panels for a few dollars. Some people use canvas glued to board. Small panels dry faster, carry easier, and a small panel forgives you for spending only two hours on it. I have painted a 12x16 outdoors exactly once, in May, and it was a struggle from the first minute to the last.

Wet panel carrier. The unsung hero. A box with slots so two wet panels can ride home face to face without touching. Mine holds four. Before I had one I drove home with a wet 8x10 on the passenger seat and every one of them had a dog hair in the sky.

Brushes. Four or five hog bristle flats, sizes 4 to 10, and one small round for the last marks. That's all. I had twenty and used four.

Solvent. Odorless mineral spirits in a small screw-top jar with a coil in it, never turpentine anymore; I got tired of the headache and the smell in the car. A little bottle of medium (linseed oil, cut with the spirits) for later passages.

The rest. Paper towels; a plastic bag for the used ones (never leave them; they can start a fire in the sun). A palette knife. A viewfinder, which is a piece of gray matboard with a rectangle cut out, to decide what's in the picture. Bug spray. A hat. Water. A folding stool I mostly don't sit on. And a little notebook where I write the date, place, time and weather on the back of every panel before I pack up, because you will not remember.

I carry five tubes:

That's a warm and cool of each primary, minus a cool yellow I decided I didn't need, plus white. No green in a tube; every green in a landscape gets mixed from the yellow and the blue, and grayed with a touch of red, and that is exactly why the greens stop looking like a golf course after the first season. No black either; ultramarine and alizarin make a dark that's alive.

The reasons for a small palette are practical and then, later, not. Practical: five tubes weigh nothing, the palette stays organized, you never hunt for a color. The other reason is that with five colors every mixture on the panel is related to every other one, and the painting holds together in a way that I could not get when I had twenty tubes and used all of them. Mixing a color also makes you look harder at the thing you're mixing it for. That's the part that took me two years to understand.

I lay the paint out the same way every time, white in the corner and the colors in that order along the edge, so that my hand knows where things are when my eyes are on the field.

The thing nobody tells you and the thing that changes everything: the light moves. The sun is going across the sky the whole time you're painting, and the shadows go with it. The barn that had its whole east wall lit and a long shadow across the yard at nine o'clock has, by eleven, a shadow half the length pointing somewhere else, and by one the wall you were painting is in shade. If you keep painting what's in front of you for four hours you end up with a painting of four different mornings, and it looks like it.

So: two hours, roughly. In the first fifteen minutes decide where the light is coming from and where the big shadows fall, and put them down. After that the shadows on the panel are the ones you keep, no matter what the field does. When the light has moved so far that the subject is a different subject, stop. If the painting isn't done, come back tomorrow at the same hour, or start a second panel. (Monet, famously, worked on a row of canvases and moved from one to the next as the day went on. I have four slots in my wet panel carrier, which is my version.)

Some other things about light that I now do without thinking:

• Squint. Squinting collapses everything into three or four values — the sky, the lit things, the shadowed things, the darkest darks — and those big value shapes are the painting. The details are not the painting. This took me a long time to believe.
• Block in the whole panel in the first twenty minutes with thin paint, big brush, no detail. Sky, land, the shadow shapes. Then work back into it thicker. A panel that's covered edge to edge in twenty minutes can be finished in two hours. A panel where you've spent forty minutes on a fence post cannot.
• Morning or late afternoon. Low sun makes long shadows and warm light, and the color of the light against the color of the shadow is the whole music. Midday is flat and hot and I eat lunch then.
• Keep the sun off the panel. If the sun hits the painting while you work, you mix everything too dark and get home to a muddy picture. Face the box away from the sun, or clip an umbrella to the tripod (about which, see wind, below).
• The sky is not blue at the bottom. It's pale and warm near the horizon and gets bluer as it goes up. Everybody's first fifty skies are one flat blue.

Wind is the enemy, ahead of rain. Rain sends you home; wind lets you stay and ruins everything. A pochade box on a tripod is a sail. Things I've learned the hard way, which is the only way I've learned anything about wind:

• Hang the shoulder bag from the center of the tripod. The weight settles it.
• A bulldog clip on each side of the panel. Without them I have seen an 8x10 leave the box, fly forty feet and land face down in a pasture, and the cows were interested.
• The umbrella that shades the panel is a bigger sail than the box. On a windy day, skip the umbrella and turn your back to the sun instead.
• Paper towels weighted under a rock or they end up in the river. Tube caps in your pocket, not on the box lid.
• Wind dries the paint on the palette while you're looking at the sky. Mix a little more than you think and work faster than you'd like.

Bugs. Gnats love wet oil paint and a wet panel is a sticky trap. They fly into the sky and stay there. You can pick them out with the point of the knife while the paint's wet and touch the spot up, or you can let it dry and flick them off with a fingernail, which leaves a little hole that mostly doesn't show. There is a deer fly in one of my July skies that I decided to leave; it's over the barn and looks like a bird if you don't get close. Mosquitoes at dusk near the river will end a session faster than the light does. Long sleeves, spray, and get down there earlier.

Heat makes the paint soft and slippery and makes you stupid by noon. Cold (April, October) makes the paint stiff and your hands stiffer; I paint in fingerless gloves and put the tubes in my pocket. Rain: a car is a fine studio and the windshield is a decent viewfinder. I've painted three panels through the glass this year and one of them is the best thing I did in June.

There's a red dairy barn on County Road 12, on the Halvorsen place, that sits on a little rise with a stand of maples behind it and a pond in front. I painted it first in April because it was the first thing I saw when I got out of the car. I have now painted it twelve times, and I intend to keep going until the snow.

The idea, which is not my idea, is that if you paint the same subject over and over, the subject stops being interesting and you're forced to paint what's actually interesting, which is the light. Barn number one is a picture of a barn. Barn number twelve is a picture of a cold, clear September morning that happens to have a barn in it. Between them:

• No. 1, April 10, morning. Bare maples, brown fields, a thin sky. Too much red on the barn; I painted the barn I knew instead of the barn in the gray light.
• No. 2, April 24, morning. Same spot. Fixed the red. The pond too blue.
• No. 3, May 8, late afternoon. First time from the other side, sun behind the barn, the whole thing a dark shape against a lit field. Best of the spring.
• No. 4, May 22, noon. Flat, dull. I keep it to remind myself about noon.
• No. 5, June 5, morning, after rain. Everything washed and saturated, the maples in full leaf for the first time. The greens gave me fits.
• No. 6, June 19, evening. Long shadows, warm wall, cool shadow. The one I'd frame.
• No. 7, July 3, morning, haze. Everything pale. Learned that a hazy day is a painting about the sky.
• No. 8, July 17, morning. The deer fly. Otherwise fine.
• No. 9, July 31, afternoon. Thunderheads coming up behind the maples. Painted fast, quit early, got wet.
• No. 10, August 14, morning. Hay bales in the front field for the first time. Suddenly a different composition. Mr. Halvorsen came out and looked and said "huh," which I choose to take as praise.
• No. 11, August 28, evening. The corn behind the pond tall enough to hide the road. Dry summer; the field gone gold.
• No. 12, September 18, morning, cold. First color in the maples. Long shadows. Frost on the roof burning off as I set up. Two hours exactly. Done.

Twelve small panels of one barn, laid out on the floor in a row, are the most useful thing I've made in four years. I can see the whole year go by, and I can see what I got better at and what I didn't. Anyone who wants to learn this should find a barn.

Every panel I finished, or gave up on, this year. The barns are listed above and aren't repeated. Places are all within about twenty miles of the Halvorsen farm.

Twenty-five panels counting the barns, of which I'd show maybe nine to anybody. That's about the ratio every year. The rest go on a shelf in the closet and I look at them in January and learn from them then, which is what they're for.

Plans for the rest of the season: the maples, obviously, in October; the barn as long as the roads are clear; and one try at snow, if it comes early, with the tubes in my pockets. I'll add to the log until the paint gets too stiff to move.