SOME YEARS ago, after a typically vicious Chesapeake squall capsized a fishing boat off Norfolk and drowned 13 people, I asked a meteorologist what it is about the Bay that produces summer thunderstorms of such extraordinary power.

The factors are not mysterious, he said. They're the same ones that breed storms everywhere: superheated air rising rapidly from sun-baked summer landscapes, juxtaposed with cooler, moisture-laden air evaporating from a large body of water; rapidly moving weather systems, and flat terrain over which winds can sweep and build without interruption.

The Chesapeake region's geography, he said, compounds and concentrates these factors and funnels them from nine major river valleys onto the nation's largest estuary.

Getting squallbound in the rivers themselves can be just as bad: A friend once had a steel houseboat pounded almost to pieces within a few minutes by a storm that blew into the mouth of the Potomac and stacked waves six feet high and only 30 feet apart.

Anyone who spends time on the water can learn to spot squall-breeding conditions and take appropriate measures. Some things to watch for:

TIMING --

The meanest thunderstorms usually hit the Bay after 4 p.m. Those who anchor early generally can relax and watch them through the portholes.

TEMPERATURE --

Usually in the high 80s or 90s, often in conjunction with weak southerly winds from a weather system stalled in the area for several days.

SKY COLOR --

Squalls often build in bright haze when the afternoon sky to the west shows a coppery hue, often for an hour or more.

CLOUDS --

Towering altocumulus clouds (the classic thunderheads) mounting in the western sky.

None of these factors guarantees a squall, but the appearance of any of them is reason enough to closely monitor VHF weather forecasts and watch for rapidly changing conditions. A lot of static on VHF or any radio is another dangerous sign.

Squalls on the Bay usually are highly localized and very concentrated in power, and typically -- but not always -- approach from the west. If you take a compass bearing on one and it bears north of northwest or south of southwest it will probably miss you. If it bears between northwest and southwest, you should take immediate precautions: drop and secure sails, seek shelter, and have the life jackets handy.

Hug a western shore if possible and stay well clear of areas like the shipping lanes or the Bay Bridge where maneuvering in restricted visibility would be particularly dangerous.

Anchor, if possible, before the squall gets to you. If you must keep moving, motor slowly into the weather, preferably toward a windward shore. Chances are the storm will blow over in 30 minutes or less. You might take all the precautions and have some cloud that looks like it's carrying Dorothy and Toto sweep harmlessly over you.

But if you ever meet a card-carrying Bay squall head-on, you'll never again worry about being too cautious.