The TRAVELER'S JOURNAL: March seventh, sitting in a desert 35,000 feet above sea level.

The atmosphere in a modern jetliner is pressurized, warmed and filtered to make passengers feel right at home, that is assuming your home is someplace high and dry like Flagstaff, Arizona. Most travelers agree an airplane's artificial atmosphere is considerably less than comfortable. Couple dry, stale, thin air, with jet lag, motion sickness and all the other maladies that come from sealing several hundred strangers in a small space for hours on end, and it's easy to see why long flights can be a challenge, especially for those with heart or lung problems. Here are some time-tested tips for in- flight comfort.

Takeoffs and landings can be ear-popping. For most folks, changes in cabin pressure can be subdued by chewing gum, sucking mints or simply swallowing. If your head's stuffy, a decongestant taken an hour before departure may help. Dehydration is another concern in the desert dry atmosphere, so drink plenty of liquids. Bring a bottle of water, or order fruit juice or non-caffeine drinks. Go easy on the alcohol; one drink in the air is worth three on the ground.

If you suffer motion sickness, ask for a seat over the wing, where the ride's smoothest. Dramamine or scolpolomine skin patches can provide enough relief for even the longest flight. Finally, a medical rule of thumb states that flying should be safe for anyone who can walk a block or climb a flight of stairs without becoming short of breath. But if you've suffered recent heart or gastro- intestinal illness, consult your doctor before making plans to fly.